I. Human EcologyAmos H. Hawley
II. Cultural EcologyJulian H. Steward
The term “ecology,” which has its root in the Greek word oikos (household or living place), came into use in the latter part of the nineteenth century in the works of zoologists and botanists to describe the study of the ways in which organisms live in their environments. Soon two branches of ecology were distinguished: autecology, the study of the individual organism’s interaction with environment, and synecology, the study of the correlations between the organisms engaged with a given unit of environment. The latter study has prevailed, however, and has become the principal connotation of ecology, since it became evident in numerous field studies that organisms, whether plant or animal, establish viable relationships with environment, not independently but collectively, through the mechanism of a system of relationships. Bioecologists were thus led to employ a set of concepts and techniques of investigation that imparted a markedly sociological coloration to their work.
Origins and history
The ecological approach was introduced as human ecology into the field of sociology at a critical period in the development of the latter discipline. In the 1920s the reformistic phase of sociology was drawing to a close, and the subject was gaining acceptance as a respected discipline in the curricula of American universities. That the transition would have been effected so quickly without the aid of a theoretical framework lending itself to empirical research seems doubtful. Ecology opportunely provided the necessary theory. A period of vigorous research followed that was to prove instrumental in launching sociology on its career as a social science.
Sociologists made free use of analogy as they borrowed heavily from the concepts of plant and animal ecology. The Darwinian notion of animate nature as a web of life became at once a general orienting concept and a basic postulate; it directed attention to the necessary interdependence among men as well as among lower forms of life. A second concept, the balance of nature, denoting a tendency toward stabilization of the relative numbers of diverse organisms within the web of life and of their several claims on the environment, provided human ecology with its characteristic equilibrium position. The more or less balanced web of relationships, when viewed in a specific local area, presented the aspect of community, a concept with obvious appeal for students of human social life.
According to plant and animal ecologists, the community, or ecosystem, is a population comprising a set of species whose reactions to the habitat and coactions between each other constitute an integrated system having some degree of unit character. Coactions involve members behaving both with reference to their similarities in an mtraspecific relationship known as commensalism and with reference to their differences in what is called symbiosis, an interspecific relationship. The community develops from simple to more complex forms through a sequence of stages described as succession. Each stage in the sequence is marked by an invasion of a new species or association of species, the series culminating in a climax stage in which a dominant species appears. The dominant species is related to the environment in such a way that it is able to control and maintain the community indefinitely. The community, then, tends to approximate a self-maintaining, or “closed,” system.
Community and environment
The application of the concepts from plant and animal ecology to the human community carried with it the implication that the community was essentially a natural phenomenon, which meant that it had developed independently of plan or deliberation. From this it was a short, though uncritical, step to the interpretation of human ecology as a study of the biotic or subsocial aspect of human social organization (Park 1936), a view that was elaborated at some length by Quinn (1950). Not only did the subsocial characterization convey an excessively narrow concept of social organization, but it posed an operational problem for which there was no workable solution.
A somewhat different definition of human ecology, which ignored any reference to the cognitive level of events, was enunciated by McKenzie ( 1925, pp. 63–64), whose formulation of the subject as a study of the spatial and temporal relations of human beings, affected by the selective, distributive, and accommodative forces of the environment, was widely accepted as authoritative. Although McKenzie’s definition inspired a large amount of fruitful research effort, it had the unfortunate effect of concentrating attention almost exclusively on spatial distributions and correlations. In consequence, many promising implications of ecological assumptions were neglected.
Hawley, in attempting to restore a conceptual continuity with plant and animal ecologies, advanced the view of human ecology as the study of the form and development of the human community (1950, p. 68). Community, in this connection, is construed as a territorially localized system of relationships among functionally differentiated parts; human ecology, then, is concerned with the general problem of organization conceived as an attribute of a population—a point of view that has been shown to be consistent with a long-standing sociological tradition (Schnore 1958). Although the emphasis is centered on the functional system that develops in a population, it is not intended to exclude concern with spatial and temporal aspects; rather, these aspects are regarded as useful dimensions for the measurement of organization.
A further step in making the orientation of human ecology explicit within the larger context of general ecological theory was made by Duncan (1959, pp. 683–684), who described four principal variables of human ecology—population, organization, environment, and technology—that constitute an ecosystem. In other words, while any one of the four may be treated as a dependent variable for certain purposes, it is also reciprocally connected with each of the other variables. The virtue of this perspective lies in the range of problems it opens to the student of human ecology. Yet it seems unlikely tin various eco-logical studies, although hat that advantage can be fully enjoyed without a clear notion of how organization is constituted.
While sociologists were at work defining human ecology for their purposes and pursuing many of its research leads, the concept spread into various other fields of inquiry. Human geographers wondered if the term “human ecology” was not a more apt characterization of their discipline; but their historic preoccupation with the landscape and their general addiction to a macroscopic treatment of occupance led them to discard this notion. Archeologists, in their efforts to reconstruct population distributions, have made use of ecological concepts and techniques, but without attempting to give formal statement to the approach.
Studies of human evolution by anthropologists involved questions of the man-environment relationship, which, in turn, led them to describe their work as human ecology; indeed, social or cultural anthropologists have long engaged in various ecological studies, although not until recently have they so defined their activity. One definition by an anthropologist is that of Steward (1955), for whom human ecology is the study of the adaptation of specific items of culture to particular environments. This conception, which reduces ecology to something akin to a research technique, is shared by a number of Steward’s contemporaries.
The language of human ecology has also made its appearance in economics, psychology, epidemiology, and other fields. In some instances, the term is used merely as a label for an environmental emphasis; in others, it is put forward in an effort to broaden the purview of a discipline.
But in spite of the widespread diffusion of ecological concepts, responsibility for systematic development of human ecology has been left to sociologists, who have drawn heavily on related fields for both theoretical and technical aid. The writings on real estate, finance, public administration, demography, planning, history, and other areas of inquiry, in addition to the literature of the fields previously mentioned, have at one time or another been exploited in the interest of human ecology. The reason for that catholicity of taste is not hard to find; human ecology is concerned with sociological problems in their fullest breadth. It overlaps, therefore, all the spheres of learning that concern the social life of man.
Distinctive features of human ecology
As human ecology has moved toward a major concern with the general problem of social organization and thus closer to the central concern of sociology, it has retained certain distinctive features. Foremost among these is the importance attributed to environment.
The broad, positional hypothesis is that organization arises from the interaction of population and environment. Environment, however, defined as whatever is external to and potentially or actually influential on a phenomenon under investigation, has no fixed content and must be defined anew for each different object of investigation. Environment is seen both as presenting the problem of life and as providing the means for its resolution; to adopt this position is to place the problem in a time-space context.
A second distinguishing characteristic is the emphasis on population as the point of reference; organization, it is contended, is exclusively the property of a population taken as a whole and not of an assemblage of individuals. Obvious as this position may be, it has profound methodological and theoretical implications for the manner in which the ecological problem is put, the variables employed, and the data to be observed. The concreteness of population, however, in contrast with the seemingly ephemeral nature of organization, tends to beguile the student to a view of population as the independent variable in all things pertaining to organization. It is obviously more convenient to proceed from the more accessible to the less accessible by asking how a population makes a unitary response to its environment; yet that is merely a common-sense way of approaching the problem, and investigation soon makes it apparent that population is for many purposes better regarded as the dependent variable, delimited and regulated by organization.
A third characteristic is the treatment of organization as a more or less complete and self-sustaining whole. The interaction of population and environment is seen as culminating in a system of relationships between differentiated parts which gives the population unit character and enables it to maintain its identity. As the property of a population, organization lends itself most readily to a morphological, or structural, analysis. The parts are the units—individuals or clusters of individuals—that perform functions and the relationships by which these units are linked. Differing configurations of unit functions and relationships are expected to occur with differences in relationship to environment and at different stages in development.
A morphological concern does not exclude the problem of development. Presumably, any given form of organization had an earlier form and is capable of having a later form. Every organization has a history, perhaps a natural history, the knowledge of which may shed light on the nature of organizations by indicating, for example, at what points they are vulnerable to change and how change spreads through them.
Related to the conception of organization is a fourth distinctive, although by no means unique, attribute of human ecology: the central position given to an equilibrium assumption. Morphological change is assumed to be a movement toward an equilibrium state, whether through a succession-like sequence of stages or through a process of continuous modification. Unlike the equilibrium notion in some of its other applications, such as in functionalist theory, the ecological usage of the term harbors no teleological overtones; on the contrary, this usage merely implies that as an organization attains completeness, it acquires the capacity for controlling change and for retaining its form through time, although the interval need not be specified. To put it differently, to the extent that an organization possesses unit character, an approximation to equilibrium obtains.
There is a further implication that a stable relationship with environment is contingent on relative stability in the relationships between the parts of an organization. A population always remains open to environment, but the formation of organizations canalizes environmental influences and makes for increasing selectivity of response.
The term “community” has commonly been used to denote the unit of organization for ecological purposes. Operationally defined as that population which carries on its daily life through a given system of relationships, the community is regarded as the smallest microcosm in which all the parameters of society are to be found. For reasons that were largely fortuitous, human ecologists at first focused their attention on the city and its tributary area as the prototype of community; later, in an effort to encompass the antecedents of cities, they broadened their consideration to include all forms of nucleated settlement. The term “community,” however, has the disadvantage of referring to the organization of a more or less localized settlement unit that does not always approximate a self-maintaining whole. For example, in an extensively urbanized society, local settlement units are usually components of more inclusive systems; in that event, the entire system must be treated as the object of study. But the difference between a simple, compact organization and a large, diffuse one is primarily a difference in scale; accordingly, the principles of organization should be the same in each. Since the designation of both simple and complex systems as communities threatens confusion, a more neutral term, such as “social system,” is to be preferred.
Principles of ecological organization
Inasmuch as principles of organization hinge on what is meant by population, it is imperative that the concept be developed more fully.
A human population is an aggregate of individuals who possess the following characteristics. As a living organism, every individual must have access to environment, for that is the only possible source of sustenance. Moreover, the interdependence of individuals is necessary; this condition, which obtains for all forms of life, holds true to an exceptional degree in the case of man, because of the naked state in which he comes into the world and his long period of postnatal maturation. Interdependence is the irreducible connotation of sociality.
The human being also possesses an inherent tendency to preserve and expand his life to the maximum permitted by prevailing circumstances; this is a general motive of which all other motives are special cases. In its most elementary sense, expansion of life refers to the multiplication of man-years through either the leaving of progeny or the extension of longevity; but it also includes all that is involved in the realization of that objective. Another important characteristic is the indeterminacy of the human being’s capacity to adapt; there is no known restriction on the kind or extent of refinement of activity in which he can engage. Finally, the human individual, again like other organisms, is time-bound; the recurring needs for food and rest fix the fundamental rhythm of life and regulate the allocation of time to all other activities. Accordingly, the time available for movement is limited and, in consequence, the space over which activities can be distributed is correspondingly limited.
These several attributes of individuals not only define the kind of population with which human ecology deals, but they also constitute a cardinal set of assumptions from which principles of ecological organization may be deduced. The following are some of the more salient of these principles of organization.
Principle of interdependence
Interdependence develops between units on each of two axes: the symbiotic (on the basis of their complementary differences) and the commensal (on the basis of their supplementary similarities). That is, units that combine in a symbiotic union may also enter into other combinations of a commensal character; the effect of each type of union is to raise the power of action above what it would be were the units to remain apart.
The effect, however, is not the same in each case. The symbiotic union enhances the efficiency of production or creative effort; the commensal union, since its parts are homogeneous, can only react and is suited, therefore, only to protective or conservative actions. Although commensalism is an elemental form of union, it is applicable to a wide array of situations. The point of importance at present is that a population tends to be knit together through an interwoven set of symbiotic and commensal relationships.
It should be evident that interdependence has temporal and spatial implications for the units involved. Relations of functional complementation and supplementation require mutual accessibility among units; since this is contingent on the time available for movement, the distance separating related units is always subject to some limitation. In general, for every set of related units, there should be, other things being equal, an appropriate pattern of distribution in the temporal and spatial dimensions.
Principle of the key function
A second principle may be described as the principle of the key function. That is, in every system of relationships among diverse functions, the connection of the system to its environment is mediated primarily by one or a relatively small number of functions, the latter being known as the key function or functions. To the extent that the principle of the key function does not obtain, the system will be tenuous and incoherent; in the extreme case, in which no system exists, every function has the same relationship to environment. Given a functional system, then, there are always some functions or functional units directly involved with environment and others that secure access to the environment indirectly, through the agency of the key function.
The notion of key function invokes the question of how to define the notion of environment, which can refer to many different kinds of things. For present purposes, these things may be classified in two broad categories: the natural and the social. Although every organized aggregate must contend with both, the relative importance of each may vary over a wide range. In some instances, because of the inaccessibility of the settlement, activities of necessity center on the exploitation of the local natural environment, while influences from the social field are relatively infrequent or of no great consequence. In this event, the key function is the activity that extracts the principal sustenance supply from local resources. But where the product from local resources, or a substantial part of it, is exchanged for other sustenance materials, whether through trade or other distributive mechanisms, the key function is determined by the comparative importance of production and of trade as sources of sustenance.
In many such instances, no distinction is necessary because the producer is also the trader—two functions combined in one functionary. But even before the two functions appear as separate specialties, the requisites of trade or distribution begin to regulate the uses of local resources. As the reliance on exchange advances, the social environment actually displaces the natural environment as the critical set of influences. A population is never emancipated from its dependence on physical and animate matters, but the importance of locale declines with increasing involvement in a network of intersystem relations; the natural environment is extended and diffused, and contacts with it are mediated through a variety of social mechanisms. Hence, the functions that link the local system to the social environment come to occupy the key position.
Principle of differentiation
The extent of functional differentiation varies with the productivity of the key function or functions; this is the principle of differentiation. A corollary is that the size of population supportable by the system varies with the productivity of the key function. For given the simplifying assumption that each unit is fully occupied, the number of people is determined by the number of functions to be performed.
In a hunting and gathering community, for instance, productivity is usually low, even though the physical environment is richly endowed; hence, there is little time or opportunity available for the cultivation of more than a few specialized functions. Nor is it possible to support enough people to staff even a moderate extent of specialization. By contrast, where the key function is devoted to stable agriculture, the range of possibilities is much greater, while in an industrial system productivity is so great that there are no known upper limits on either the number of specializations or the size of the population that can be supported.
The productivity of the key function, then, constitutes the principal limiting condition on the extent to which a system can be elaborated, on the size of population that can be sustained in the system, and on the area or space the system can occupy.
A question of some importance has to do with the relative number of units engaged in each of several interrelated functions. That question remains unanswered at present. It may be suggested, however, that the number of units engaged in any given function is inversely proportional to the productivity of the function and directly proportional to the number of units that utilize the product of the function.
It follows, of course, that functional differentiation involves a differentiation of environmental requirements. As the materials and conditions used by diversely specialized units differ, so also will their needs for location in space and time. In general, units performing key functions have the highest priority of claim on location. Other units tend to distribute themselves about the key function units, their distances away corresponding to the number of degrees of removal separating their functions from direct relation with the key functions. The temporal spacing may be expected to reveal a similar pattern. Special location requirements, however, as for type of soil or other resource, may obscure the tendency to a symmetrical arrangement of functions by degree of indirectness of relationship with key functions.
An important implication of the principles of the key function and of differentiation is that of transitivity in the relation among functional units. Relations with environment are necessarily transitive for some units. By the same token, relations among many units are transitive. The advance of specialization increases transitive relations more than proportionally and lengthens the transitive sequences. Thus, it is possible for functional units widely removed from one another so far as direct encounters are concerned to be inextricably linked through their respective linkages with one or more units performing intervening functions. All functions, regardless of kind, are subject to the environmental nexus. They differ only in the immediacy with which influences reach them. Those which operate at or near the ends of chains of relationships may appear to have large degrees of independence of environment. The appearance is illusory, however. It is due, rather, to the time required for effects to reach them and, in complex systems, to their having positions in two or more relational sequences which expose them to countervailing influences.
Principle of dominance
Given the principles stated above, it is a simple inferential step to a principle of dominance. According to this principle, functional units having direct relations with environment, and thus performing the key function, determine or regulate the conditions essential to the functions of units having indirect relations with environment. Dominance, in other words, is an attribute of function.
But while the power inherent in a system is unevenly distributed, it is not confined to the key function unit. Power is held in varying degrees by all other units, in measures that vary inversely with the number of steps of removal from direct relation with the unit performing the key function. A single power gradient running through all the units involved in a system assumes a very simple situation, of course, one in which a single unit occupies a key function position. Before introducing complications into this overly simplified conception, it is opportune to note some further implications of the dominance principle.
Where the progress of differentiation has distinguished a relatively large number of units, they tend to form clusters or complex units. A corollary to the dominance principle sheds light on how that comes about: the greater the extent to which variously specialized units are subject to the environmental conditions mediated by some one unit, the greater their tendency to coalesce in a corporate body. The interdependence among such units is manifestly close, and their requirements for mutual accessibility are correspondingly acute. A hierarchic pattern emerges in which the number of strata corresponds to the number of degrees that the parts are removed from direct relations with the key function; thus, symbiotic subsystems appear as components of a parent system. The nuclear family, although its origins are obscure, fits this principle. But instances with more proximate origins are found in the combinations of specialists to form producing enterprises, welfare agencies, governing bodies, etc., and again in the combinations of producing enterprises, retail establishments, or governments to constitute larger, more complex units. There is no restriction of scale or complexity in the formation of symbiotic or corporate units.
On the other hand, units that are of a given functional type and therefore occupy equivalent positions in the power hierarchy may raise their power potential by the formation of categoric unions. Any threat to a function or to the conditions of its performance can provoke such a response; groups of elders, the medieval guilds, labor unions, professional associations, councils of churches, retail associations, and associations of manufacturers are examples of categoric units. A social class is at most a loose form of categoric unit.
As with the corporate union, the categoric union may be composed of units of any size or kind. It may appear as a federation of categoric units or as an association of corporate units. So long as it retains its pure categoric form, however, such a unit can do little more than react to circumstances affecting it. Nor can it have more than a transitory existence, since in order to engage in positive action of any kind and to attain some measure of permanence, it must develop at least a core of specialists. Although the categoric unit tends to assume the characteristics of the corporate unit, it remains distinctive as long as its criterion for membership is possession of a given, common characteristic. In any event, the categoric unit is a source of rigidity in a social system, and once formed, its effect is to preserve the position of a functional category in the system.
The concept of dominance has been widely employed for the purpose of delimiting the boundaries of systems. It is argued that centers of settlement, such as cities, exercise dominance over their surrounding areas, in diminishing degrees as the distance from the center increases; the margin of influence marks the boundary of the system. Empirical support for this proposition is provided by the evidence of nonrandom distribution of related functions and by the gradient pattern that appears in the frequencies with which outlying functions are involved with central functions.
Two qualifications of this conclusion are needed; first, dominance is exercised from and not by centers, since it resides in functional units rather than in the places where they are located. Further, the apparent decline of dominance results less from an effect of distance in reducing control over related functions than from the increased difficulty of establishing dominance uniformly over an exponentially widening area.
Isomorphism. A principle of isomorphism has been implied in much that has been said and needs now to be stated. Units subject to the same environmental conditions, or to environmental conditions as mediated through a given key unit, acquire a similar form of organization. They must submit to standard terms of communication and to standard procedures in consequence of which they develop similar internal arrangements within limits imposed by their respective sizes. Each unit, then, tends to become a replica of every other unit and of the parent system in which it is a subsystem.
Since small units cannot acquire the elaborate organizations of which large units are capable, they jointly support specialized functions that complement their meager organizations. For example, whereas a large unit may include among its functionaries accountants, lawyers, engineers, public relations experts, and other specialists, the small unit must purchase comparable services from units specializing in each of the relevant functions. The principle of isomorphism also applies to the size of units, at least as a tendency; that is, all units tend toward a size that enables them to maintain contact with all relevant sectors of the parent system.
Closure and social change
Operation of the several principles mentioned thus far moves a system toward a state of closure. This term must be employed here with circumspection, for it cannot have its usual connotation of independence of environment. Closure can only mean that development has terminated in a more or less complete system that is capable of sustaining a given relationship to environment indefinitely. For closure to be realized, it is required not only that the differentiation of function supportable by the productivity of the key function has attained its maximum but also that the various functions have been gathered into corporate and categoric subsystems; moreover, the performance of the key function should have been reduced to one unit or to a number of units united in a categoric federation. Then a system is highly selective of its membership and capable of exercising some control over factors that threaten change in the system.
Under these circumstances, certain conditions of equilibrium are held to obtain: the functions involved are mutually complementary and collectively provide the conditions essential to the continuation of each; the number of individuals engaged in each function is just sufficient to maintain the relations of the functions to each other and to all other functions; and the various units are arranged in time and space so that the accessibility of one to others bears a direct relation to the frequency of exchanges between them. Needless to say, equilibrium as thus defined is a logical construct; the conditions express an expected outcome if the principles of organization are allowed to operate without any external disturbances.
Origins and effects of change
Every social system is continuously subject to change, for since the environment is always in some state of flux, the equilibrium that can be attained is seldom more than partial. A system founded on nonreplaceable resources is faced with “immanent change”; sooner or later it will either pass into decline or shift perforce to a different resource base. Such, for instance, has been the experience of innumerable mining communities; in a similar manner agricultural communities often alter the soil composition of their lands by the uses they practice, with the ultimate result that the lands will no longer support the systems as they are constituted. Instances of maladaptation, such as the reliance of the Irish on the potato as a food staple in the nineteenth century, can lead to catastrophic consequences.
In general, however, change has an external origin, a proposition that follows by definition from an equilibrium position. Some influences emanate from the physical or biotic environment, such as variations in the growing season or invasion by parasites. To the extent that episodic occurrences of that order fail to modify one or more functions comprised in the system, their effects are transitory; the system returns to its original form. But where a function, particularly a key function, is substantially modified, the system must be reconstituted. The disappearance of a game supply, the silting of an estuary, or the eruption of a volcano may render a key function inoperative in its usual location. The population must relocate and work out a new system of activities. Unless there is an increase of productivity, new ways of acting will merely displace old ways, and the change will not be cumulative.
Cumulative change, or growth of the system, presupposes an increase in the productivity of a key function. Only in this way is it possible to multiply specialization, to employ a greater variety of techniques, and to support a larger population. The probability of the occurrence of disturbances having that effect rises with the number of points of contact with a social environment; location, therefore, is an important factor. A site on a traveled route is more exposed to external influences than one situated at a distance from an avenue of movement; a site at a conflux of routes is much more vulnerable to disturbances from without. Any location that fosters frequent meetings of people from diverse backgrounds is a gateway for the infusion of alien experiences and techniques into the social system centered there. Whether change is released through a deposit of numerous small additions to the culture or through a simple, dramatic innovation is immaterial.
The process of cumulative change may be generalized as a principle of expansion. Expansion is a twofold process involving, on the one hand, the growth of a center of activity from which dominance is exercised and, on the other hand, an enlargement of the scope of the center’s influence. The process entails the absorption and redistribution of the functions formerly carried on in outlying areas, a centralization of mediating and control functions, an increase in the number and variety of territorially extended relationships, a growth of population to man a more elaborate set of activities, and an accumulation of culture together with a leveling of cultural differences over the expanding domain. After the revival of trade in Europe, from the tenth century onward, the favorably situated village with its narrow vicinage grew into a market town that served as a center of an enlarged territorial organization. That gave way, in turn, to the emergent city capable of exercising an integrating influence over an area of regional scope. Most recently, the metropolis has superseded the city and has brought under its dominion a vast interregional territory. Although the process is as old as recorded history, it has not advanced in a simple linear progression. It has moved and then stalled in one place, only to surge ahead in another; it has faltered and even on occasion has seemed to turn back upon itself, but it has always resumed its course with renewed vigor.
The limits to expansion are sometimes fixed by the facility of movement between center and periphery—by the maximum distance over which the exercise of dominance is feasible. More often than not, however, the limits are drawn at the points of juncture with the expanding domains of social systems in neighboring regions. As a system encounters its limits, however they are fixed, it loses capacity to absorb further change, and equilibrium tendencies begin to assert themselves once more.
In many instances, however, the first symptoms of change are experienced at the periphery of a system. Since the effects of dominance grow more uneven with increased distance from a center, boundaries are apt to be permeable at many points. Yet, in the degree to which a system is integrated, events at the boundary are transmitted directly to the key unit, from which they are communicated to ancillary units; it is always at the boundary that one system begins its absorption of another. Expansion may be resumed at any moment and in any of the systems that hem in one another. An innovation, even though it might present itself to all systems about the same time, gains admission to one or another by virtue of a more favorable location for its use, a more appropriate organization for its acceptance, or some other local advantage. The renewed expansion encroaches upon the territories of adjoining systems, sometimes reducing them to mere components of a single, greatly enlarged system.
Under conditions of closure, environmental effects, and particularly cultural innovations, would be expected to enter a system through the key function, for the obvious reason that it has the most direct connection with environment. And, presumably, change would spread through the system by affecting units successively in the order in which they are removed from direct relation with a key function. But in a system centered on a convergence of routes, many units may have direct, although not equal, access to the outside world. Change may therefore enter the system at many points, at least until its structure is fully developed with the parts systematically arrayed relative to a key unit. An expanding system, in other words, is an open system, and it remains so until the limits to expansion are reached.
Burgess’ hypothesis of city growth
A hypothesis of city growth, stated by E. W. Burgess ( 1961, pp. 37–44), pertains to a special case of the more general principle. According to that proposal, city growth takes the form of expansion from a zone centered on a highly accessible location. Growth involves increasing density of occupance of the central zone and, at the same time, a redistribution of activities or land uses scattered around the center to conform to a gradient pattern of variation of intensity of land use according to distances from the center. Redistribution results from increasing pressure at the center and a consequent encroachment by high-intensity uses into the spaces occupied by lower-intensity uses in a succession-like manner. Alternating periods of redistribution and stabilization of distribution, and those of growth and partial equilibrium, create a wavelike effect more or less visible in a set of concentric zones. By venturing to describe, in rather specific terms, the content of the zones and by thus reifying a set of statistical constructs, Burgess diverted attention from a growth process and caused it to be fixed on a specious distribution pattern. His argument, therefore, seemed to acquire a historical limitation that it need not have had.
The miscarriage of the import of the Burgess hypothesis is evident in criticisms that have opposed the preindustrial city to the industrial city as a qualitatively different phenomenon. Whereas Burgess suggested that the social-economic status of residents is higher in each successive concentric zone, critics have shown that in the preindustrial city the social-economic gradient runs in the opposite direction. Useful as that finding may be, it misses the essential point. That is, the significant gradient is one of dominance; units tend to distribute themselves over space in a way that reflects their relationships to the dominant unit. In this respect, both industrial and preindustrial cities are similar. The qualitative difference lies in the kinds of units that exercise dominance. In the preindus-trial city, all functions are carried in familial or household units, and power is unevenly distributed among them. But in the industrial society, the household unit has been relegated to a minor position; specialized functions are performed by extra-familial units, and the separation of functions from the household has involved a spatial separation as well.
Furthermore, the notion of a monocentered system is applicable only to the simplest instances. All others include a constellation of settlement nuclei, that is to say, subsidiary service centers within cities, and villages, towns, or cities within hinterlands. Each serves as a locus of influence over a localized area, varying in scope with the types of functions centered in the nucleus. Thus, as Christaller observed, the constellation of nuclei forms a hierarchy by size and number of places and by order of functions performed (1933). Small places provide low-order, or ubiquitous, functions, whereas each larger place performs, in addition to low-order functions, higher-order functions for broader domains. At the apogee of the hierarchy is the metropolis, in which the integrating and coordinating functions for the entire system are domiciled. Thus, dominance is exercised downward and outward through nested sets of subsidiary centers [seeCentral Place].
The least satisfactory aspect of the theory of change concerns its temporal incidence. The idea of succession, borrowed initially from bioecology, lingers in the dictionary of human ecology. Change as cyclical in form, consisting in movements between equilibrium stages, is clearly the most intelligible conception. Nevertheless, apart from the difficulty in empirically identifying an equilibrium stage, there are unsolved conceptual problems of the spacing of stages, and of the factors governing the intervals between stages. Thus far, succession has been applied only in retrospect. Its utility will remain uncertain until it can be projected into prediction. Quite possibly, that may have to wait for more extensive work on social system taxonomy.
The limits of human ecology
Human ecology has progressed since its inception from an effort to apply the concepts of plant and animal ecology to human collective life, through an extended period of preoccupation with spatial configurations, to an increasing concern with the form and development of territorially based social systems. In the last phase, human ecologists have sought to clarify the assumptions of ecology and to draw out their implications for organization. Although the results of that work are far from complete, it seems clear that they indicate the direction in which human ecology will continue to develop.
As with most approaches in social science, human ecology has limited objectives. It seeks knowl-ledge about the structure of a social system and the manner in which the structure develops. Hence, it is not prepared to provide explanations for all of the manifold interactions, frictions, and collisions that occur within the bounds of a social system. The findings of human ecology, however, define the context in which all such phenomena take place, and which is therefore pertinent to their full understanding.
Human ecology is not qualified to deal with the normative order in a social system. Yet consistent with its position is the expectation that a normative order corresponds to and reflects the functional order. The two are different abstractions from the same reality.
Amos H. Hawley
Burgess, Ernest W. (1925) 1961 The Growth of the City: An Introduction to a Research Project. Pages 37–44 in George A. Theodorson (editor), Studies in Human Ecology. Evanston, Ill.: Row, Peterson.
Christaller, Walter 1933 Die zentralen Orte in Siiddeutschland: Eine okonomisch-geographische Untersuchung iiber die Gesetzmdssigkeit der Verbreitung und Entwicklung der Siedlungen mit stddtischen Funktionen. Jena (Germany): Fischer.
Duncan, Otis Dudley 1959 Human Ecology and Population Studies. Pages 678–716 in Philip M. Hauser and Otis Dudley Duncan (editors), The Study of Population: An Inventory and Appraisal. Univ. of Chicago Press.
Hawley, Amos H. 1950 Human Ecology: A Theory of Community Structure. New York: Ronald.
McKenzie, Roderick D. (1924) 1925 The Ecological Approach to the Study of the Human Community. Pages 63–79 in Robert E. Park, Ernest W. Burgess, and Roderick D. McKenzie, The City. Univ. of Chicago Press. -* First published in Volume 30 of the American Journal of Sociology.
Park, Robert E. (1936) 1952 Human Ecology. Pages 145–158 in Robert E. Park, Human Communities: The City and Human Ecology. Collected Papers, Vol. 2. Glencoe, Ill.: Free Press. → First published in Volume 42 of the American Journal of Sociology.
Quinn, James A. 1950 Human Ecology. Englewood Cliffs, N.J.: Prentice-Hall.
Schnore, Leo F. 1958 Social Morphology and Human Ecology. American Journal of Sociology 63:620–634.
Steward, Julian H. 1955 Theory of Culture Change: The Methodology of Multilinear Evolution. Urbana: Univ. of Illinois Press.
Cultural ecology is the study of the processes by which a society adapts to its environment. Its principal problem is to determine whether these adaptations initiate internal social transformations or evolutionary change. It analyzes these adaptations, however, in conjunction with other processes of change. Its method requires examination of the interaction of societies and social institutions with one another and with the natural environment.
Cultural ecology is distinguishable from but does not necessarily exclude other approaches to the ecological study of social phenomena. These approaches have viewed their special problems—for example, settlement patterns, the development of agriculture, and land use—in the broad context of the complexly interacting phenomena within a defined geographical area. Explanatory formulations have even included the incidence of disease, which is related to social phenomena and in turn affects societies in their adaptations. This modern concept of ecology has largely superseded other concepts, such as “urban,” “social,” and “human” ecology, which employed the biological analogy of viewing social institutions in terms of competition, climax areas, and zones.
Cultural ecology is broadly similar to biological ecology in its method of examining the interactions of all social and natural phenomena within an area, but it does not equate social features with biological species or assume that competition is the major process. It distinguishes different kinds of sociocultural systems and institutions, it recognizes both cooperation and competition as processes of interaction, and it postulates that environmental adaptations depend on the technology, needs, and structure of the society and on the nature of the environment. It includes analysis of adaptations to the social environment, because an independent tribe is influenced in its environmental adaptations by such interactions with its neighbors as peaceful trading, intermarriage, cooperation, and warfare and other kinds of competition, in the same way a specialized, dependent segment of a larger sociocultural system may be strongly influenced by external institutions in the way it utilizes its environment.
The cultural ecological method of analyzing culture change or evolution differs from that based on the superorganic or culturological concept. The latter assumes that only phenomena of a cultural level are relevant and admissible, and it repudiates ”reductionism,” that is, consideration of processes induced by factors of a psychological, biological, or environmental level. The evolutionary hypotheses based upon this method deal with culture in a generic or global sense rather than with individual cultures in a substantive sense, and they postulate universal processes. Cultural ecology, on the other hand, recognizes the substantive dissimilarities of cultures that are caused by the particular adaptive processes by which any society interacts with its environment.
Cultural ecology does not assume that each case is unique. Its method, however, requires an empirical analysis of each society before broader generalizations of cross-cultural similarities in processes and substantive effects may be made. Cultural ecologists study highly diversified cultures and environments and can prescribe neither specific analytic formulas nor theoretical or ideal models of culture change; there can be no a priori conclusions or generalizations concerning evolution. The heuristic value of the ecological view-point is to conceptualize noncultural phenomena that are relevant to processes of cultural evolution.
Empirical studies disclose that among the simpler and earlier societies of mankind, to whom physical survival was the major concern, different social systems were fairly direct responses to the exploitation of particular environments by special techniques. As technological innovations improved man’s ability to control and adjust to environments, and as historically derived patterns of behavior were introduced, the significance of both the environment and the culture was altered and the adaptive processes not only became more complex but also acquired new qualities.
The ecological concept of interacting phenomena draws attention to certain general categories of relevant data. The resources, flora, fauna, climate, local diseases and their vectors of occurrence and many other features of the environment constitute potential factors in one part of the interacting system. The nature of the culture, especially its exploitative and adaptive technology but also features of the internal and external social environment, constitutes the other part. The interaction involves the social arrangements that are required in land exploitation; population density, distribution, and nucleation; permanence and composition of population aggregates; territoriality of societies; intersocietal relationships; and cultural values. In each case the empirical problem is whether the adaptation is so inflexible as to permit only a certain pattern or whether there is latitude for variation which may allow different patterns to be developed or borrowed.
Explanations in terms of cultural ecology require certain conceptual distinctions about the nature of culture. First, the various components of a culture, such as technology, language, society, and stylistic features, respond very differently to adaptive processes. Second, sociocultural systems of different levels of integration profoundly affect the interaction of biological, cultural, and environmental factors. Societies having supracommunity (state or national) institutions and the technological ability to expand the effective environment beyond that of the local or primary group can utilize resources within and outside the area controlled by the larger society. The adaptive responses of complex societies are thus very unlike those of a tribal society, which adapts predominantly to its own environment.
Culture history and ecological adaptations
The historical processes by which a society acquires many of its basic traits are complementary to studies of adaptive processes. The historical processes include the extensive borrowing of many cultural traits and trait complexes from diverse sources; the migrations of people; the transmission of cultural heritages to successive generations; and local innovations or inventions. Recognition of these historical processes, however, does not relegate environment to the circumscribed role of merely permitting or prohibiting certain cultural practices so that all origins must be explained by such history. It is obvious that fishing will have minor importance in desert areas and that agriculture is impossible in arctic regions. It is equally clear that abundant fishing resources cannot be exploited without appropriate techniques and that agricultural potentials cannot be utilized without domesticated plants. The presence of gold, oil, and uranium is unimportant until the society has demands for them and means of extracting them.
The Indians of the Southwest had at one time been predominantly food collectors, but the introduction of food crops, largely from Mesoamerica, provided the basis for the development of the more complex Pueblo village culture. Many details of this culture, such as ritual elements, were borrowed from the south, but the development of Pueblo sociocultural patterns are comprehensible only in terms of the processes of population growth, extension of biological families into lineages, eventual consolidation of lineages into larger settlements, and the appearance of many village institutions that cut across kinship groups.
Dissimilar ecological adaptations may also occur among societies that have been subjected to similar historical influences. The Indians of California and of the semiarid steppes and deserts of the Great Basin shared substantially the same devices for collecting wild foods and for hunting, but the vastly greater abundance of flora and fauna in California supported a population thirty times that of the Great Basin. The California Indians lived in fairly permanent villages which had developed some social elaborations, whereas the Great Basin Indians were divided into independent family units which foraged over large territories during most of the year and assembled only seasonally in encampments that did not always consist of the same families.
The investigation of cultural ecological processes must consider the possibility that basic sociocultural patterns may have diffused or been carried by migrations from one kind of environment to another, but it also must examine whether these patterns have been modified. Assessment of modifications requires a distinction between the outer embellishments of the culture, such as ritual elements, art styles, and kinds of architecture, and those social patterns which are human arrangements for self-perpetuation.
Man’s adaptations to his natural environment cannot be comprehended in purely culturological terms because man not only shares basic biological needs with all animals but also has distinctively human characteristics. All activities are culturally conditioned, but ecological factors cannot be wholly distinguished from inherent biological and psychological factors which are the basis of behavior. Analyses that ascribe importance to inherent human qualities bear directly upon the validity of common hypotheses and raise new problems.
It has generally been assumed that the nuclear family consisting of parents and children has always been the irreducible social structure because the sexes serve complementary functions in meeting procreational and subsistence needs and in caring for and training the young during their long period of dependency. This assumption is generally valid for all ethnographically known societies, especially for primitive societies, among whom there is a clear sexual division of labor in subsistence, maintenance of the household, and child rearing. In contemporary industrial societies, however, the biological family has surrendered many of its functions, and cultural differences between sex roles have been diminished. Conceivably, the nuclear family might be reduced to little more than a procreational unit if its cultural functions are lost.
In primitive societies, larger suprafamilial bands or other groups that developed in response to cooperative needs, especially in subsistence activities, had a biological basis, especially through lineages. The patrilineages of hunting bands, for example, had the advantage that particular skills, knowledge of the environment, and obedience to authority were transmitted from father to son in a male-dominated group. Biological relationships have so long been fundamentally important in culture history that they may be ascribed excessive importance, as when the position of head of a family or even kingship is transmitted through primogeniture. The biological basis survived because of the lack of patterned alternatives.
Another basic biological factor in cultural ecological adaptations is the prolonged period of human growth. The factors of age and environment interact in many ways to affect behavior patterns. The Shoshoni child may help in rabbit drives but not in big game hunting; the east African boy may herd cattle but not go to war; modern American youths attend school for a required number of years; in areas of precarious subsistence the aged may be abandoned to die, while in other areas they may be accorded special consideration; and everywhere the marriage age group is determined by biological development, culture, and adaptations to the environment.
There are, however, many problems concerning the interaction of biological and cultural ecological factors. One of these arises from the assumption that the nuclear or biological family is a basic part of every human society. This assumption might rest on the obvious biological nature of the family or upon the universal cultural functions the family is believed to serve. The higher primates usually live in fairly small bands that consist of females and their dependent offspring, a single powerful male, and subordinate adult males. In certain depressed segments of modern society, however, the matrifocal family or kin group, which consists of women and dependent children and somewhat subordinate males who are loosely attached to the family, appears to be the irreducible social unit. This family represents an adaptive response to restricted territoriality, low income, and lack of opportunity for improvement.
Although the transition from a precultural primate society to the nuclear family has not been explained, the contemporary existence of the matrifocal kin group raises the question of origin. How did the nuclear family develop from an early kind of society which may have lacked the cultural ecological processes that created and have subsequently supported it?
Speculation about the origin of the family must be based on what is known archeologically about the origins of technology and on ethnographic evidence. In all ethnographic cases of pref arming societies the nuclear family is based on clear complementarity of the sexes in subsistence activities. Women are food collectors because they must not leave their children, who are their inescapable responsibility, whereas men may spend long periods away from their families hunting large game or fishing, if the technology makes this possible. Prior to the invention of such distinctly male-associated hunting devices as spears, bows, nets, and traps, both sexes may have collected food in so similar a way that they were no more differentiated than were the higher primates. If so, there may have been a long transitional stage when some type of matrifocal kin group persisted. The hand ax, which was long the principal weapon, may attest to men’s role in protecting the group but not specialization in hunting, for its use was in close combat.
Settlement patterns and the adaptive process
A culture that is introduced into a wholly new environment must adapt in some ways to local conditions, but only empirical research can determine whether the adaptations are unalterably fixed or whether there is latitude for variations. In some instances, the case for narrow limits of variation seems clear. Simple societies that exploit only sparse and scattered resources obtained by food collecting must obviously fragment into small groups, for members of the society are in competition with one another. More abundant resources, such as rich areas of vegetable foods, large game herds, fisheries, or intensive farming, may permit variation in some features and thus allow latitude for borrowing or for local innovations.
There can, however, be no a priori conclusions: whether a society is dispersed over its land or clustered in large settlements may result from adaptive factors, from historical patterns, or from both. The Indians of California and the Northwest Coast had very similar native population densities, but the dispersed acorns and game in California led to considerable dispersal of the villages, whereas the rich salmon fisheries of British Columbia and Alaska required the people to concentrate on the main streams in fairly large settlements. Similarly, agricultural areas dependent upon irrigation concentrated the population within the network of canals. On the rainless and barren deserts of coastal Peru, the people had to live in dense settlements along or near the rivers. The large communities on the coasts of British Columbia and Peru, however, were parts of unlike sociocultural systems. Each developed from a different historical background, and each had a distinctive local adaptation; there were fisheries on the Northwest Coast and intensive, irrigation farming together with ocean resources in Peru.
A dense population supported by a rich economy is not always concentrated in large centers. The native agricultural peoples of the northern Andes of Colombia and the Araucanian Indians of the central valley of Chile had very similar population densities, but the northern Andean peoples were organized in strong, class-structured chiefdoms with communities of five hundred persons or more, whereas the Araucanians were dispersed in many closely spaced villages, each a patrilineage of no more than one hundred persons. This contrast is not explained by farm productivity or other technological factors involved in maintaining large communities. The northern Andean chiefdoms were the result of a diffused cult-complex of temples, priests, warriors, and human sacrifice, which may have originated in Mesoamerica but was re-adapted to the diversified environment of high mountains and deep valleys. Presumably, the Araucanians could have supported chiefdoms, but the nucleating factors were absent. A limited pastoral nomadism based on llama herding may have been an ecological deterrent to community growth, Although this factor did not inhibit the development of states and empires in Peru.
Nontechnological features of culture may also affect the adaptive arrangements through the external social environment of the society. Peaceful interaction of societies through marriage, trade, visiting, and participation in ceremonies, games, and other activities may take various patterns within the limits imposed by the environment. Intergroup hostilities, however, may have a decisive influence, because fear of warfare often causes peoples to cluster in compact settlements that are not required by subsistence patterns and sometimes do not give optimum access to resources. Thus, the palisaded villages of parts of the Amazon some-what inhibited farming of the frequently shifting slash-and-burn farm plots. To judge by the pre-history of the Pueblo Indians, the nature of farming permitted considerable variation. Early settlements were small and widely scattered, but in later periods the population declined as the settlements became large and tightly nucleated. This was the result of increased precariousness of farming in an area that had frequent droughts and was inhabited by marauders. In Tanganyika, people did not dare disperse along the fertile lands near the waterways until colonialism largely eliminated warfare. Throughout much of history, warfare has been a major factor in the interaction of culture and environment.
Exploitation of an environment by means of certain cultural devices may also drastically affect the environment, which again reacts upon the culture. In native California, as in many other areas, the Indians deliberately fired the grass in order to kill seedlings of brush and trees and thus increase the grazing land for game. Deforestation, overgrazing with concomitant soil erosion, the damming and rechanneling of rivers, drainage of swamps, and conversion of rural areas to urban or suburban land-use patterns also alter the environment. In such instances as the firing of grasslands, the development of irrigation works, or reclamation of swamps, the culture increases its basic resources. In others, such as overgrazing, lack of conservation, or the exhaustion of basic minerals, the culture destroys or impairs certain aspects of its local foundations.
Cultural variables versus holism
Although the culture of any society constitutes a holistic system in which technology, economics, social and political structure, religion, language, values, and other features are closely interrelated, the different components of a culture are not similarly affected by ecological adaptations. Technology, which exemplifies progress in man’s control of nature, tends to be cumulative. A language, unless replaced by another one, slowly but continuously evolves into divergent groups of languages. Humanistic and stylistic cultural manifestations may retain their formal aspects during social transformations but acquire new functions. Societies change through a series of structural and functional transformations.
Social structures respond most clearly to environmental requirements. This basic structuring is related most immediately to cooperative productive activity, and it is manifest in community and band organization and in essential kinship systems. Among simple societies, any interpersonal or inter-familial arrangements necessary for survival in particular areas are virtually synonymous with social organization. Because food collecting, such as in the case of seed gathering, is competitive, societies in unproductive environments tend to become fragmented into nuclear family units. Societies of hunters are more productive under cooperative arrangements and attain various patterns of cohesion. Societies that depend primarily upon farming tend to have permanent community organization, whether in dispersed or nucleated settlements, because cooperation in such activities as clearing plots and irrigation projects facilitates production. Increase in productivity is delimited by the environmental potentials, crops, and farm methods, but it may lead to larger communities and to internal specialization of role and status.
Productive increases achieved by social cooperation and improvements in exploitative technology became the bases of the transformation of small, homogeneous societies into larger societies that were internally specialized by occupational role and social status. Structural aspects of cooperation are reasonably well known, but quantitative statements of productivity, potential and actual surpluses, manpower hours that might be available for pursuits other than production of basic necessities, numbers of persons engaged in special occupations, and other measurable activities are rarely available. These serious lacunae in our data leave open the question of the part that sheer quantities of people and things played in the transformations from simple to complex societies.
Even within the category of culture subsumed under social patterns, it cannot be assumed that all features are equally fixed by a given ecological pattern. Among simple societies, residence, kinship, and subsistence patterns are more fundamental and less alterable or variable than clans, moieties, religious and secular associations, and other elaborations which are secondary embroideries on the basic social fabric. Distributional evidence in many parts of the world clearly shows that such elaborations have diffused widely across different kinds of environments and to fundamentally different social structures adapted to use of the environments. In the Southwest, for example, matrilineal clans occurred among the western Pueblo farm villages and the seminomadic pastoral Navajo, and moieties were a secondary adjunct of the patrilineal hunting bands in southern California and some of the eastern Pueblo agriculturalists.
A final problem concerning the response of different aspects or components of culture to adaptive processes is whether old or traditional forms may acquire new functions. Among the Sonjo of Tanganyika, the men’s age-group that served as warriors in precolonial times now spends several years performing wage labor in the new nonmilitaristic, cash-oriented context. Clan-owned or lineage-owned land and traditional types of ownership and inheritance of herds may persist under production for an outside market. Although traditional structures and trait-complexes tend to perpetuate themselves in all stages of cultural development, the evidence seems clearly to indicate that the new functions of the changing context eventually alter old forms beyond recognition, or that these forms wither and are replaced by new ones.
Levels of sociocultural integration
The adaptations of a complex or highly developed society differ in many ways from those of a simple society. The internal specialization that developed after the agricultural revolution and more so after the industrial revolution has affected the adaptive processes of the local segments of states and nations. Land use has increasingly reflected the importance of external economic institutions rather than local subsistence goals. Trade, improved transportation, mechanization, and other factors related to industrialization have made each local social group a more highly specialized and dependent part of the larger sociocultural system. There is a tendency toward monocropping in areas best suited to production of crops for external markets and toward specialization of local extractive industries, such as mining, oil, and timber, which have little intrinsic value to the local subsociety. Larger sociocultural systems have also created metropolitan and industrial centers and set aside special recreational areas.
Owing to technological achievements, the impact or conditioning effects of nature upon society are far less direct and compelling in a complex society than in a simple one. Culture increasingly creates its own environment. Foods and other necessities can be transported, water diverted, and electric power transmitted great distances. Reasonable comfort can be provided even for those who live for a time in Antarctica.
In a more fundamental sense, however, the distinction between primitive and developed societies is not merely one of social complexity or of technological knowledge. Among primitive peoples, the family, extended kin group, or fairly small village or band adapts directly to its own territory. Among more complex societies, there are suprafamilial and supracommunity institutions that are impersonal in that they do not involve the total cultural behavior of the people connected with them. State economic institutions—corporations, for example, that extract new materials and manufacture and distribute the products—serve the varied needs of the many subcultural groups within the state. They also extend the interaction of culture and environment far beyond state boundaries through the exploitation of distant resources and through extensive commerce. These vast economic extensions of the more developed societies penetrate local societies, and by creating new uses for land and other resources and imposing outside political and economic patterns, they fundamentally transform the local societies.
Within a modern, complex state, there is increasing specialization of land use within environmental potentials, but distinctiveness of local subcultures is partly leveled by the impact of national influences. The subcultures of ranchers in Nevada, sugar-cane workers in Puerto Rico, tea-plantation workers in Kenya, and other subsocieties who live on the land are shaped by the outside economic institutions to which they are linked, as well as by the local environment. As technology develops and sub-societies become more dependent upon the larger society, direct adaptation to the local environment decreases. The way of life, or subculture, of business executives of New York City, for example, has become dichotomized: in part it is derived from their highly specialized occupational role in a city and in part from their residence in suburbia, where the family and neighborhood culture differs from that of their profession and neither is closely adapted to the local environment. Even more remote from direct environmental impact are certain urban institutions found in contemporary Nevada, where the divorce, gambling, and entertainment complexes are linked to special aspects of the total national culture rather than to the semideserts of the Great Basin.
Some substantive applications
The concept of cultural ecology may be clarified by a number of substantive applications. These cases range from simple to complex societies, from the most elementary level of sociocultural integration to higher levels.
The precursors of man lacked culturally derived devices for killing game and for gathering, storing, and preparing food. They were food scavengers, and any food they collected was preserved without culturally prescribed techniques. Because animals can best forage a known habitat, they may have lived in groups or bands that were somewhat territorially delimited. Any social structuring was probably biologically oriented around the male’s dominance and protection of the group and the female’s role of caring for the young.
When culture provided more efficient techniques for survival, the essential human biological facts of sex, age, and kinship continued to affect the nature of society but were patterned in various ways by cultural ecological adaptations. Where the principal resource was seeds, roots, fruits, insects, small mammals, or shellfish, which occur sparsely, food gathering was necessarily competitive rather than cooperative because the yield decreases with the number of families that exploit the same site. If the local occurrence of principal resources varied each year, as it did among the Shoshoni Indians of Nevada, a multifamily society of permanent composition was not possible. Each Shoshoni family was linked with other families through intermarriage, through largely fortuitous association at a winter campsite, and through brief cooperation during antelope and rabbit hunts wherever there happened to be game. There were no bands of constant membership and no claims to territory or to resource areas. These families were not free wanderers, however, for any human society more effectively exploits a territory familiar to it.
The Chilean archipelago is completely unlike the Great Basin, but the erratic occurrence of shellfish, which was the principal food of the Alacaluf Indians, was functionally similar to Great Basin resources in preventing the formation of permanent social units larger than the nuclear family. That the nomadic, food-collecting family was the irreducible social unit in both cases is a function of the biological factors underlying marriage, division of labor, and child rearing. The independent nuclear family, however, is rarely reported ethnographically, and its occurrence in history is an empirical question rather than a matter of deduction from evolutionary principles.
Many societies fragment seasonally into family units of food collectors, but if the resources are sufficiently abundant and their whereabouts is predictable, the same families maintain contact with one another in loose groups that associate for supra-familial activities. Examples are the Indians of California, who subsisted on acorns, and the Indians of Lake Superior and the upper Paraguay River, who gathered wild rice. It appears that such groups tended to consist of extended families or lineages, although the evidence is not clear on this point.
Hunting generally requires cooperation, but there are various kinds of hunting societies, each reflecting special ecological adaptations. The Central Eskimo, whose population density was one person to 250 square miles or more, lived in small, some-what isolated family clusters. Since some cooperation was necessary in arctic hunting, nuclear families could not well have survived alone.
In societies that hunted large herds of bison, the concentrated resources supported bands of several hundred persons that exceeded the size of traceable lineages and consisted of many unrelated kin groups. The clans among certain Plains Indians were fictitious kinship extensions and an embellishment of the basic subsistence group. The Plains bands were more tightly integrated than the hunting bands of Canada, because game herds were larger, cooperative hunts were more highly developed, and warfare, especially after horses were acquired, became a major factor. By contrast, hunters of small and less-migratory game herds cooperated in bands of thirty to sixty persons who constituted a patrilineal lineage that controlled territorial resources, married outside the band, and brought wives to the husband’s band.
The basic social structure of these types of bands is essentially an interfamilial arrangement that is a necessary—or an optimum—organization for survival, depending on the conditions. A historical explanation that they originated elsewhere and were introduced to the areas through diffusion or migration is not credible, especially in the case of the patrilineal hunting bands that occurred in South Africa, Australia, Tasmania, southern California, and Tierra del Fuego. Each is a special kind of evolutionary development.
Societies of early farmers seem also to have been small, except where intercommunity warfare forced the people to concentrate in protected villages, and each probably consisted of lineages that tended to bud off as long as land was available. Although few analyses of the ecological adaptations of such farmers have been made, several types are suggested. In tropical rain forests such as the Amazon basin, where root crops were staples, slash-and-burn cultivation together with rapid soil exhaustion required the frequent shift of plots and sometimes villages. In addition, dietary needs made riverine protein foods of major importance, and canoe transportation facilitated the concentration of the population along rivers. In temperate areas, such as the United States, there was a marked contrast between the small villages of woodland farmers, who contended with prairie sods or forests, and the river-bottom farmers, as in the Mississippi system, whose fertile soils, once cleared of vegetation, supported large communities that were able to adopt the temple-mound complex from Mesoamerica.
In several temperate, and areas, early farming was restricted to flood plains or to the rainy high-lands. Increased population density in the valleys—made possible by irrigation in such cases as Peru —led to theocratic and militaristic states, whose power extended over wide areas, and to urban centers, which were the containers of civilization. It is not now clear to what extent trade, based upon increasingly specialized local production in adjoining areas, religious cults, and militaristic controls regimented the people to achieve maximum production and fill special statuses and roles.
Animal domestication entailed other kinds of ecological adaptations. Herds require more land for subsistence purposes than farming, have to be tended and moved about, and may be subject to theft. Claims to exclusive grazing areas were difficult to enforce until barbed wire was mass-produced in the last century. Cattle stealing and consequent intergroup hostility could therefore develop, as they did among many cattle breeders of east Africa. Free-roaming livestock, moreover, is a threat to crops, and until recently, farmers had to fence out the animals.
When societies became mounted, for example, the horse or camel nomads, other special adaptations occurred. They became more efficient hunters or herders and could transport foods a greater distance, thus extending their areas of exploitation and permitting larger social aggregates to remain together. They could also engage in predatory activities against one another and their settled neighbors. Mounted predators even created several ruling dynasties in the empires of the Middle East, China, and India.
After the agricultural revolution, the development of supracommunity state- or imperial-level institutions extended the areas of resource exploitation beyond those occupied by any of the subsocieties that were welded into the larger sociopolitical unit. Each local subsociety, although adapted in some measure to its terrain, became a specialized part of a new kind of whole to which it contributed different foods, raw materials, products, and even people—all for state purposes.
The industrial revolution enormously expanded the areas of exploitation through its improved transportation, mass manufacturing, communications, and economic and political controls. Its technology also gave importance to many latent resources. Modern nations and empires embrace highly diversified environments and draw upon areas beyond their political boundaries. Their technology may modify environments to meet their cultural needs, and, above all, any localized subsociety reacts to a complex set of state institutions, to a diversified social environment, and to a large number of goals other than survival. Although people necessarily live in particular places, members of highly industrialized nations must be viewed as increasingly nonlocalized to the extent that their behavior is determined by a great overlay or elaboration of cultural patterns that are only remotely connected with particular environments and are even minimizing some of the child-rearing and economically complementary functions of members of the nuclear family. It can be imagined that nuclear power, hydroponic and synthetic food production, and other technological developments might create wholly artificial environments, as in a permanent space station.
Julian H. Steward
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Helm, June 1962 Ecological Approach in Anthropology. American Journal of Sociology 67:630–639. → Includes a review of the literature of cultural ecology.
Lattimore, Owen (1940) 1951 Inner Asian Frontiers of China. 2d ed. Irvington-on-Hudson, N.Y.: Capital.
Sahlins, Marshall D.; and Service, Elman R. (editors) 1960 Evolution and Culture. Ann Arbor: Univ. of Michigan Press.
Steward, Julian H. 1955 Theory of Culture Change: The Methodology of Multilinear Evolution. Urbana: Univ. of Illinois Press.
Washburn, Sherwood L. (editor) 1961 Social Life of Early Man. Viking Fund Publications in Anthropology, No. 31. Chicago: Aldine.
Introduction to insect ecology
Imagine a world without insects. At first thought, this might seem wonderful. No biting flies. No termites to destroy homes. No malaria, yellow fever, sleeping sickness, or any of the multitudinous diseases vectored by insects. No need to worry about insects eating crops and no need to take special precautions to keep insect pests out of stored food or other supplies.
Certainly, there would be tradeoffs. There would be no elegant butterflies, including the migratory monarch Danaus plexippus that has captured the imagination of generations of humans. We would not hear crickets chirping at night. We would have no bees and so no honey. These are trivial losses compared with the benefits we would experience, however, and they might seem like a reasonable price to pay for a world without insect pests. Unfortunately, there would be other tradeoffs. Earth is home to approximately 950,000 described species of insects, more than half of the 1.82 million known species of plants and animals combined. Estimates of the total number of insect species on Earth, including undiscovered species, range from two million to 80 million, with five million commonly accepted as a reasonable estimate. Insects play indispensable roles in maintaining many familiar aspects of life on our planet. The story of these diverse and numerous roles is the story of insect ecology.
The niche theory of ecology
Ecology is the study of interactions between living organisms and both the living and nonliving components of their environment. Any introduction to ecology would be incomplete without an explanation of the concept of a niche, which is the organizing theme for the study of ecology. Although the term niche often is used in its historic sense to refer to a physical space in a habitat, that interpretation is restrictive and incomplete. There is a better definition, particularly for understanding the niches of insects. A niche is the suite of ecological strategies by which a given species maintains a balanced energy budget while converting available energy to the maximum number of offspring. These offspring carry genes into the next generation, perpetuating successful ecological strategies. Thus, an ecological niche includes not only the choice of a physical place to live and forage but also behavioral strategies for avoiding enemies, finding food, locating and selecting mates, and producing offspring. The niche of a given species is defined by the specific ecology of that species, embracing all the interactions between that species and its biotic (living) and abiotic (nonliving) environment.
This definition of an ecological niche has both predictive and explanatory powers. For example, niche theory predicts that under adverse environmental conditions an attempt at reproduction will be abandoned if the energy cost of producing offspring could deplete the energy budget so severely that both offspring and parent are likely to die. This explains why, in adverse conditions, plants abort seeds and animals abort or abandon their young. If parent and offspring both die as the result of environmental stress, neither of them will pass genes to the next generation; thus, under stressful conditions, some parents sacrifice their offspring, conserve energy, and make another attempt at reproduction later, when conditions are more favorable.
Insects exhibit several variations on this theme of sacrificing individuals to balance the energy budget. One variation occurs when immature insects eat one another, including their own siblings. The larvae of monarch butterflies do this. Another grisly example of balancing the energy budget via cannibalism is that of the female praying mantis consuming her mate, sometimes during the mating act. The male's body provides nutrition for development and provisioning of the eggs he fertilized. This behavior makes perfect sense in the context of niche theory. The ultimate goal of life is to pass on genes; therefore the strategies that make up an ecological niche are centered on producing the maximum number of offspring without going into energy debt. Maintaining a balanced energy budget is a crucial facet of ecology, even if it means forfeiting some offspring—or even oneself.
The importance of insect eating habits
The key to understanding insect ecology lies in analyzing the energy budget of each species of interest, and this starts with their dietary habits. All the energy available to insects is derived from their food, so the first entry in the energy budget of a species depends on the food resources exploited by that species. Insects feed on nectar, pollen, leaves, fruits, and many other plant parts, including wood. They also eat dung, fungi, and both vertebrate and invertebrate animals.
Insects are extraordinarily numerous, both in terms of numbers of species and numbers of individuals, and so their eating habits have a significant influence on the global ecosystem.
Some insects, such as bees, butterflies, and some flies, feed on nectar. In the process they often pollinate the plants supplying nectar. Imagine a world without insects, and you will have to imagine a world without many of the foods we take for granted. Approximately 60% of all crop plants require animal pollination agents, most of which are insects. There is no doubt that the human diet would be impoverished dramatically without insect-pollinated plants.
The species of insects that feed on nectar and transport pollen are vastly outnumbered by the species that feed directly on the leaves, stems, roots, flowers, and fruits of plants. The feeding habits of insects have had profound effects on most of the major ecosystems on Earth. Plants evolve adaptations in response to insect damage, and it is believed that insect feeding behavior has affected the diversity of plant species dramatically. Insects have had a huge influence on the biodiversity of this planet.
Other kinds of ecological interactions, also related to insect feeding habits, would be lost if there were no insects.
For example, insects remove carrion and dung. When an animal dies, the body often is still warm when the first insects arrive and begin dismantling and recycling the body, returning its materials and stored nutrients to Earth's ecosystem. Flies lay eggs on the body, and the resulting maggots are an important factor in the rapid conversion of dead flesh to new life. Some beetles specialize in feeding on carrion. Some dead animals, such as mice, are so small that sexton beetles (family Silphidae) dig a hole under the body, bury it, and prepare it as food for themselves and their offspring. Decomposition of dead animals would take much longer if it were dependent solely on the action of bacteria and fungi. Insect undertakers speed the process and increase the efficiency of nutrient recycling.
Some kinds of insects eat dung. This takes place in most climates, but it is particularly spectacular in the tropics, where dung is so coveted by insects that often it is still fresh and warm from the animal when the first scarab beetles converge and claim portions as their own. The world would be piled high with dung if insects did not remove and recycle animal wastes.
The importance of insects on Earth
There are innumerable specific examples of the integral roles of insects in life as we know it on this planet. One distressing consequence of a complete loss of all insects from Earth would be the loss of many vertebrate animals that eat insects. Birds, reptiles, and many mammals, including some primates, rely on insects for part or all of their diets. Some human societies consume insects, which are an excellent source of protein and fat. Although biting flies are a bane of human existence, both because of the nuisance and, more important, because of their capacity to carry disease, the immature stages of these flies often are water dwelling and are vital sources of food for fish. Our streams, rivers, lakes, and ponds would be depleted of fish and other aquatic life if all insects vanished from the Earth.
Ecology is the study of interactions between living organisms and their environments, and comprehending the critical and integrated role of insects on Earth is the philosophical underpinning for understanding insect ecology. Insight into both the classic and the unique roles of insects in Earth's ecological systems enhances an understanding and appreciation of the intricacy of life on this planet.
Unique characteristics of insects
The ecological interactions of insects are dictated by their unique characteristics, many of which are completely different from the human experience. Comprehending these differences, which are based on size, physiological characteristics, and morphological features, lays the groundwork for understanding insect ecological interactions. Essential characteristics of insects that dictate their interactions with the environment are small size, exoskeleton, metamorphosis, high reproductive capacity, genetic adaptability, and, for most kinds of insects, their ability to fly. Only insects have this unique suite of characteristics. These features act in concert to define the niches of insects. Some fundamental concepts related to these characteristics are worth reviewing in the context of a discussion of insect ecology.
Insects are tiny; this fact is enormously significant in their ecology. Among the smallest known insects are beetles in the family Ptiliidae. Adults are less than 0.04 in (1 mm) in length and could crawl comfortably through the eye of a sewing needle. Adult wasps in the family Mymaridae are less than 0.02 in (half a millimeter) in length. At the other end of the insect size scale are the tropical walkingsticks, such as the Australian phasmid Palophus (order Phasmatodea), which can grow to 10 in (25 cm) in length. The largest of the giant silk moths (family Saturniidae) are the atlas moths, with wingspreads up to 12 in (30 cm). In terms of mass, the Goliath beetle, an African scarab (family Scarabaeidae), is one of the largest, weighing about 3.5 oz (100 g). The largest insects are diminutive, however, when compared with most mammals and other vertebrates.
Central to understanding insect ecology is recognizing that their absolute energy requirement is small, commensurate with their size. An organism that requires only a modest energy budget to be successful has an advantage over an organism that demands a large budget; there are more opportunities for insects with small energy budgets than for
Another important consequence of small size is that insects have a relatively large ratio of surface area to volume. This means that they lose water and heat much more rapidly than would a larger animal. Many of their ecological strategies have evolved because of these specific vulnerabilities. To a large extent, insect ecology is structured around the need to obtain and conserve water, and this challenge is enhanced by their small size. In contrast to sophisticated morphological and behavioral controls on water loss, insects have adapted to heat loss by not having control of body temperature. The energy cost of maintaining a constant warm body temperature for such a small animal would strain the energy budget, so in contrast to mammals which maintain a constant body temperature, insects are poikilothermic. This means that a critical feature of their ecology is that their bodies can function at approximately the ambient environmental temperature.
Another ramification of this aspect of insect ecology is that developmental rate is determined by temperature. Each insect species has a lower temperature threshold below which development will not proceed and an upper temperature threshold above which the insect dies. Between these two limits, the developmental rate increases with rising temperature. Immature insects grow faster in warm conditions. This variable development time has implications for allocation of energy and thus is an essential concept in insect ecology.
Because of their small size, insects live in a world dominated by forces we rarely contemplate, while at the same time they are comparatively free of some of the forces and influences that beset humans and other large vertebrates. For example, the force of gravity rules many aspects of human life, and gravity is a significant factor in determining the energetic cost of locomotion for large animals. In contrast, the tiny force exerted by the surface tension of water is relatively insignificant for humans and other large vertebrates, particularly in the context of locomotion.
The world experienced by insects is exactly the reverse: gravity is comparatively less influential than are the cohesion forces between molecules. It is partly because of this reversal that insects are able to have exoskeletons. Most insects do not have a large enough physical mass to be injured in a fall, but tiny insects can be trapped and rendered helpless by a drop of water because the cohesion forces between water molecules are stronger than their legs and wings. Therefore, cohesion forces, which barely affect human ecology, play a dominant role in insect ecology. Molecular cohesion allows insects to walk upside down on ceilings, and if an insect happens to fall from the ceiling, the relatively smaller effect of gravity means that the insect is unlikely to get hurt. Adaptations related to these physical forces play key roles in the energy budget.
Insects are notorious for changing form as they develop from immature stages to reproductive adults. Termed "metamorphosis," this change in morphological features during development also may entail modifying habitat, food preferences, and behavior. An understanding of insect metamorphosis is fundamental to understanding their ecology, because a dramatic change of form allows insects to switch their ecological interactions from one life stage to the next. It is an evolutionary advantage to be able to make a radical shift in ecological strategies to best suit the needs and goals of each life stage.
Insects have phenomenal powers of reproduction. Female insects routinely produce hundreds of eggs. Although abandoning one's offspring is considered the height of irresponsibility for humans, it is widespread among insect species. Insects typically produce a batch of eggs and immediately desert them, though sometimes with minimal provisions for their nourishment and safety. Nutritional provisioning may take the form of leaving the egg on an appropriate host plant selected by the mother. Or, in some odd cases, provisioning takes the form of arranging for another organism to provide care. There are walking stick insects that provide their eggs with a lipid-rich appendage called a "capitulum." This appendage attracts ants, which eat the capitulum and disperse the eggs to relatively safe locations, where they can incubate and hatch.
However, parental care among insects is uncommon. The few exceptions appear in diverse taxonomic groups, including the orders Hemiptera (family Belostomatidae), Orthoptera (family Gryllidae, subfamily Gryllotalpinae), Coleoptera, Isoptera, and Hymenoptera. The last two orders listed are of particular interest, as they include social species. Social insects represent a group placed in an extraordinary situation in which the energy budget of the ecological niche includes a large allotment for care of the young. For most species of insects, the energy budget for reproduction does not include caring for the young. Producing huge numbers of offspring, sometimes many hundreds per female, is one of the special ecological features of insects. Because of their enormous reproductive potential, insects in favorable environments can generate huge populations with stultifying rapidity. Anyone who has seen a plant colonized by aphids or a container of grain invaded by weevils or grain moths is familiar with this phenomenon.
The genetic adaptability of insects is tied closely to their innate capacity to reproduce in great numbers and also is a function of their typically short life spans. This concept requires an understanding of Darwin's principle of "survival of the fittest," one of the most profoundly misunderstood ideas in biology. It frequently is interpreted as a struggle between individuals, with those that are stronger, faster, and more aggressive surviving. In the Darwinian sense, however, "fitness" means number of offspring. This concept relates the definition of fitness closely to the definition of an ecological niche.
An individual that produces many offspring has a high fitness rating, whereas one with no offspring has a fitness of zero. Strength, speed, and aggression are relevant only if they contribute to an individual's ability to reproduce. Nature essentially provides a selective breeding program in which the natural hardships of the environment and the successful strategies of the ecological niche act instead of a sentient breeder. This process of differential survival and reproduction is called "natural selection."
From one generation of insects to the next, changes in the genetic make-up of the population are wrought by the processes of natural selection and take place with relative speed. Think of it this way: in less time than it takes for a newborn human to grow old enough to learn to walk and talk, a mosquito species that transmits malaria can go through more than 40 generations. Under favorable conditions, each female mosquito is capable of producing 100 to 150 eggs, and each individual could complete its life cycle in about two weeks.
With the potential to produce more than 100 eggs per female and more than a dozen generations per year, mosquito populations can build up with staggering speed. Each generation is subjected to natural weeding out of the unfit individuals, such as those susceptible to the poisons that humans use to control their populations. Many mosquitoes succumb to insecticides and may die without ever reproducing; their genes simply vanish from the population. Those individuals able to detoxify insecticides, however, will survive and ultimately generate a population of insects resistant to the insecticides that have been used against them. These genetic changes take place over many generations of mosquitoes, but dozens of mosquito generations go by in considerably less time than the span of one human lifetime. Therefore, from the human perspective, insecticide resistance evolves rapidly.
Remarkably, few groups of animals have evolved wings and the capacity for flight, despite several obvious advantages of flying. Like fitness, flight is connected closely to the energy budget and therefore to the ecological niche concept. Flight is useful for escape, foraging, and dispersal. Although flight has special advantages, it also comes with disadvantages. Larger animals have to contend with the relationship between weight and stalling speed; they have to accelerate more quickly and fly faster to take off and remain aloft. A mosquito can stay aloft at a speed of 0.2 mph (0.3 kph). The largest insects need achieve only about 15 mph (24 kph), whereas many small birds have to reach a minimum speed of about 20 mph (32 kph) to stay in the air. This increases the demands on the energy budget, making unassisted flight too expensive for most larger animals.
Insects have a special advantage: their particular combination of small size and relatively low energy investment per individual offspring means that they can easily afford both flight and reproduction without straining their energy budget. For example, some beetles complete their entire larval development while feeding on one seed. In contrast, a single seed probably would not supply the energy expense of takeoff for a large bird.
There is another fuel-related consideration: because insects are poikilothermic, they do not expend large amounts of energy trying to keep warm in a cold place. This has a double advantage in terms of the power of flight. First, a poikilothermic lifestyle allots less energy to staying warm, freeing up part of the energy budget for other investments, such as flight. Second, the general trend toward cooler temperatures higher in the atmosphere is less of a concern for insects. Instead of needing additional energy to keep warm while flying high, insects can simply let their body temperature drop and thereby conserve energy.
Insects often exploit their small size and simply allow themselves to be caught in wind drafts, reducing the cost of flight almost to zero by permitting themselves to become the aerial equivalent of plankton. It is common to find tiny insects being swept along high in the air. This efficient form of dispersal spreads insects from one habitat to another with relatively little energy expenditure.
Classic themes in ecology
Most ecological strategies can be reduced to two basic goals: the acquisition of energy from the environment and the balanced allocation of energy for all life processes. Within each of these two goal areas, there are broad patterns common to all living organisms. These patterns are classic themes in ecology, often studied using insects as paradigms. Insects also have special ecological features not common among other living organisms; those aspects are reviewed under "Special features of insect ecology." Classic themes in ecology include competition, feeding ecology, reproductive strategies, succession, biodiversity, predator-prey interactions, and parasite-host interactions, which are discussed in the following sections. Mutualism, another classic ecological pattern, is considered under "Ant plant interactions."
The concept of a species-specific energy budget is an organizing principle in ecology. Sometimes the strategies involved in acquiring and allocating energy are modified as the result of overlap of resource exploitation by one or more species; this is specifically true in the case of a limited resource. When one or more individuals suffer negative effects on fitness from niche overlap and subsequent niche modification, the situation is referred to as "competition." There cannot be competition if there is no niche overlap or if the shared resource is unlimited. Under the defining conditions, however, competition has the potential to have a negative impact on the energy budget of the competing organisms. Competition takes two broad forms: intraspecific competition, or competition among members of the same species, and inter-specific competition, or competition among individuals belonging to different species. The energy budget of any living organism begins with food; therefore, an obvious arena for competition is communal exploitation of a limited source of food energy. Other potentially limiting resources include water, mates, and habitat space.
Intraspecific competition can include overlap in the use of both energy resources and mates. Competition for mates often is an evolutionary force driving behavioral characteristics, such as territoriality among dragonflies. Generally, intraspecific competition affects members of one life stage (often adults), but it can embrace competition between insects at different life stages. It is worth noting that metamorphosis has the potential to minimize or eliminate competition between life stages for those insect species that utilize different niches at distinct developmental stages.
Interspecific competition can be an evolutionary force driving niche specialization or speciation. As competition increases, niche specialization also may expand, resulting in fine-tuned partitioning of resources. A classic example is the variety of different feeding guilds among the many insect species found on a single tree: gall formers, leaf miners, leaf eaters, and root feeders all can share the same plant, each specializing in a particular mode of obtaining energy.
Insects can be divided into different groups based on their feeding habits, and this categorization can be accomplished via two distinct but overlapping systems. One system classifies insects on the basis of the kinds of food they eat: carnivores, herbivores, or omnivores. A second system categorizes insects as either generalists or specialists, depending on the degree of specialization of their feeding habits. Each of these categorization systems provides insight into ecological interactions, but no system presently in use aligns perfectly with current taxonomy, so there is no easy way to integrate these categories with phylogenetic groupings.
Herbivores, for example, the larvae of moths and butterflies (order Lepidoptera), feed almost exclusively on plants. A special consideration related to feeding on plants is the tendency of plants to produce defensive chemical compounds intended to deter feeding or even to poison aspiring herbivores. Insects can evolve their own chemicals to surmount these defenses, usually in the form of detoxifying enzymes, although they may store toxins in their bodies without detoxification and even recruit these chemicals for their own defense. Insects also can evolve types of behavior to avoid exposure to plant toxins. An example of this is trenching, in which insects chew through plant veins, allow sap to drain out, and then consume the plant material that thereby has been deprived of toxin-containing sap.
Another special consideration of feeding on plants is that, with the exception of seeds, most plant material is relatively rich in carbohydrates and low in proteins. To be successful, plant-feeding insects need to evolve mechanisms for ensuring adequate nitrogen intake. These mechanisms must fit into the ecological niche of the species without resulting in energy-budget imbalances.
Carnivorous insects face a reverse problem: their animal prey tends to be replete with protein, so nitrogen is not a limiting resource, but an imbalance between nitrogen and other nutrients can cause problems with internal osmotic balance and also with the energy requirements of processing amino acids. Some amino acids, such as arginine and histidine, are rich in nitrogen atoms. These amino acids require such a large energetic investment during digestion that it is more efficient to discard the excess than to attempt to break these amino acids down chemically. For example, blood-feeding tsetse flies (family Glossinidae) discard the amino acids arginine and histidine from their food because the cost of metabolizing these molecules is greater than the energy yield from digestion.
Omnivorous insects have the advantage of the potential to balance their nutritional requirements, particularly nitrogen and sugars, but have a special disadvantage: they may need to maintain a larger array of digestive and detoxifying enzymes, and this makes large demands on the energy budget. This issue leads to the second major system for categorizing feeding habits: generalists versus specialists. In the context of the energy budget, specialists have restrictive feeding habits and thus can be more efficient in their production and use of digestive and detoxifying enzymes. The tradeoff is that specialist feeders are limited in their repertoire of possible food sources and may expend too much energy searching for appropriate food. Generalists, in contrast, have the advantage of a much larger repertoire of food resources, but there is an associated expense: these insects may have to invest more energy in digestive and detoxifying enzymes. Some insects employ inducible enzymes. They have the genetic capabilities to synthesize appropriate enzymes but do so only when induced by exposure to relevant chemicals in their food. This is a compromise that allows exploitation of a wide variety of food resources while retaining an energy-efficient strategy for digestion and detoxification.
A key part of the definition of a niche is the need to produce the maximum number of offspring while maintaining a balanced energy budget. Only those offspring who survive to produce offspring of their own, however, can count as successful reproductive efforts. This caveat has implications for reproductive strategies. It is necessary but not sufficient to produce offspring. They have to survive and successfully reproduce.
Reproductive strategies fall into two broad patterns. The reproductive strategy of producing many offspring with a relatively tiny investment in each is referred to as an r strategy, with the r representing the intrinsic rate of increase of a population. This recognizes that population increase has the potential to follow an exponential curve when population growth is unchecked.
In contrast, the reproductive strategy of producing only a few offspring but putting a large investment into each is referred to as a K strategy. The K represents the carrying capacity of the environment for that species and implies a limiting factor suppressing population growth. This recognizes the limits of the energy budget that defines the niche of a particular species. Very specifically, in the case of a species that produces few offspring, the designation K reflects the heavy emphasis on reproductive costs in the total energy budget. These organisms consign large amounts of energy into producing few offspring and enhancing their survival. Thus, K species tend to have relatively low mortality rates early in life. In contrast, r species allot only small amounts of energy to producing and nurturing individual offspring, and this is reflected in relatively high mortality rates early in life.
Insects characteristically use an r strategy. For most insects, sheer numbers take precedence over parental care: if enough offspring are produced, some are almost certain to survive. At one extreme, insects such as giant silk moths (family Saturniidae) may produce as many as 300 eggs, each with minimal provisioning. Most of these offspring die as eggs or larvae. Very few survive to become reproductively mature. At the other extreme, insects such as tsetse flies and bat flies (family Streblidae) produce only a few young but invest energy in parental care, thereby ensuring that a high percentage of the offspring survive to maturity.
There are rare and special cases in which certain insects can reproduce as larvae. This phenomenon is called paedogenesis and has an ecological correlation. Species exhibiting paedogenesis usually live in transient and somewhat isolated habitats with limited resources as well as limited time to exploit those resources. The ability to reproduce as larvae allows these insects to produce a maximum number of offspring rapidly before the source of their energy budget is used up. This unusual phenomenon is known to occur in only a few kinds of insects and some mites (class Arachnida).
Ecologists have long observed that the kinds of organisms found in a particular ecosystem changes over time. This process of change tends to be predictable and directional and is called succession. Although sequences of succession vary among different habitats, the broad features include initial rapid colonization by species that use predominantly r reproductive strategies, followed by the slower establishment of species that use predominantly K reproductive strategies. Ultimately, this leads to the establishment of an ecological system known as a "climax community." An entomological example of succession is the directional and predictable series of insects that arrive at, colonize, and exploit a large animal carcass, such as a dead deer in the woods. Another example is the change of component species of the insect community on a plant as the plant ages.
Early stages of succession are characterized by large numbers of a few species, so the emphasis is on colonization with low biological species diversity. Some of the initial colonists depart during the process of succession and are not found at later stages. The climax community, however, often includes a mix of r and K species, with the r species exploiting resources that tend to be transient or patchy, while the K species invest in permanent structures that allow them to exploit more constant resources. The hallmark of a climax community is its species richness, known as biodiversity.
The measure of richness of different species of living organisms per unit of geographic area is summarized as biodiversity. Usually, biodiversity is correlated directly with the complexity of the habitat and the geologic topography as well as climax community structure. Complex habitats with a high index of biodiversity generally are more ecologically stable and resistant to disturbances. Less complex habitats tend to have a low index of biodiversity and typically are less ecologically stable. Monoculture croplands are prime examples of the latter type of ecosystem. Human manipulation often creates ecosystems with a low index of biodiversity, and this has serious implications both for loss of species and for the spread of pest species. Encouragement of biodiversity and preservation of endangered species revolves around preservation of their required habitats with enough geographic area to allow for the retention of ecosystem complexity.
The consumption of one animal by another is a classic theme in ecology and derives much of its importance from the concept of the energy budget. When an animal is killed and consumed, it is referred to as predation. Predators contribute significantly to the flow of energy through an ecosystem: when they eat, they annex and mobilize the entire energy content of a living animal, and they do this many times during their life span as they consume many prey individuals. Insects can be predators, prey, or both.
Insects also fill a special subcategory of predators known as "parasitoids." Parasitoids differ from predators because they generally live inside a single prey individual and consume it from within, ultimately killing it. Unlike predators, parasitoids typically are highly specialized with regard to prey species. Both predators and parasitoids can operate in groups. A pack of predatory lions—or even ants—can eat a single prey individual, or a group of parasitoid fly larvae can eat a single caterpillar. Predators and parasitoids are critical in the regulation of population growth and therefore are important in evolutionary selection. Examples of predatory insects include various families of beetles and true bugs as well as the praying mantids familiar to organic gardeners.
Like predators, parasites consume living animals. A crucial difference is that parasites do not usually kill the "prey" out-right, although they may debilitate a host seriously enough to contribute to its death. Parasites live either on or inside their hosts and cause generalized weakening by draining the host of nutrients. In general, a parasite is a more specialized feeder and has a smaller energy budget than a predator, and this is significant in terms of numbers of species. There are relatively more ecological niches available for parasites than for predators. Many insect species have adopted a parasitic lifestyle. The combination of small size, modest energy requirement, and propensity for rapid speciation predisposes insects to this ecological strategy. Examples of parasitic insects include flies, fleas, and lice.
Special features of insect ecology
The unique suite of behavioral, morphological, and physiological features possessed by insects results in highly specialized ecological phenomena. Special features of insect ecology include marine insects, plant-insect interactions, sociality, and diapause. In addition, the special application of insect ecology to the human issue of pest control is discussed in the context of integrated pest management (IPM).
One of the few ecosystems in which insects are notably scarce is the marine ecosystem, although there are many freshwater aquatic species of insects. A brief review of the unique characteristics of insects, analyzed in light of the need for a balanced energy budget, provides a ready explanation for this dearth of marine insect species.
Marine habitats have salty water. For insects, with their small size and high surface-to-volume ratio, this translates into an enormous risk of severe dehydration: the salt in this environment pulls fluid out of their bodies. Moreover, salts can invade the insects' bodies and upset their internal osmotic balance. Some insects use energetically expensive physiological mechanisms, actively excreting excess salt from their bodies to reduce dehydration in marine habitats. These adaptations, however, drain the energy budget of insects that live in saltwater. Although a few species have found a way to maintain a balanced budget despite this expensive lifestyle, marine insects are quite rare. The small size that gives insects an advantage in most other habitats is a bane in the sea.
Interactions between insects and plants can be positive or negative. Negative interactions occur when insects eat plants; this is called herbivory. More than half of all known insect species feed on plants, with most of these species coming from eight major orders. For example, approximately 99% of all known species of moths and butterflies (order Lepidoptera) are herbivorous. Plants often evolve toxic chemicals, which are believed to serve primarily as either feeding deterrents or outright poisons intended to reduce herbivory. Insects, in turn, evolve ways to detoxify or avoid ingesting the toxins. When this happens, the negative interaction between insects and plants is reciprocal.
A landmark paper by Paul Ehrlich and Peter Raven, published in 1964, speculated that these reciprocal negative interactions between insect herbivory and the evolution of plant toxins constituted a special subcategory of evolution. Using a term first coined by C. J. Mode in 1958, they referred to this special form of evolution as coevolution. Their theory states that herbivory by insects is an evolutionary pressure that selects for plants with toxins, and plant toxins, in turn, provide evolutionary selection for varieties of insects capable of detoxifying the plant substances. This process is believed to result in evolution of new species of plants and insects. The theory of coevolution largely explains the great diversity of flowering plants (angiosperms) and insects found on Earth.
Coevolutionary interactions between insects and plants also can be positive. Pollination is an obvious example of a positive interaction and includes cases of coevolution between plants and their insect pollinators. Plants produce nectar specifically to attract insects, because often when an insect lands on a flower to partake of nectar, it either inadvertently or deliberately collects pollen from the flower. Plant fertility is tied closely to ecological niches of insect pollinators.
Insect pollination is unquestionably the most efficient pollination system other than self-fertilization. Insects benefit by obtaining nectar and pollen as food, and plants benefit when pollen is transferred efficiently between flowers. This positive ecological interaction can become reciprocal and can direct the processes of evolutionary change. Coevolution could produce new species that would not have evolved without these tight associations of reciprocal positive interactions. It seems likely that without insects, there would be fewer species of plants. In particular, there would be only a fraction of the number of species of flowering plants currently found in the wild. Not all pollination systems, however, are examples of coevolution in the true sense of long-term reciprocal evolutionary selection.
The efficiency of insect pollination is related directly to insects' unique suite of characteristics and their energy budget. Pollen is not indefinitely viable; it dies, sometimes within hours, once it leaves the parent plant. This puts a premium on pollination systems that emphasize speed. Utilizing a living pollination agent with the power of directed flight is like sending pollen air mail rather than surface mail: it goes faster but costs more. Because insects can easily afford flight in their energy budget, they require only a small allotment of nectar and are the cheapest and most efficient available living pollen carriers. Plants that use insect pollinators expend less energy producing nectar and therefore have an easier time balancing their energy budgets while producing offspring. Many species of plants rely on the efficiency of insect pollination, and the diversity of plants on Earth would be reduced drastically if insects were eliminated.
Mutualism and ant-plant interactions
Ecologists define a variety of interspecific interactions, including the broad categories of competition, predation, parasitism, and mutualism. Mutualism is a special case of symbiosis in which individuals from different species participate in an association that is beneficial to both. Insects provide some of the prime examples of ecological mutualism. Prime among these are ant-plant associations, which are examples of tight coevolution as well as mutually beneficial symbiotic relationships.
One classic example of ant-plant interactions comes from the associations of plants in the genus Acacia and ants in the genus Pseudomyrmex. An estimated 10% of Central American Acacia species recruit ants as their bodyguards. These plants have enlarged, modified thorns in which the ants make their homes. The Acacia plants provide the ants with nectar and protein, the latter produced in special structures that grow at the tips of new leaves. In exchange for room and board, ants aggressively protect Acacia plants. They fend off herbivores that might otherwise eat the Acacia plants, and they also chew off the growing tips of nearby plants, thus suppressing the growth of potential competitors of their hosts. For both Acacia plants and ants, the investment of energy in maintaining their association yields a sufficiently high return to justify this expense in the energy budget. A similarly favorable return on the energetic investment is the evolutionary force driving other types of mutualism, and other insect examples can be found among fig wasps, ants that cultivate aphids for honeydew, termites and their intestinal microorganisms, among many other associations of species.
Division of labor is a strategy that increases efficiency of energy use. Insects that live solitary lives do not have the luxury of specializing in one or just a few tasks, but some insects have evolved social systems that feature division of labor. Other key characteristics of insect sociality are overlap of generations, cooperation in care of young, and intraspecific communication. Overlap of generations is found in many insect species, and both sexual signaling and aggregation chemical signals are common between males and females of the same species. Only social insects, however, have a combination of generation overlap, communication of information, and communal brood care. Social insects are in the orders Hymenoptera and Isoptera.
The success of sociality as an ecological strategy can be measured by the number of individuals produced by the system: approximately 75% of insect biomass on Earth comes from social insects—ants and termites. This success is largely due to the extreme efficiency of cooperative care of young. The investment of care increases the proportion of young that survive, and this investment is made possible by the increased efficiency that comes with division of labor. Social insects have individual colony members specialized for reproduction, defense, foraging, brood care, and even housekeeping. Sometimes this job specialization depends on age, so that younger adults have one set of chores and older adults have a different set of chores. For example, the common honey bee (Apis mellifera) worker spends time cleaning the hive and maintaining the comb or caring for brood or foraging at different stages of adult life.
Other species of insects, notably ants and termites, have morphologically distinct castes. Each caste has its own job and a set of morphological characteristics to match; soldiers have large jaws and are specialized for protecting the colony; some kinds of ants are specialized for storing liquid food in their bodies. These ants, called honey-pot ants, have enormously swollen abdomens and, when given the correct communication, will regurgitate an edible droplet of sweet liquid for their nest mates. Some ants domesticate other insects, such as aphids or scale insects, and are rewarded with sweet liquid in exchange for protection. This herding behavior is unique to ants.
Insects, like other living organisms, must evolve strategies for adapting to climatic challenges. Some insects adapt to specific and predictable climatic conditions, such as cold seasons or dry seasons, with a strategy called diapause. Insect diapause is a genetically hardwired set of physiological changes and adaptive behaviors that serve to synchronize the life cycle of an insect species with predictable seasonal changes in its environment. Diapause is characterized by reduced metabolic activity and suppressed reproductive behavior, but it varies from one insect species to another and is not simply an insect version of hibernation. The unifying feature of this genetic program occurs during a species-specific stage of the insect's life cycle. Once it has begun, it proceeds according to a programmed plan and continues until it has run its course; it cannot be interrupted or shut off.
Diapause is initiated by triggers called token stimuli. These stimuli usually are environmental cues that herald a seasonal change that presents an ecological challenge. For example, winter is invariably preceded by a period of about three months during which each day is slightly shorter than the day before. In temperate and Arctic zones, winter is a period of harsh ecological challenge. Temperatures drop sharply, food becomes scarce, and both the biotic and abiotic environments undergo dramatic alterations. Insects that experience winter diapause detect shortening day length and use it as a cue to begin initiating physiological and behavioral changes in preparation for diapause. The physiological control and the visible manifestations of diapause, however, are as complex and diverse as the many species, life cycles, and environments of insects. The ability to undergo diapause confers on an individual insect a higher probability of surviving predictable adverse environmental challenges and thereby mediates evolutionary adaptation of that species.
The role of ecological information in pest control
There is no question that insects rank high on the list of animal pests, but the definition of what constitutes a pest species is entwined inextricably with ecology. A butterfly might be a desired visitor in a flower garden, but if its larva feeds on a crop plant is it actually a pest? A beetle that helps recycle dead trees in a forest, speeding degradation of dead wood and thereby returning nutrients to the soil, probably would not be considered a pest. The same species of beetle degrading the dead wood of a fence or a building definitely would be thought of as a pest. A bee pollinating a fruit crop is not a pest; the same species of bee nesting in a school playground would be unwelcome. Thus the definition of a pest is based solely on context and human values. Pest control is, in essence, an ecological problem. It is built on guiding principles that are grounded in an understanding of the ecology of the pest: its natural history and its interactions with both the biotic and abiotic features of the environment.
All the governing principles of pest control can be reduced to a single concept: pests are controlled by making the habitat so hostile that it is incompatible with the maintenance of a balanced energy budget. While this concise statement is an excellent distillation, it leaves much to be desired in terms of providing guidance on how control is to be accomplished. If the guiding principle is to alter the environment to make it totally unsuitable for the insect pest, then the corollary is to manage this without also rendering the environment unsuitable for those living organisms that are considered desirable. Thus, the basic issue in pest control is how to manipulate many environmental interactions in such a way as to create a hostile habitat for the pest while still allowing desirable species to flourish. This technique is called integrated pest management (IPM). It uses a variety of control methods (chemical, mechanical, cultural, and biological) that target a pest species' ecological interactions. The success of any IPM program is dependent upon a thorough knowledge of the pests' life history and ecology.
For example, a farmer practicing IPM could use information about the lower threshold temperature required for development of an insect pest species to predict when that pest will become active in the spring. A plant variety that germinates early in the season can mature and be past its vulnerable stage before pests are active. Introduction of pest-specific predators and parasites into the field can further deplete the pest population. Removing all plant debris at the end of the growing season can eliminate overwintering pests that have entered diapause. In all cases, effective pest control begins with acquiring basic life history information and studying the ecology of the species for which control is desired.
Insect ecology could be called the study of the energy budget of insects. The unique characteristics of insects provide a framework for understanding their interactions with both the biotic and abiotic components of their environments. Insects can maintain a balanced energy budget in most of the habitats available on Earth, but they are understandably most common in ecosystems where they can obtain large supplies of energy with relatively minimal expenditures. Their unique characteristics and special ecological adaptations play an indispensable role in almost every ecosystem on the planet.
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Martha Victoria Rosett Lutz, PhD
The term "ecology" designates one of the basic divisions of biology and was first employed by Ernst Haeckel, the German zoologist, in the 19th century. In the mid-20th century ecology emerged as one of the primary scientific and ethical concerns of humanity. The word comes from the Greek root oikos ("house" or "dwelling") and refers to the systemic relationship of abiotic environmental factors with biotic components such as plants, animals, and microbes. Ecology, therefore, is the study of the structure and dynamics of the web of life, i.e., the biological processes that compose and sustain the earth's ecosystem.
In contrast with the Newtonian view that nature is simple in structure, mechanistic in behavior, and static in form, an ecological interpretation stresses that nature is diverse in structure, reciprocal in behavior, and dynamic in form. Basic premises of an ecological interpretation include the following: (1) All forms of life exist in interdependent relationships. Life is sustained by reciprocity and mutuality among organisms. (2) Nature is dynamic rather than static. In its adaptation and growth nature shows a constantly changing face. (3) The stability of nature depends upon diversity. Heterogeneous environments have greater possibilities for change and adaptation than homogeneous environments. (4) Nature is fragile and finite. Through intentional or unintentional intervention the ecological balance can be so disturbed as to be irremediable. Moreover, nature has limits, and its supply of resources is not infinite.
In recent years ecologists have shown in considerable detail that many of the organisms and species on earth are either being irretrievably destroyed or are now in danger of extinction because of damage by human beings to the earth's ecosystems. Numerous ecological perils threaten humans and other living beings today. A litany of these usually includes: the rapid destruction of forests; massive erosion of soil; disappearance of sources of fresh water; desertification; pollution of land, air and water; extinction of species; global warming; and the thinning of the stratospheric ozone layer. Overarching all of these is the burgeoning human population which, along with patterns of excessive consumption, compounds every ecological problem the earth faces today.
Theology of Nature. In response to the ecological crisis one of the major developments in contemporary theology has been the attempt to reformulate a theology of nature. Six key themes can be discerned. (1) The biblical doctrine of creation (Gn 1) stresses the goodness of the whole created order. In the divine perspective nature has an implicit value of its own in that it manifests the goodness of God and joins in universal praise of its Creator. Nature, therefore, cannot be reduced to an exclusively instrumental status in the service of humanity. (2) Humans are an integral part of the web of life and not an exception to it. So close is the kinship of the human species to its natural environment that the two live in an inescapable reciprocity. Ecology traces the bonds between the two. While emphasizing human continuity with the nonhuman natural world theologians and scientists also call attention to the unique capacities of humans to reflect upon and to project the future of the natural world. Humans are to be responsible caretakers of the earth (Gn2.15). Within the created order only humans have the capacity to transcend time and place and hence to exercise stewardship with respect to the creation before the Creator. (3) Human sin is illustrated in the fact that the terrestrial sector of creation bears the consequences of our irresponsibility. The ecological crisis calls our attention to the biblical assertion that "the whole creation is in travail" (Rom 8.23). (4) The incarnation affirms the value of an individual's personal being before God by affirming simultaneously the significance of earthly life and its natural environment as the context for God's revelation. The Word that became flesh has identity and continuity with the creative Word which called all things into being (Jn 1:3). The New Testament authors extend the logos doctrine into a cosmic view in which Christ is the consummation of all things (Colossians) and the restoration of the cosmos through sanctification (Hebrews). The incarnate Christ restores to the creation its reality and value. (5) Some theologians, notably those influenced by teilhard de chardin or American process philosophy, emphasize a new formulation of the doctrine of divine immanence. Rather than acting from without, God is seen as the source of constant gracious creativity acting, in a manner consistent with the doctrine of the Holy Spirit, from within the on-going world process. In process thought distinctions are made between God's primordial and his consequent nature. According to process theology the world process is included within the life of God, who at the same time transcends the world. In this sense it can be said that God is the world's "ultimate environment." (6) The God of creation is also the Lord of history. Environmental scientists and theologians are concerned that ecological values be seen in their social as well as natural context. An ecological ethic requires the reordering of economic values. An adequate theology of nature will see the natural and the social worlds as existing inextricably together.
Ecological Theology. Official religious bodies, not unlike other institutions, have paid little attention to ecological issues until recently, but they are now doing so more explicitly than ever before. Moreover, ecology has increasingly come to engage the attention of religious thinkers, including Christian theologians. The objective of ecological theology is to spell out, in this case from within the context of Christian tradition, precisely why people of faith should care about the nonhuman natural world. Ecological theology is especially appropriate at a time when some prominent environmentalists are claiming that religion, and particularly Christianity, is indifferent if not inimical to the well-being of nature. Critics often cite the controversial thesis of historian Lynn White, Jr., that the Bible, by giving humans "dominion" over the earth (Gn 1.26), has sanctioned our "domination" of the natural world. Or they appeal to philosophers like John Passmore, who argues that Christianity will never contribute substantially to ecological ethics without ceasing thereby to be Christian. Preoccupation with the supernatural and with immortality, they argue, has led believers to focus so intensely on the "other world" that they pay little attention to this one.
Many theologians agree that the Christian tradition is somewhat ambiguous in its evaluation of nature, but that it still has the resources for a fresh ecological vision. Since the survival of nature was not a major issue during the emergence of biblical religion we should not expect the latter to come with pre-packaged remedies for our ecological problems. Still, even though concern for treating the earth as our home has not been a prominent feature of Christian spirituality, the central teachings of the faith can now be shown to be powerfully relevant to ecology.
It is not immediately obvious, though, precisely how Christianity can be said to be ecologically significant. Like some other religious traditions, it cherishes a spirituality that at times seems to have made terrestrial reality less important than ecological ethicists would require. Like some other religious traditions it has fostered a spirituality in which humans are in via, on a long journey of homeless detachment. And this ideal of religious home-lessness easily lends itself to translation into an environmentally noxious cosmic homelessness. Can the religious call to live homelessly be made compatible with the ecological imperative to treat the natural world as our home?
In the Bible, Abraham, the common ancestor of Judaism, Christianity, and Islam, was summoned to leave his ancestral home in order to pursue God's promise. His willingness to endure homelessness for the sake of the promise remained an ideal of Israel's religion which also understood itself in terms of an Exodus journey. Jesus called his own followers to a life of homeless pilgrimage. The Letter to the Hebrews states that faith in the promise makes us "strangers and aliens on earth," seekers of "a better homeland, a heavenly one" (11.13–16) Numerous other Christian writings, hymns, and prayers down through the ages have echoed the same theme: sojourning, the sense of not yet being at home, is central to Christian faith. Thus many Christians, perhaps even the majority of them, find it difficult to see the earth as "home." They sometimes even interpret it as though it were little more than a "vale of soul-making."
This spiritual interpretation of terrestrial existence turns the natural world into little more than a way-station on an exclusively human path to salvation. It robs earthly reality of intrinsic value, and although the doctrines of creation and incarnation clearly exalt the goodness of nature, Christian spirituality has been largely indifferent to the long-term thriving of the earth as a good in itself. francis of assisi, ignatius loyola, hildegaard of bingen, meister eckhart, thomas aquinas, and many others have emphasized the value of all created things, but concern for the long-term welfare of nature has not been a very explicit part of Christian preaching and teaching.
Nevertheless, some concerned theologians have begun to explore the ecological relevance of Christian faith. They are convinced that ecological ethics requires at least some kind of religious grounding and that religious homelessness need not be turned into a cosmic homelessness. While at first sight a pure naturalism may seem to be the only possible framework in which we could embrace the earth as our true home, under examination naturalism fails to demonstrate in sufficient depth precisely why our natural environment is inherently, and not just instrumentally, a good to be preserved and cherished. On the other hand, Christian theology, in spite of its historical ambiguity on the issue, can provide such a foundation.
For convenience one may distinguish three ways in which this theological premise is now being developed. These are the apologetic, sacramental, and eschatological approaches to ecology. None of the three can claim adequacy by itself, and there is some tension among them. But taken together they constitute at least the beginnings of an effective theological response.
The Apologetic Approach. The apologetic approach to ecological theology claims, either explicitly or implicitly, that Scripture and tradition together provide an adequate religious foundation for ecological ethics. Examples are the World Day of Peace Message by Pope John Paul II entitled "The Ecological Crisis: A Common Responsibility" (1990), the American Catholic Bishops' pastoral, "Renewing the Earth" (1992), and the World Council of Churches' statements on "Justice, Peace and the Integrity of Creation." In addition, a growing body of theological articles and books on ecology voice a similar apologetic concern.
The distinguishing mark of apologetic ecological theology is its sometimes unstated conviction that Christian tradition does not need to undergo drastic revision in order to constitute a sufficiently solid basis for environmental ethics. Its summons to responsible stewardship is a clear signal of biblical religion's concern for the natural world. So our vocation as responsible stewards of creation should provide enough of a religious incentive for Christians to take up the cause of ecology today. Numerous scriptural and traditional texts, many of them passed over before, also demonstrate the considerable extent to which Christian faith quite directly obliges us to protect the environment that we share with all other species of living beings.
This apologetic type of ecological theology also looks for support to the traditional emphasis on timeless religious virtues. Without the practice of love, humility, justice, detachment, and gratitude no alleviation of the ecological crisis is even conceivable. Since human habits of immoderation and injustice have contributed so obviously to pollution and the drain on nonrenewable resources, nothing less than a return by all of humanity to the pursuit of virtue will ultimately restore the earth to health. For this conversion to take place in an effective way people the world over must embrace their role as faithful stewards of the creation, representing God's goodness and care toward all other forms of life.
This approach may be called "apologetic" because it places a somewhat defensive shield over traditional religious teachings, claiming that they do not deserve the complaints they sometimes receive from secular ecologists. Apologists imply that Christianity is in essence immune to criticism since environmental abuse stems only from our disobedience to the dictates of faith and not from any deficiencies inherent in Christianity itself. Consequently, the ecological crisis calls less for the readjustment of Christianity than for a straightforward retrieval of its forgotten teachings about stewardship, justice, and other virtues. If we would only allow the eternal values set forth in Scripture and tradition to shape our environmental policies, we could avert the possible calamity that threatens the earth today. The fault is not with the sources of faith but with our failure to accept their message.
The often strident criticisms of Christianity by some ecologists would seem to justify something like an apologetic response. In spite of the well-known arguments concerning the "religious origins" of our current ecological crisis, it is by no means evident that religion is itself the main culprit. The widespread destruction of ecosystems may stem much less from specific religious attitudes than from irreligious habits and policies uncensored by a healthy sense of human limits and gratitude for the gift of creation.
Nevertheless, although apologetics must be one aspect of any Christian ecological theology today, a growing number of critics from within the Christian community now consider it to be quite inadequate. They concede that its focus on Scripture and tradition is helpful in bringing to our attention many ecologically significant texts and teachings (e.g., the Wisdom Literature, the Noachic covenant with "every living creature," or the psalms that glorify nature as God's creation, not to mention many texts from the New Testament or from early and medieval Christian writers). However, questions still linger about the religious sufficiency of the theme of stewardship and about the general obliviousness to the cosmos that characterizes so much traditional and modern theology. To an increasing number of theologians the ecological crisis requires that we go beyond apologetic theology.
The Sacramental Approach. The ecological crisis, some theologians argue, is so novel and momentous that it calls for a much more radical transformation of Christian faith than the apologetic approach proposes. These theologians seriously doubt that Christianity can adequately confront the problems facing the natural world simply by calling on such classic themes as stewardship and the practice of virtue, important though these may be. Even the most impressive display of scriptural and traditional texts about God and nature may not be enough to demonstrate Christianity's essential involvement with ecology. Theology needs to undergo a new and unprecedented internal change in its whole approach to nature. In brief, according to this second approach, the ecological situation requires that theology develop a much more profound "sense of the cosmos," especially after several modern centuries in which it has focused its attention almost exclusively on themes of history, subjectivity, society, and freedom—usually to the exclusion of nature. Theology is now being challenged to bring the universe back to the center of its concern.
Advocates of this cosmological transformation of theology base their position on what they take to be the sacramentality of nature, a theme already explicit in Scripture and tradition but often subordinated to the biblical emphasis on salvation history. While Scripture and tradition are still essential sources of ecological theology, nature itself is seen here also as powerfully disclosive of God. A fresh acknowledgment of the sacral quality of the cosmos itself is taken as the main reason for our valuing nonhuman nature. The inherently revelatory character of nature gives it an intrinsic, even "sacred," value that should shelter it from exploitative technological and industrial projects undertaken in the name of development and "progress."
Thomas Berry is one of the most prominent advocates of this sacramental approach. In his widely influential writings he argues that Scripture and tradition are by themselves an incomplete foundation for ecological spirituality and theology. Instead, he suggests that we base our ecological perspective on the sense that the universe itself is the primary revelation of God. His theology claims a strong pedigree in the sacramental emphasis of Catholic tradition, both western and eastern. However, advocates of both the apologetic and eschatological theologies are unhappy with the subordinate role he gives to the Bible.
The sacramental theme is also taken up into the "creation-centered" theology associated especially with Matthew Fox. This theology goes far beyond apologetics. Claiming that the traditional call to stewardship and virtue is not nearly enough, it argues that the ecological crisis requires a more radical rethinking of what it means to be Christian within the framework of the entire earth-community. Our inherited texts and teachings are not alone capable of leading us through the needed shift in our religious thinking and practice. The rhythms and powers of the universe must also be allowed to guide us. A reattunement to nature requires also that we attend to the voices of native peoples who have always lived close to the earth. All of the traditional teachings of Christianity—if we expect them to be effective in an age of ecological sensitivity—need to be recast in a sacramental, cosmological, relational, non-hierarchical, non-patriarchal, and non-dualistic fashion.
More than anything else, however, the biblical theme of creation must now be brought to the very center of Christian theology. According to creation-centered theology, this most ecologically compelling of all doctrines has been eclipsed by the tradition's unbalanced exaggeration of a "fallen" world and the need for human redemption. According to Fox, a one-sided Fall/Redemption theology diverted our religious attention away from the intrinsic, original goodness of nature. As long as nature seemed to be vitiated by our own sinfulness we failed to greet its sacramental effusiveness with an appropriate reverence. Moreover, a predominantly Fall/Redemption interpretation of the Bible led us toward an anthropocentrism that distracted us from concern about the nonhuman natural world.
Creation-centered theology, therefore, requires a less human-centered understanding of the cosmos than we find in the apologetic approach. Its relativization of the human even carries over to the notion of sin. Sin refers not only to our estrangement from God and other humans, but as well to nature's alienation from us and from God. Likewise "reconciliation" refers not only to the restoration of interhuman bonds, but more fundamentally to the renewal of the entire earth-community to which we belong much more completely than it belongs to us. In this theology Christ is much more than a personal historical savior. First and foremost he is the heart of the whole cosmos, the Word in whose image all of nature was fashioned, and the goal toward which the entire universe moves in its evolution. A cosmic Christology, with its roots in the New Testament writings of John and Paul, as well as in Irenaeus and Teilhard de Chardin, is the deepest foundation of a specifically Christian sacramental approach to ecology.
Countering the ecologically problematic cosmic homelessness of some of the world's religious traditions, creation-centered theology encourages an enjoyment of the natural world as our true home. Accordingly it moves beyond those spiritualities that fostered a sense of discomfort with our embodied existence. It is especially critical of the dualistic strains in Christian tradition that have sanctioned negative attitudes toward nature, women, and the body.
To those who accuse this new theology of advocating a licentious brand of "neo-paganism," sacramental ecologists remind us that their ecological preoccupation exacts a much more difficult kind of renunciation than did the puritanical dualism that defined so much Christian morality in the past. An ecological spirituality imposes upon humans the very strict spiritual discipline of taking into account the implications of all of their actions for the entire natural world and future generations. And while an ecological asceticism does not seek to detach us from the natural world, it does require that we forsake the ideal of autonomous, isolated selfhood with which we have become so comfortable since the Enlightenment. Sacramental ecology is equally intolerant of the privatization of religion. Taking into account the implications of our being intimately intertwined with the wider earth-community, and not just with human society, this ecospirituality demands personal sacrifices that we have never made before.
Such an ethic also calls for a wider understanding of justice than we find in most previous Christian moral teaching. Its emphasis on "eco-justice" reminds us that we cannot respond appropriately to any social inequities without attending also to the prospering of the earth's eco-systems. Likewise a truly "pro-life" ethic goes beyond focusing only on issues of human fertility and takes into account the need to protect the earth's complex life-systems without which there will be a complete and final "death of birth" (McDonagh).
Finally, since a sacramental perspective on ecology discovers in nature an inherent value, it radically questions utilitarian or naturalist attitudes toward the physical world. Inasmuch as nature is essentially the sacramental manifestation of an ultimate goodness and generosity, its value transcends that of simple raw material at the service of purely human projects. Thus the nurturing of a sacramental vision is one of the most important contributions Christianity can make to the grounding of ecological ethics.
The Eschatological Approach. Nevertheless, an accentuation of the theme of nature's sacramentality may not yet be the most distinctive endowment Christian faith can make to ecology. While any attempt to construct a Christian ecological theology today must build on the sacramental interpretation of nature, several theologians (notably Jürgen Moltmann) have asked whether biblical religion's most fundamental theme, that of a divine promise for future fulfillment, is itself of any relevance here. In other words, does eschatology have a significant role to play in shaping an ecologically sensitive theology?
To some ecologists a concern for the eschatological future, as it has been traditionally understood in Christian theology, is ecologically problematic. Discourse about the end of the world or about life beyond death seems to distract us from engagement with present ecological emergencies. On the other hand, since the theme of promise is the backbone of biblical faith, it is doubtful that we could have a distinctively Christian ecological theology without making eschatology central to it.
By "eschatology" theology today no longer means simply the religious concern for a personal destiny beyond death. Instead eschatology refers primarily to the patient, shared hope in God's promise that underlies the stories about Abraham, Moses, Israel's messianic expectations, Jesus' parables of the Reign of God, and the early Christian community's longing for the coming of Christ. Eschatology is not speculation about another world so much as it is the anticipation of God's always surprising and restorative appearance out of the future. It is not a vision that pulls us off the face of the earth, but one that looks toward the renewal of the earth and all of creation. The main theme of eschatology is not escape to the other world, but a new creation of this world, culminating in "the kingdom that will have no end."
Therefore, when viewed eschatologically all of reality, including the natural world, is permeated with promise. Even in all of its ambiguity the entire universe hints at future fulfillment. Authentic faith constantly scans the horizon for signs of the coming of a new future into the world, not for a removal of humans from the earth. Biblical faith looks not only for a God sacramentally revealed in present natural harmony but even more for the future coming of God in the eschatological perfection of creation, which of course includes the resurrection of the dead. This eschatological sense of promise may also help ground a Christian ecological theology.
An eschatological approach to ecology looks upon the natural world itself as essentially a promise of future fulfillment. Thus it is not only the world's sacramental character, but its being permeated by promise that bids us to care for it. If a theology of nature is to have a close connection to biblical religion, then the theme of promise must be made central and not subordinated to other theological criteria. Seen eschatologically, the present cosmos is an installment of the ultimate perfection announced by the good news of God's coming. Consequently, nature is not something from which to separate ourselves in order to find a final fulfillment, but a reality to which we are everlastingly related and whose new creation we constantly await.
Standing on the promissory character of nature an eschatological ecology does not displace but instead gives a distinctively futurist orientation to the sacramental contribution discussed above. Sacramentalism has the felicitous effect of bringing the wider cosmos back to the attention of theology. But in the Bible sacramentality is taken up into eschatology. Biblical hope does not look for a complete and final epiphany of the sacred in any present manifestation of natural beauty. Such a revelation of God awaits the eschatological future which even now relativizes all present cosmic realities, including the natural world in all its splendor. Nature's value then consists not only of its being transparent to God, as the sacramental approach rightly argues, but also of its being a promise of the future unfolding of God's vision for the world. Thus, human violence toward nature is by implication not only a sacrilege against the alleged "sacredness" of life. It is also despair, the turning away from a promise.
As the American Catholic Bishops' pastoral "Renewing the Earth" notes, the fundamental ecological virtue is hope. A genuinely biblical perspective requires that our ecological theology remain deeply connected to the sense of promise. If the sacramental approach seeks to recosmologize Christianity, then the Bible demands that we always embed our cosmology in eschatology. Present cosmic reality is not the conclusive symbolic revelation of God but an intense straining toward a new creation not yet fully manifest.
In its vision of redemptive fulfillment the Bible in fact explicitly includes the entire cosmos, now groaning in the birth pangs of new creation. Following the spirit of Paul in Romans 8.18–22, one may say that the universe is not a mere point of departure for the homeless religious pilgrimage, but in all of its evolution a participant in the human journey into God. Religious homelessness does not have to turn into a cosmic homelessness. The cosmos is not left behind as the children of Abraham pursue the promise. Rather it accompanies us in all of our striving. Nature shares eternally in our fate, and God's incarnational embrace of the world makes the whole universe a perpetual participant in the salvation for which Christian faith hopes. Our own religious longing for future fulfillment, therefore, is not a violation but a blossoming of the cosmos. This way of looking at things should make a difference in how we treat the earth's fragile ecosystems.
Viewing nature as promise, eschatological ecology also allows the universe to have a future that far transcends our purely human aspirations. The cosmic future includes much more than the goals that we humans might formally sketch. According to eschatological ecology, any realization of our plans for the human future must be accomplished in a manner that does not interfere with the promise for a transhuman future that the present cosmos may be carrying within itself. We are ethically obliged to preserve all the diversity of the earth's life systems, irrespective of their value for us, since to destroy them is not only to diminish our own future but that also of the larger world that includes us. As the Wisdom Literature implies—and especially the Book of Job—we are not ourselves the authors of the divine vision that embraces and moves all of creation.
An eschatological interpretation of nature carries two additional implications for ecological theology. In the first place, when the cosmos is viewed as promise nature can claim our respect and conservation without requiring that we prostrate ourselves before it. Sacramentalism, on the other hand, if not carefully tempered by a sense of the future, tends to sacralize nature, at times almost to the point of divinization. Eschatology allows for deficiencies in nature that a purely sacramental ecology may not easily tolerate. When it is taken as promise rather than solely as the present symbolic mediation of God, nature is allowed to be less than perfect. When we do not require the universe at this moment to be fully revelatory of God we will be less surprised and discouraged when it turns out to be bloody as well as beautiful. A sacramental ecology cannot easily accommodate the dark side of nature, whereas an eschatological posture, looking more toward the future than the present for the completion of creation and the final coming of God, can acknowledge the unfinished status of the world.
In the second place, an assimilation of ecology into eschatology allows for, and even demands, a way of thinking about personal life beyond death that will avoid a sense of our final separation from nature. The traditional interpretation of death and beyond was exceedingly problematic from an ecological point of view. In spite of the teaching about bodily resurrection, it has usually pictured human destiny in terms of an immortal human soul abandoning the body on an otherworldly journey to a realm completely beyond nature. This picture could hardly avoid placing the entire natural world, of which the body is a part, in a negative light.
What then would an ecologically satisfying notion of personal immortality look like? Karl rahner proposed that a person's death need not imply a separation from the earth and the universe, but rather the possibility of entering into a deeper relationship with nature. Though Rahner's language was still somewhat dualistic, he speculated that in death the soul takes on a "pancosmic" relationship to the world rather than becoming completely detached from it. If it is as persons that we die, Rahner implied, any "personal" survival of death could be construed as a deepening rather than a severing of our connections with the cosmos. In death the person is set free from a shallow relationship to the cosmos and to God in order to assume a more profound one. Such a view is consistent with Christian teachings about resurrection, continuous creation, and divine incarnation, as well as with the sense that nature is filled with promise.
The dualistic anthropology presupposed by much traditional piety, on the other hand, encouraged us to prepare for death by detaching ourselves as thoroughly as possible here and now from the world and the body. Allegedly this ascesis would make us ready for the final flight of the soul from the earth. A spirituality chastened by ecological concern, however, would prepare us for our personal death by having us always cherish and deepen our relationship to the cosmos. We prepare for death not by reducing the degree of our connectedness to the earth-community, but by heightening it. Spiritual discipline should under no circumstances mean a weakening of our sense of being intricately related to nature. Thus, in an ecological spirituality asceticism is not so much a matter of leaving things out of our lives as it is the habit of embracing the otherness around us, including the wildness of nature.
For Christian faith, of course, the archetype of such inclusiveness is Jesus himself. The Gospels present him as one who constantly sought out deeper relationships, especially with those who were no longer connected to life: the outcasts, the sick, the sinners—and the dead. Jesus' life, whose central motif is that of including the unincluded, can serve also as the model of our ecological concern. Ecological ethics then is the extension to all beings of the divine spirit of inclusiveness made manifest in Jesus. It is not essential that the historical Jesus himself have made any references to an "ecological crisis" in order to function as the model of our own ecological spirituality today. It is his eschatological spirit of inclusiveness, whose shape may vary from age to age, that is all important. Jesus' radically relational life is the sacrament of a responsive God whose preservative care and concern for life is the ultimate paradigm of our own ecological ethics.
Implications for Environmental Ethics. In light of the three versions of ecological theology discussed above, Christian theology is able to respond substantively to the suspicions voiced by some critics that it is indifferent to the ecological crisis. The apologetic, sacramental, and eschatological strains of ecological theology not only vigorously dispute such a suggestion, but together they make a strong case that Christian faith is inseparable from concern for ecological integrity.
From the apologetic approach ecological theology learns the significance of stewardship and the need for ecologically sustaining virtues. In spite of naturalistic suspicion of the notion of stewardship as being too "managerial," it would be irresponsible for humans now to abandon their vocation as caretakers of nature. Moreover, recent exegesis has shown that the Bible in no way sanctions the human exploitation of nature that Lynn White, Jr., declared to be the main historical cause of the environmental crisis. The biblical theology of human dominion and stewardship was never intended to make humans anything other than bearers of the image of a just and compassionate God in our relationship to the rest of creation.
On the basis of the sacramental approach ecological theology can make the case that Christian faith is essentially, and not just accidentally, bound to the preservation of nature. The loss of nature leads directly to a loss of our sense of God. It is useful to ask what our religions would look like if we lived on a lunar landscape (Berry). From the beginnings of religious history on earth the mystery of the sacred has been revealed through such natural phenomena as clean water, fresh air, fertile soil, clear skies, bright light, thunder and rain, living trees, plants and animals, and life's fertility. Nature, viewed in sacramental perspective, is not primarily raw material to serve human purposes but essentially the showing forth of a divine goodness and generosity. As such it commands a care and concern that a utilitarian view cannot provide.
Finally, an eschatological emphasis allows us to revere nature without compelling us to worship it. It treasures nature as a promise open to future perfection, and in this way provides a distinctively biblical direction to ecological concern.
Bibliography: i. g. barbour, ed., Earth Might Be Fair: Reflections on Ethics, Religion and Ecology (Englewood Cliffs, N.J.,1972). t. berry, The Dream of the Earth (San Francisco 1988). c. birch and j. b. cobb, jr., The Liberation of Life (Cambridge 1981). c. birch, w. eakin, and j. mcdaniel, eds., Liberating Life (New York 1990). h. e. daly and j. b. cobb, jr., For the Common Good (Boston 1989). d. edwards, Jesus the Wisdom of God: An Ecological Theology (Maryknoll, N.Y., 1995). f. elder, Crisis in Eden: A Religious Study of Man and Environment (New York 1970). j. f. haught, The Promise of Nature (New York 1993). d. t. hessel, ed., Theology for Earth Community: A Field Guide (Maryknoll, N.Y.,1996). b. hill, Christian Faith and the Environment: Making Vital Connections (Maryknoll, N.Y., 1998). s. mcdonagh, The Greening of the Church (New York 1990). j. moltmann, God in Creation, trans. m. kohl (San Francisco 1985). c. f. d. moule, Man and Nature in the New Testament: Some Reflections on Biblical Theology (Philadelphia 1967). j. nash, Loving Nature (Nashville 1991). m. oelschlaeger, Caring for Creation: An Ecumenical Approach to the Environmental Crisis (New Haven 1996). k. rahner, On the Theology of Death (New York 1961). r. r. ruether, ed., Christianity and Ecology: Seeking the Well-Being of Earth and Humans (Cambridge 2000). p. santmire, The Travail of Nature (Philadelphia 1985). p. smith, What Are They Saying about Environmental Ethics? (Mahwah, N.J., 1997). l. white, jr. "The Historical Roots of our Ecological Crisis," Science 155:1203–1207.
[j. c. logan/
j. f. haught]
The ecology of invertebrates consists of all the external factors acting upon that organism. These factors may be either physical or biological. The physical or abiotic environment consists of the nonliving aspects of an organism's surroundings, including temperature; salinity; pH (a measurement of acidity or alkalinity); exposure to sunlight; ocean currents; wave action; and the type and size of sediment particles. The biotic environment consists of living organisms and the ways in which they interact with one another.
Invertebrate species have colonized all types of aquatic habitats. For example, sponges of the class Calcarea are restricted to firm substrates. They are also restricted by physical factors that affect their skeletons, limiting their habitats to shallow zones. Hexactinellida sponges colonize soft surfaces; they prefer to live in deep water. Demosponges can live on such different substrates as rock, unstable shell, sand, and mud; in some cases they burrow into calcareous material. They are found in a variety of underwater habitats ranging from upper intertidal to hadal depths (below 20,000 ft or 6,100m). The ecological dominance of the Demospongiae reflects their diversity in form, structure, reproductive capabilities and physiological adaptation. Cnidarians and ctenophores are mostly marine; however, a few groups have successfully made their way into freshwater habitats.
Most lower metazoans are either sessile polyps, which means that they are attached at the base to the surface that they live on, or planktonic carnivores. Some, however, employ suspension feeding and many species harbor symbiotic intracellular algae that supply them with energy. Hydroids, scyphozoans, and anthozoans live in seas around the globe, from polar to tropical oceans. Most lower metazoans, however, live in coastal waters.
Sunlight has an important role in both terrestrial and marine environments, powering the process of photosynthesis that provides energy either directly or indirectly to nearly all forms of life on earth. The diel, or 24-hour cyclical migrations of epipelagic species, are at least in part active responses to changing light levels. Epipelagic refers to the upper levels of the ocean that are penetrated by enough sunlight for photosynthesis to occur. Aurelia aurita approaches the surface during the day, at both midday and midnight, or only at night, and becomes scattered throughout the water column at night or during the sunlit days. Diel migrations probably do not occur in the bathypelagic zone (about 3,280–6,562 ft or 1–2 km); migration in the mesopelagic zone (about 656–3,280 ft or 200–1,000 m) depends on the levels of available light in that zone. Sunlight is necessary for vision as well as photosynthesis. Many animals rely on their vision to capture prey, avoid predation, and communicate with one another.
Bioluminescence is a type of visible light produced by marine animals such as scyphozoans, hydrozoans, ctenophores, squids, thaliaceans, and fishes. It may be used for counterillumination or as ventral camouflage. Another possibility is that bioluminescence is a useful defense mechanism against potential predators.
Turbidity refers to the cloudiness of sea water caused by the suspension of sediment particles and organic matter. High concentrations of suspended particles in the water over offshore coral reefs are considered a stress factor for coral colonies because they reduce the amount of light for photosynthesis and smother coral tissues. Nevertheless, many reefs with large growths of coral are found in relatively turbid waters, such as the fringe reefs around the inshore continental islands in the Great Barrier Reef lagoon. This finding suggests that turbid water is not necessarily harmful to coral. Fine, suspended particles provide a large surface area for colonization by microorganisms that produce organic nutrients. By limiting light penetration, turbid water also limits the distribution of both benthic algae and phytoplankton, which are at the base of the web.
All lower metazoans are ectothermic (sometimes referred to as "cold-blooded"), which means that they retain the same temperture as their surroundings. Because of this restraint; invertebrate physiology has evolved to operate in a specific temperature range for each species. Most organisms can tolerate only a narrow range of temperatures; changes above or below this critical range disrupt their metabolism, resulting in a lowered rate of reproduction, injury, or even death. Since temperatures change less rapidly in the open sea than in shallow waters, species in shallow waters can tolerate a wider range of temperature than deep water species. Temperature often influences the distribution, reproduction, and morphology (form and structure) of these organisms. Colonies of Obelia geniculata and Silicularia bilabiata living in cold water develop long, branching hydrocauli (stalks), whereas colonies of these species living in warm waters have short stems with few branches. Gametogenesis in the hydrozoan Sertularella miuresis begins when the temperature reaches 50°F (10°C) and stops when it reaches 64°F (18°C). Coryne tubulosa reproduces asexually at around 57°F (14°C), but produces medusae when the temperature falls to 35°F (2°C). The acclimation temperature of Chrysaora quinquecirrha polyps is about 51°F (10.50°C), but the upper lethal temperature dose, defined as the temperature at which 50% of the test animals die (LD-50), is 95°F (35°C).
Salinity, or the level of salt content in seawater, can affect invertebrates. Species that have evolved to live in freshwater can rarely live in salt water, and few marine species can tolerate low salinities or freshwater. This becomes quite apparent when one studies species richness (number of species) as one moves down a river into an estuary. Species richness (number of species) is relatively high in freshwater, then decreases considerably as salinity increases to about 5ppt, where most freshwater species cannot exist. Species richness then increases with salinity as more low-salinity-tolerant species are encountered. Species richness is at its greatest at the mouth of the estuary, where fully marine species occur with estuarine species.
Salinity can effect the morphology of organisms. For example, the shape, number, and size of tentacles of Cordylophora caspia polyps is affected by salinity. The scyphozoan medusae of Rhopiena esculenta can survive at levels of salt concentration as low as 8 parts per thousand (ppt), the scyphistomae to 10 ppt and the planulae to 12 ppt. The estromatolites of Phylloriza peronlesueri, however, form in hypersaline (very salty) waters. A rise in the salt content of the Baltic Sea allowed A. aurita and C. capillata to expand into northern waters, and allowed Rhizostoma pulmo and A. aurita to move from the Azov Sea into the Black Sea. The hydroid Laomedea flexuosa increases its production of gonozooids when the seawater concentration is around 30–40 ppt; at higher concentrations, however, the colonies begin to degenerate. The cephalo-chordate Branchiostoma nigeriense becomes opaque above salinities of 13 ppt.
Ocean currents and turbulence
Moving water is essential to lower metazoans because it supplies food and dissolved gases; prevents the accumulation of sediment; and disperses waste products, medusae, and larvae. Aglaophenia picardi resorbs the tissues of its hydrocauli into the hydrorhiza when the surrounding water is relatively stagnant, but regenerates them when the water begins to move more rapidly. The speed and direction of current flow affect
the form and size of some lower metazoans. The size of hydroids is usually inversely related to the speed of water movement; large specimens are found in calm water and smaller specimens in rougher water. Aglaophenia pluma develops unbranched hydrocauli about 0.6 in (1.5 cm) tall in shallow, turbulent water, but produces branched hydrocauli as high as 19.6 in (50 cm) in deeper water with bidirectional currents. Planar (flat) forms such as A. pluma, Plumularia setacea, and Eudendrium rameum are most abundant where the current tends to flow in one direction, while radial or arborescent (treelike) forms such as Lytocarpia myriophyllum, Nemertesia antenna, and E. racemosum flourish in bi- or multidirectional currents. The distribution of species that inhabit coral reefs and display highly specific patterns of tolerance is greatly affected by water movement. Morphological differences in hydroids and anthozoans are also regarded as indicators of distinct patterns in water movement.
The majority of organisms are not able to survive in great depths (below 3,281 ft or 1,000 m). In general, the number of invertebrates is highest in shallow water communities and decreases as water depth increases. However, species diversity may be quite high at great depths on the abyssal plain where the environment has been extremely stable for millennia. Though diversity can be high, biomass may be low in these deep benthic habitats, because the lack of light prevents any primary production. Therefore, these habitats are usually limited by food and depend on organic input from sunlit seas above.
The lack of mixing and primary production result in oxygen-minimum layers in the ocean, and many species are either adapted to lower oxygen concentrations or avoid these areas. The scyphozoans Periphylla periphylla and Nausithoe rubra show high levels of the anaerobic enzyme lactate dehydrogenase, probably as an adaptation to moving at depths between 1,312 and 4,921 ft (400–1,500 m), which has minimal levels of oxygen.
Pollution may result from contamination by sewage, hydrocarbons, polyvinyl biphenals (i.e., PCBs), pesticides (e.g., DDT), and heavy metals such as cadmium, copper, lead, mercury, and zinc. Experiments have revealed that exposure to pollutants can lead to sublethal effects in hydroids, including changes in the curvature or branching of the hydrorhiza; loss of hydranths; stimulation of gonozooid production; or changes in the rate of growth. Low concentrations of metal ions such as copper and mercury may inhibit growth regulation in hydroids while increasing the growth rate in Laomedea flexuosa and Clavopsella michaeli. In Elefsis Bay, a polluted area of Greece, populations of Aurelia aurita have multiplied to rates of more than 1,500 medusae per 10 m3. Certain species of Rhizostoma have survived in parts of Madras Harbor that have been polluted by diesel oil; however, the presence of crude petroleum in the waters of Alaska has caused a reduction or cessation in the strobilation in the polyps of A. aurita, and the production of ephyra and polyps with both morphological and behavioral abnormalities. Pelagia noctiluca, a scyphozoan from the Mediterranean Sea, acquires high concentrations of cadmium, lead, mercury and zinc. Individuals of the species Chrysaora quinquecirrha have been found to have highly concentrated levels of the herbicide pendimethalin in their tentacles; they show no change in behavior at concentrations of the pesticide that are lethal to fishes such as perch. The dumping of raw sewage in may tropical areas of the world destroys coral reefs by increasing turbidity that prevents light penetration, increasing sediment loads that smother corals, and increasing nutrient loads that encourage algae growth that can out-compete corals.
Competition occurs when organisms require the same limited resources, such as food, living space, or mates; or when two groups of organisms try to occupy an ecological niche in the same location at the same time. Competition may either be interspecific (between different species) or intraspecific (within the same species). Hydroids, which have a stoloniferous growth pattern, demonstrate two different growth strategies. The first, a guerrilla strategy, is characterized by extensive hydrorhizal growth with little branching and sparsely spaced hydrocauli or polyps; this pattern is exhibited by L. flexuosa. Guerilla behavior is an opportunist-style strategy for reducing interspecific competition for space and the possibility of overgrowth by other organisms. The second strategy, phalanx; results in highly branched hydrorhizae with dense hydranths carried on large hydrocauli. The phalanx strategy is exemplified by Podocoryne carnea and Hydractiniaechinata, which grow on the shells of hermit crabs. Intraspecific variation in the allocation of resources that lead to hydrorhizal growth can be observed in the colonies of H. echinata. This species usually shows either little outward hydrorhizal growth combined with a high rate of reproduction, or extensive hydrorhizal growth combined with a lower rate of reproduction and correspondingly greater competitive ability. Contact with another colony leads the hydrorhiza to produce an abnormally large number of stolons (shoots or runners) armed with nematocysts, which sting and kill the tissue of other hydroids. Guerrilla growth strategies have adaptive value in situations where there is relatively little space available, as on shells occupied by other hydroids, while the phalanx strategy is more advantageous for expanding the colony to shells that are inhabited by hydroids.
Competition for space is of prime importance in the coral reef ecosystem. Most of the aggressive species are small and have slow growth rates, while the less aggressive coral species have faster growth rates and are able to outpace their competitors through rapid growth. The ability to maintain either rapid growth or aggressively dominative practices, but never both, explains why no single species of coral is able to dominate a coral reef. One possible outcome of competition is the extinction of the less successful competitor.
A niche can be subdivided into two or more small niches with minimal overlap, allowing competing organisms to share a resource. Examples of resource partitioning may be found on coral reefs. Small ecological niches can be occupied by similar species if the anatomy, feeding behavior, and territory of each species are only slightly different from those of another. The hydrozoans Hydractinia (retained gonophores), Stylactis (medusoids), and Podocoryne (medusae) have similar morphologies but different reproductive strategies, which allows them to occupy similar niches on the shells of hermit crabs. The competitive ability of colonies may also depend on their size. Podocoryne carnea hydroids show a greater selective advantage in aggressiveness in relation to H. echinata in interspecific competition. In instances of intraspecific competition among different colonies of P. carnea, however, the colony most likely to lose out is the one with the slowest rate of growth.
Feeding mechanisms and behavior
Lower metazoans demonstrate a remarkable variety of feeding mechanisms. Most sponges are suspension feeders that subsist on such fine particles as bacterioplankton and dissolved organic matter. Sponges acquire food and oxygen from water that flows through them; this flow is actively generated by sponges beating their flagella (microscopic whiplike structures). This process also acts as a means of waste removal for sponges. The movement of water through sponges is aided by ambient currents passing over raised excurrent (providing outward passage) openings, which creates an area of low pressure above these openings. Sponges are also capable of regulating the amount of flow through their bodies by narrowing or partly closing off various openings. The volume of water passing through a sponge can be enormous—as much as 20,000 times its volume over a 24-hour period.
Sponges are size-selective particle feeders. Their aquiferous systems create a series of "sieves" of varying mesh size. The largest diameter of incurrent openings is usually around 0.002 in (50 µm), which keeps larger particles from entering the aquiferous system. A few species have larger incurrent pores, reaching diameters of 0.006–0.0069 in (150–175 µm). Some sponges trap roughly 90% of all bacteria in the water they filter. Other sponges also take significant amounts of dissolved organic matter into their aquiferous systems. In some demosponges, 80% of the organic matter that is filtered through their aquiferous system is too small to be seen by light microscopy. The other 20% is composed primarily of bacteria and dinoflagellates. Other sponges harbor symbionts such as green algae, dinoflagellates (zooxanthellae), or cyanobacteria, which also provide them with nutrients.
Many invertebrates are predators that feed on protozoans, other invertebrates, and fishes. The discovery of several Mediterranean species of sponges that capture and digest entire animals came as a surprise to marine biologists. These species of the family Cladorhizidae have no choanocytes or aquiferous systems, but anatomic and biological analysis revealed the presence of spiky filaments with raised hook-shaped spicules. These carnivorous sponges capture and hold small crustaceans with their spicules, which act like Velcro® tape when they come in contact with the crustaceans' exoskeletons. Once captive, the crustaceans cannot free themselves. They struggle for several hours, which indicates that the spicules do not produce any paralyzing or toxic secretions. Cells then migrate around the helpless prey, and digestion takes place outside the cell walls.
Most cnidarians are carnivorous, using cnidocytes on their tentacles to capture prey. Polyps, the sessile stage of cnidarians, are generally believed to be passive predators, feeding on animals that blunder into their tentacles. Some cnidarian medusae possess sensory structures resembling primitive eyes; they are active predators. Many corals and anemones feed by suspending thin strands or sheets of mucus over the surface of their colony. The sticky mucus collects fine particles of nourishment from the water; cilia present on the organisms drive the food-laden mucus into the mouths of coral or anemones. Many species have developed complementary adaptations such as ectodermal ciliary currents on their tentacles, oral discs or columns, or the ability to position themselves strategically within the water's flow pattern.
In sea anemones, the presence of nearby food evokes behavior that has two phases: a prefeeding, and a feeding response that leads to the ingestion of prey. The prefeeding response consists of the expansion of the oral disc, the movement of its tentacles, and both the extension and swaying of the column. This prefeeding behavior increases the chances of catching nearby food. The feeding response, which takes place after the prey has made contact with the anemone's tentacles or oral disc, includes the discharge of nematocysts and ingesting movements. Sea anemones are able to detect prey from the prey's emission of small dissolved molecules of
amino acids, tripeptides, and vitamins. The cnidae, which ensure the capture of the prey, are spirocysts and nematocysts. The numerous spirocysts on the tentacles of sea anemones appear to have an adhesive function. The contact of solid food with the tentacles leads to a massive discharge of spirocysts, which hold the prey while the nematocysts inject their toxin. Ingestion is directed by the chemical and mechanical stimuli produced by the immobilized prey. After ingestion, the prey is enclosed by filaments whose cnidoglandular tracts contain nematocysts but no spirocysts. The penetrating filaments of these nematocysts inject more toxin. The prey is then subject to the action of secretory cells that ensure its extracellular digestion.
The prey of sedentary cnidarians is composed of small motile animals such as zooplanktonic larvae, isopods, amphipods, and polychaetes. Sea anemones found closer to shore may complete their diet with larger sessile prey dislodged by wave action or foraging predators. The size of the prey is generally small considering the diameter of the sea anemone, and many species may be considered microphagous. Of the nine common species of Caribbean reef sea anemones, seven are planktivores. Condylactis gigantea and Stoichactis helianthus, which eat macroscopic prey such as gastropods and echinoids, probably depend on heavy wave action in the reef to supply them with prey.
Comb jellies are entirely predatory in their habitats. The long tentacles of ctenophores have muscular cores with an epidermal cover that contains colloblasts, or adhesive cells. The tentacles trail passively through the water or are twirled about by various circular movements of the body. Upon contact with the prey, the colloblasts burst and discharge a strong sticky material. In ctenophores, which bear very short tentacles (orders Lobata and Cestida), small zooplankton are trapped in mucus on the body surface and then carried to the mouth by ciliary currents (along ciliated auricular grooves in ctenophore lobates and ciliated oral grooves in cestids). Under conditions of starvation in an aquarium, lobate adults often swim vertically through the water only to descend with their lobes extended; in this position the width of the lobes may be as much as 116% of the animal's length. The lobe width decreases to about 79% of the body length after food is placed in the aquarium and the animal begins to feed. Once the digestive tract is full, these adults still continue to feed by entangling their prey in mucus, which produces a bolus or clump near the mouth. Quite frequently they will either spit out this ball of food or completely empty their digestive tract and continue to feed. This behavior pattern can continue for several hours until the concentration of food is reduced to the point at which all the prey that have been captured can be ingested.
Most pelagic ctenophores and cnidarians feed primarily on copepods. There are a few intriguing examples of pelagic cnidarians that feed exclusively on one type of prey. The siphonophore Hippopodius hippopus feeds only on ostracods; the hydromedusae Bougainvillia principis feeds mainly on barnacle nauplii; and Proboscidactyla flavicirrata eats only veligers (mollusk larvae). Some cnidarians and ctenophores feed specifically on gelatinous prey or fishes. Ctenophores of the genus Beroe offer well-known examples of selective feeding on gelatinous prey: B. cucumis feeds exclusively on the ctenophore Bolinopsis vitrea, and B. gracilis feeds only on Pleurobrachia pileus. When beroids prey on animals larger than themselves, they appear to attach themselves to the prey and suck its tissues into their mouths. Beroids lack tentacles; however, they do possess some 3,000 macrocilia that are hexagonally arranged and form a ciliary band around the inside of the mouth that beats inward, and forcing tissue from the prey into the beroid's pharynx. Gelatinous species that include high proportions of soft-bodied prey in their diets often eat fish eggs and larvae when they are available.
The diets of gelatinous predators generally show some selectivity, and are dependent on factors including the prey's size; the width and spacing of the predator's tentacles; the predator's swimming behavior and speed; water flow; and the prey's ability to escape. In general, species that catch large prey have few and widely spaced tentacles, while those that feed on small prey have numerous closely spaced tentacles. Most gelatinous predators move while feeding, which allows them to make use of water currents that will bring prey toward their tentacles. Cnidarians that are ambush predators are able to catch large, fast-moving prey; cruising predators prefer small, slow-moving prey. For example, siphonophores, which are ambush predators, tend to select large and relatively swift prey, while Aurelia aurita, which is a cruising predator, selects slow-moving organisms. Swimming offers the advantage of allowing predators to scan larger volumes of water; however, it also has the disadvantage of alerting the prey to the predator's presence. Some predators deal with this disadvantage by remaining stationary while they are "fishing."
Swimming is not the only way for prey to escape. Bivalve veligers will close themselves off when they are disturbed by the medusae of Chrysaora quinquecirrha; 99% of captured veligers are ingested alive.
Ecological significance of predation
The feeding rate of predators can be expressed in terms of clearance rate (volume of water × predator −1 × time −1), or in terms of numbers or biomass of prey captured (predator −1 × time −1) and are often combined with estimates of predator and prey densities in order to estimate the effects of predation on prey populations. One characteristic of gelatinous predators is that clearance rates tend to be constant even at extremely high prey densities. In species with few tentacles (e.g., Pleurobrachia spp.), however, ingestion is limited by prey handling. Saturation feeding at typical prey densities rarely occurs in situ. Gelatinous predators feeding on small prey seldom fill themselves at natural prey densities. They may not reduce populations, but they do keep them in check. Predators that consume ichthyoplankton often appear in areas of intense spawning activity and are major causes of fish egg and larva mortality. In some areas large schyphomedusae consume high numbers of commercially important fish larvae, and they also compete with fishes for food. Swarms of jellyfish may be so dense that they clog and damage fishing nets. Many hydromedusae species, including Porpita spp., Velella spp., and Physalia spp., also occur in huge concentrations, particularly in tropical seas.
Some species have such a significant ecological effect that they are considered "keystone species." If these species disappear or appear in an environment, the entire habitat can shift dramatically. For example, active predation by sea stars can significantly affect prey populations. The crown-ofthorns sea stars (Acanthaster spp.) feed on coral polyps in tropical reef habitats worldwide. Occasionally, population explosions of A. planci occur that can have devastating effects on coral reefs. Thousands of square miles (kilometers) of bare coral skeletons can result. Species composition and diversity of other inhabitants are affected secondarily as algae and other organisms colonize the reef. Organisms that depend on live coral for survival either leave the area or die.
Aggressive and defensive behavior
Gastropods, pycnogonids, sea stars, sea urchins, fishes, and sea turtles are predators that feed on invertebrates. Moreover, jellyfishes and comb jellies can be predators of other cnidarians and ctenophores. Sea turtles, especially Dermochelys coriacea, feed on scyphomedusae such as A. aurita. In addition, birds may add scyphozoans to their diets. Some benthic animals like the nudibranchs may feed on the scyphistomae of Cyanea capillata and A. aurita. A single nudibranch can consume as many as 200 polyps per day; however, not all nudibranchs are able to eat scyphozoans. As in the pelagic environment, scyphozoans in their benthic stages may eat one another; for example, the scyphistomae of A. aurita eat the planulae larvae of C. capillata as well as the larvae of their own species. In addition to natural predation, scyphozoans may be affected by fisheries.
In general, these species defend themselves against predators by the production of physical structures or the emission of various chemicals. Sponges are the most diverse source of marine natural products; some of these compounds offer potential pharmacological benefits. Compounds isolated from sponges vary widely in structural complexity; they include sterols, terpenoids, amino acid derivatives, saponins, and macrolides. The toxins produced by temperate and tropical sponges have been shown to deter predation by fishes, asteroids, and gastropods. The organic component of sponges consists primarily of NaOH-soluble and insoluble protein, NaOH being the chemical formula for sodium hydroxide, or lye. About 56% of Antarctic sponges are toxic. Leucetta leptorhapsis and Mycale acerata are highly toxic; however, the asteroid Perknaster fucus antarcticus is a specialist and is able to feed on M. acerata without succumbing to its toxins. This finding suggests that some asteroids have evolved physiological mechanisms that neutralize or sequester (compartmentalize) sponge toxins. In many cases these secondary metabolites are not necessarily toxic, but may make the sponge distasteful to predators and may be more effective in deterring predation. These compounds not only help the sponges avoid predators, but also prevent infection by microorganisms. In addition, they allow the sponges to compete for space with other sessile invertebrates such as ectoprocts, ascidians, corals and even other sponges. Clinoid sponges are among the most common and destructive endolithic (living embedded in rock surfaces) borers on coral reefs worldwide. Cliona, Anthosigmella, and Spheciospongia of the order Hadromerida; and Siphonodictyon, of the order Haplosclerida are siliceous sponges known to bore into hard substrates. Such sponges are able to excavate galleries in calcareous material by removing small fragments of the mineral by specialized archaeocyte cells. The cells secrete chemicals that dissolve the calcareous substrate. When infested corals are split open, clinoid sponges appear as brown, yellow, or orange patches lining the corroded interiors of the coral skeleton.
The sponge Cinachyra antarctica has distinctively long spicule tufts that emerge from the spiral conules on their surface. This species, found throughout Antarctica at depths of 59–2,496 ft (18–761 m), uses its spicules to protect itself from predators. When spicules are removed, C. antarctica is made vulnerable to predators.
Defensive and feeding activities are closely associated in most cnidarians; the tentacles of most anemones and jellyfishes serve both purposes. In some cases, however, both functions are performed by separate structures. Sea anemones and corals have developed several specialize structures used to defend against territorial invasion. Three of these structures, namely acrorhagi (special fighting tentacles), catch tentacles, and sweeper tentacles, are modified feeding tentacles. The mesentenic filament is another modified defensive structure. The acrorhagi are located at the margin of the anemone's body column. When these anemones make physical contact with one another, usually with their tentacles, the acrorhagi expand and apply themselves on the target organism. The ectodermal tissue of the acrorhagus lifts away from its underlying mesenclyme (cellular jellylike material) while the acrorhagus discharges its nemactocysts, and the ectoderm then clings to the target organism. This process is called peeling. As a result of continued discharges from the nematocysts, the victim's tissue beneath the acrorhagial peel becomes necrotic and dies. Catch tentacles (in sea anemones) and sweeper tentacles (in scleractinian corals) develop from feeding tentacles that undergo a morphological change when the organism comes into contact with appropriate other species. In response to weeks of contact, the feeding tentacles alter their form, structure, and complement of cnidae. Catch and sweeper tentacles do not adhere to potential food objects; when they are touched by prey, they actually retract. Sweeper tentacles emerge at night; as their name implies, they flail about or undulate. They can reach 5–10 times the length of feeding tentacles. The coral Montastrea cavernosa is a mildly aggressive coral, capable of destroying the tissue of a variety of subordinate coral species with its mesenterial filaments. But its own mesenterial filaments can be destroyed by M. annularis when both are placed together. These species have sweeper tentacles, which are multipurpose structures with the capacity to regulate the distance between colonies, thus functioning as organs of competition.
Although the ability of hydroids to resist predation is often attributed to their nematocysts and associated toxins, the chemical compounds make them much less attractive to a potential hydroid predator. Some species of hydroids secrete chemicals that deter feeding by the pinfish Lagodon rhomboides. After having been treated with potassium chloride, which forces them to discharge their nematocysts, both Halocordyle disticha and Tubularia crocea become palatable; this suggests that these species rely on nematocysts to defend themselves against predators. However, species such as Corydendrium parasiticum, Eudendrium carneum, Hydractinia symbiologicarpus, and Tridentata marginata, remain unpalatable after their nematocysts have been discharged.
Arai, N. A. A Functional Biology of Scyphozoa. London: Chapman and Hall, 1997.
Bergquist, P. R. Sponges. Berkeley: University of California Press, 1978.
Nybbaken, J. W. Marine Biology: An Ecological Approach. New York: Harper and Row, 1982.
Fautin, D. G. "Reproduction of Cnidaria." Canadian Journal of Zoology 80 (2002): 1735–1754.
Gili, J. M., and R. G. Hughes. "The Ecology of Marine Benthic Hydroids." Oceanography and Marine Biology: An Annual Review 33 (1995): 351–426.
Harbison, G. R., and R. L. Miller. "Not All Ctenophores Are Hermaphrodites. Studies on the Systematics, Distribution, Sexuality and Development of Two Species of Ocyropsis." Marine Biology 90 (1986): 413–424.
Kass-Simon, G., and A. A. Scappaticci, Jr. "The Behavioral and Developmental Physiology of Nematocysts." Canadian Journal of Zoology 80 (2002): 1772–1794.
McClintock, J. B. "Investigation of the Relationship Between Invertebrate Predation and Biochemical Composition, Energy Content, Spicule Armament and Toxicity of Benthic Sponges at McMurdo Sound, Antarctica." Marine Biology 94 (1987): 479–487.
Purcell, J. E. "Pelagic Cnidarians and Ctenophores as Predators: Selective Predation, Feeding Rates and Effects on Prey Populations." Annales de l'Institut Océanographique 73, no. 2 (1997): 125–137.
Reeve, M. R., and M. A. Walter. "Nutrionatal Ecology of Ctenophores — A Review of Recent Research." Advances in Marine Biology 15 (1978): 249–287.
Stachowicz, J. J., and N. Lindquist. "Hydroid Defenses Against Predators: The Importance of Secondary Metabolites Versus Nematocysts." Oecologia 124 (2000): 80–288.
Van-Praet, M. "Nutrition of Sea Anemones." Advances in Marine Biology 22 (1985): 65–99.
"Spiky sponge Cinachyra antarctica." Underwater Field Guide to Ross Island and McMurdo Sound, Antarctica. Scripps Institution of Oceanography Library. <http://scilib.ucsd.edu/sio/nsf/fguide/porifcra16.html>
Alexandra Elaine Rizzo, PhD
Eliane Pintor Arruda, MSc
Dennis A. Thoney, PhD
Ecology is commonly seen as a lineal descendant of traditional natural history extending back to such classical figures as Aristotle, Theophrastus, and Pliny. Notable persons in this tradition include the Swedish botanist, Carl von Linné (Carolus Linnaeus; 1707–1778), who coined the phrase "economy of nature" in 1749. Gilbert White (1720–1793), a British cleric, made astute ecological observations of his parish in The Natural History and Antiquities of Selborne (1789). Charles Darwin's (1809–1882) work on evolution, published in 1859, was acknowledged as the stimulus for coinage, in 1866, of the term ecology. Henry David Thoreau (1817–1862), poet and naturalist, in 1860 anticipated a key phenomenon of ecology, and its name, in an article, "The Succession of Forest Trees." In 1864 George Perkins Marsh (1801–1882), an American diplomat, anticipated the environmental crisis that was widely recognized a century later, in his book Man and Nature in America, which described the deleterious impact of humans on the earth.
The ubiquity of such observations was explicit in the observations of the historian Clarence Glacken (1967), who said that ecological theory originated in the design argument of nature and that every thinker from the fifth century b.c.e. to the end of the eighteenth century had something to say about one or more of the ideas about environments. Even this extended attribution omits consideration of the detailed, and commonly insightful, traditional natural history knowledge of nonliterate aboriginal cultures the world around. The premier British ecologist Charles Elton dubbed ecology as scientific natural history in 1927. Increasing recognition of the extended history of ecological insights, anticipating a formal science of ecology, called up the apt term protoecologist (protoecology) in 1983.
The undisputed source of the term ecology is the eminent German zoologist, Ernst Haeckel (1834–1919), who coined it in 1866. It is well to revert to Haeckel's expanded definition in 1869 as translated by Allee and others:
By ecology we mean the body of knowledge concerning the economy of nature—the investigation of the total relations of the animal both to its inorganic and to its organic environment; including above all, its friendly and inimical relations with those animals and plants with which it comes directly or indirectly into contact—in a word ecology is the study of all those complex interrelations referred to by Darwin as the conditions of the struggle for existence. (Allee, 1949)
Haeckel's definition illustrates the continuing tendency to distinguish animal and plant ecology. His emphasis on Darwinian evolution was echoed by numerous early ecologists, and later historians, and persists as evolutionary ecology.
The term ecology appeared sparingly in the scientific literature until the 1890s. In 1893 the president of the British Association for the Advancement of Science described ecology as a branch of biology coequal with morphology and physiology and by far the most attractive. Also in 1893, the Madison Botanical Congress, a large meeting of professional botanists, formally adopted the term ecology. A chair of ecological botany was established at Uppsala University in Sweden in 1897, and in 1904 Oscar Drude, a German plant geographer, described the sudden recognition of ecology in a talk at a Congress of Arts and Sciences at the Universal Exposition in St. Louis. Charles E. Bessey, a prominent American botanist, commented in 1902 that ecology had become a fad—a slight exaggeration, as the "fad" was largely confined in America to a few Midwestern universities and state agencies. The first named textbook of ecology was published in Danish by a Danish botanist, Johannes Eugenius Bülow Warming, in 1895; and the first doctorate in ecology in the United States was granted to Henry Chandler Cowles in 1898 by the University of Chicago for his work on the dunes of Lake Michigan.
In Great Britain, plant ecology was initiated by botanical surveys done by members of hundreds of local natural history societies. Arthur S. Tansley, a pioneer British plant ecologist, suggested formation of the British Vegetation Committee, which brought the scientific leadership of the natural history societies together to study British vegetation.
Continental ecology was stimulated by the experience of European biologists in tropical colonies. Several of these, notably Drude, Andreas Franz Wilhelm Schimper, and Warming produced significant works of plant ecology that influenced British and American ecology.
Ecology in America was much influenced by early state-run natural history agencies, notably in the Midwest. Stephen A. Forbes was director of the Illinois State Laboratory of Natural History for many years, influencing numerous ecologists employed there as well as producing many of the most insightful ecological articles published before 1900. Edward A. Birge was director of the natural history division of the Wisconsin Geological and Natural History Survey and a pioneer in ecological studies of lakes. Bessey organized the Botanical Survey of Nebraska, which produced the major theorist of American plant ecology, Frederic E. Clements (1874–1945). American ecologists were influenced by federal surveys of the trans-Mississippi West. Ronald Tobey traced the development of plant ecology in the Midwestern grasslands, influenced by the developments of prairie agriculture and the Clements school of ecology.
Formal ecological societies were established in Britain in 1913 and in the United States in 1915. Academic ecology developed notably in Midwestern universities. In its century as a recognizable science, ecology acquired new and expanded significance. This is evident in the proliferation of journals of ecology. English language journals numbered four in 1940, increasing to twelve by 1980. The Institute for Scientific Information (ISI) included 102 journals under ecology in 2000, fifty of which had a clear reference to ecology in the title. The journal Ecology went from quarterly to monthly and the number of pages per year increased from approximately 750 in 1940 to about 2,250 in 2000. This proliferation of journals is a mixed blessing, making keeping up with the literature well nigh impossible at a time that ecologists are becoming involved in increasingly diverse enterprises.
Thomas Kuhn's concept of paradigm, introduced in The Structure of Scientific Revolutions (1962), changed the common view of how science progresses. A paradigm is a set of overarching principles and methods shared by a scientific community within which its adherents conduct "normal science." Science advances by changing its paradigms in revolutions.
The earliest putative ecological paradigm, the Clementsian organismic paradigm was a descendant of the traditional design or balance-of-nature concept, presuming a stable or equilibrium state as a norm. It is associated with Nebraska botanist Frederic E. Clements, who developed a concept of community or "association" as an organism or superorganism (1916). Clements envisioned the community as developing to converge on a "climax" or a stable endpoint determined by climate. This paradigm dominated early-twentieth-century ecology in America and was evident in major animal ecology references and general textbooks.
Some scholars describe a "revolution," or paradigm change, in ecology in the 1950s, with the revival and widespread acceptance of the "individualistic concept" of H. A. Gleason (1939). The shift from Clementsian ideas of equilibrium, homogeneity, and determinism to Gleasonian ideas of nonequilibrium, heterogeneity, and stochasticity greatly increased the difficulty of ecology and has dominated its recent development.
Another paradigm in ecology, population regulation, has a long natural-history tradition and was introduced into ecology in the 1920s, described by some as the "Golden Age" of theoretical mathematical ecology. Raymond Pearl resurrected the earlier "logistic curve" dN rN (K N ) and introduced it as a "law" of population growth. The equation includes r as the rate of population growth, K as the limiting maximum population, N as the number of individuals, and d signifying change. Subsequently the physicist Alfred J. Lotka, who joined Pearl's laboratory, and Vito Volterra, a mathematician, expanded the logistic to two species cases, especially competition. Mathematical population ecology persisted in the face of extended criticism and subsequent concerns about the basic equation. It remains to the present in much-elaborated forms with the assurance that populations at some scale are regulated but the mechanisms remain elusive.
Early ecology recognized living (biotic) and nonliving (abiotic) aspects of nature but conventionally treated them separately. Environment acted on organisms, and organisms reacted on environment, according to Clements's familiar usage. The term ecosystem was coined by the British ecologist Arthur S. Tansley in 1935 to treat organisms and environment as a unit system. Tansley defined ecosystem as the whole system (in the sense of physics) including not only the organism complex, but also the whole complex of physical factors forming what we call the environment of the biome—the habitat factors in the widest sense.
Tansley's concept was particularly useful in aquatic ecology. It was used by Raymond Lindeman (1942) in a pioneer study of a lake ecosystem. Lindeman adapted the familiar food-chain, or trophic-structure concept of ecology and emphasized the energy and nutrient relations in a pyramid of production diminishing at higher levels and relating it to a succession of lakes. A major stimulus to ecosystem ecology was the influential textbook by Eugene Odum, Fundamentals of Ecology (1953).
The ecosystem concept was widely hailed as a new ecology in the 1970s, particularly when designated as "systems" ecology, which changed the emphasis in ecology from organisms to "ecoenergetics," the flux of energy in the ecosystem, along with the flow of chemicals through the ecosystem. Systems ecology flourished as ecology was turned into "big biology" by its first venture into heavily funded research in an International Biological Program (IBP) principally directed to formulating mathematical models of large-scale ecosystems. Another approach to ecosystems was the Hubbard Brook Program, begun in 1963, an intensive program of studies of a forested watershed, which examined nutrient flow and biomass accumulation. The program used computer simulations to model the complexity of natural ecosystems.
Opinions as to the merits of systems ecology in its philosophical and mathematical format vary, but the ecosystem persists as a major aspect of ecology and is frequently cited in the conservation, environmental, and even economic and political arenas. It is widely considered as producing ecosystem services, the valuable consequences of the multitudinous activities performed by biological systems to the great advantage of humans and often in spite of them. In 1988 "ecosystem" was designated "most important ecological concept" in a survey of members of the British Ecological Society, and it remains significant today.
Ecology as a science developed largely in academia and in state, federal, and private conservation agencies until it came to the consciousness of the general public in the context of the environmental crisis of the 1970s in the guise of environmentalism. This is clearly evidenced in Mohan Wali's "Ecology Today: Beyond the Bounds of Science." Wali collected terms using "eco," "ecological," and "ecology" by professional ecologists and by nonecologists, and found that nonecologists use the term ecology as ideology, metaphor, allegory, myth, or gospel. Furthermore, while terms coined by professional ecologists numbered 276, those coined by nonecologists numbered over a thousand. Wali categorized the latter as General Ecologies (e.g., metaphysical ecology), Eco-business (e.g., eco-pornography), Eco-Health (e.g., gyn-eco-logical), Eco-types (e.g., eco-freak), Ecosport (e.g., ecogolf), Ecophilosopy (e.g., deep ecology), and Eco-religion (e.g., ecological sin). The problem for the unwary reader is that environmentalism is exceedingly diverse and for most of the users of these terms ecological science does not exist.
Another concern is the growth since around 1980 of environmental history, which takes a more encompassing view of environmental and ecological science and sometimes confounds it with ecologism, a loose construction of philosophy and ecology. Environmental history includes many works attending to the human environment with due cognizance of ecology and history. Other works, claiming to be histories of ecology, completely ignore the science and attribute diverse human and political foibles to ecology. One such cites no ecological science journals, but asserts that ecologists call for complete social and economic change. It links ecological ideas to Marxism, anarchism, Boy Scouts, anti-Semitism, fascism, and the German disease—Nazism.
The gross extension of the term ecology by nonscientists prompted the director of the public affairs office of the Ecological Society of America (ESA) to draft a letter to disclaim equating ecology and environmentalism, which, the letter said, diminished the credibility of ecology, and to urge members of the ESA to send it to offending publications such as the Wall Street Journal.
Ecology has long been recognized as complex. One discouraged ecologist suggested, "ecology is not only more complex than we think, it is more complex than we can think." Ecology has assembled an extended body of information about the earth's ecosystems, but consensus on a general theoretical foundation remains elusive and some question its likelihood.
A notable effort to provide a theory of ecology was G. E. Hutchinson's formalization of the niche concept. A species niche is its response to all variables, biological and physical. Hutchinson influenced Robert MacArthur (1969), who pursued a theory of ecology predicated on competition that was widely accepted but subsequently questioned, as other factors such as predation and disturbance were shown to influence species relations.
An early consensus on "dynamic" ecology, focusing on the ubiquitous process of succession or change in ecosystems, regarded succession as "primary" if beginning on a previously unoccupied area, or "secondary" if following disturbance. This remains a factor in complex ecology. Disturbance by biotic or abiotic factors at varied intervals, intensities, or different area sizes is common and some disturbances (e.g., fire) are integrated into the development of an ecosystem, adding to its complexity.
One of the difficulties of assessing regularities in ecology is that ecological entities exist, and functions occur, at different scales of size, time, and rate, and the perspective of history is essential. Consideration of scale is widely evident in recent ecology, increasing its inherent complexity. The traditional ecological entities, population and community, are now expanded to metapopulations, populations of a species connected by migrations among them, and landscape ecology, an extended area including diverse communities. Taxonomic populations are also considered as guilds, species with similar functions apart from their systematic relations. At an extreme the biosphere, the whole-earth system, may be considered, and has been transformed into the GAIA hypothesis, treating the earth as an integrated superorganism transcending conventional ecology.
Another approach to ecological complexity is offered in hierarchy, an effort to deal with the scale problem in ecology. Ecological systems are considered as hierarchies in which processes at higher levels are predictable in some degree based on processes at lower levels. Some properties of the whole are said to be emergent and must be considered at the appropriate hierarchical level.
Evolutionary Ecology and Conservation Biology
Ecology was initially linked with Darwinian evolution. These ideas persist in evolutionary ecology, which explores the distribution and abundance of organisms and the control of their numbers, as in the interest in invasive species. Conservation biology is a following, overlapping discipline which focuses on the preservation of species incorporating genetic diversity. No feature of the earth is more striking than its enormous number of species. In ecological parlance number of species is called richness, number of species weighted by their proportionate abundance (number of individuals) is called diversity, and an amalgamation of all biological qualities is called biodiversity. In much ecological and common usage, however, biodiversity is simple number of species. Biodiversity has long charmed naturalists but in recent decades it has been considered a key ecological concept or paradigm and entered into environmental and political discourse as concerns grow about declining biodiversity. The emerging concern about loss of biodiversity has created urgency about its putative relation to stability and functioning of ecosystems.
Increasing recognition of the complexity of ecological systems called forth new mathematical considerations of fractals, chaos, and complexity. It led to the formation of a National Center for Ecological Analysis and Synthesis (NCEAS) on the West Coast of the United States. The NCEAS funds studies of large areas using meta-analysis of existing data and has had substantial impact on recent ecology.
Early ecology had links with sociology. Patrick Geddes, British botanist-turned-sociologist, provided a classification of science in 1880 that included ecology in sociology. Much early plant ecology was called phytosociology, and a pioneer animal ecologist, Warder Clyde Allee, published Animal Aggregations: A Study in General Sociology in 1931. Early sociology at the University of Chicago was influenced by the ideas of the plant ecologist, Frederic E. Clements. An early animal ecologist, C. E. Adams, boldly anticipated convergence of animal and human ecology.
Both the British and American ecological societies were reluctant to engage in advocacy, and a president of the British Ecological Society said the society cannot have a corporate opinion on practical affairs lest its credibility over scientific aspects be damaged. Although many ecologists, notably Paul Sears in the United States and Arthur S. Tansley in Britain, were active proponents of environmental concerns and conservation, there was a common reluctance among ecologists and their societies to become involved in political affairs and public policy. Widespread recognition of the environmental crisis in the 1970s and the popularity of Earth Day emboldened many ecologists, their societies, and their journals to substantially change their position in respect to advocacy. In spite of reservations, ecologists and their organizations are increasingly becoming involved in matters of public policy if not politics.
One indication of increasing involvement of ecologists in public affairs is the Millennium Ecosystem Assessment, in which ecologists join with other scientists and social scientists from sixty-six countries to address the relations between ecosystems and human well-being on a global scale. These assessments (1) address the current and future capacity of ecosystems to provide services to humans, (2) determine human responses to changes in ecosystems, and (3) consider how assessments can be conducted at scales from villages, to river basins, countries, regions, and globally. It is clear that ecological science is becoming increasingly involved in the realm of public policy. Some ecologists have noted the failure of our educational system to train students to participate in science-policy discussions. Ecologists and economists, in order to change the status quo, are attempting to provide innovative courses.
An extremely difficult problem for contemporary ecologists is to respond to the suggestions that ecology extends beyond the bounds of science. This grandiose view of ecology is a contradiction of Ralph Waldo Emerson's assertion in "Nature" of an essence unchanged by human beings: "But his operations taken together are so insignificant, a little chipping, baking, patching, and washing, that in an impression so grand as that of the world on the human mind, they do not vary the result." Ecology confirms the fear that humans vary the result decisively.
See also Biology ; Environmental Ethics ; Environmental History ; Evolution ; Natural History ; Nature ; Wildlife .
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Glacken, C. J. Traces on the Rhodian Shore. Berkeley: University of California Press, 1967.
Graham, Michael H., and Paul K. Dayton. "On the Evolution of Ecological Ideas: Paradigms and Scientific Progress." Ecology 83 (2002): 1481–1489.
Graham, Michael H., Paul K. Dayton, and Mark A. Hixon, eds. "Special Feature: Paradigms in Ecology: Past, Present, and Future." Ecology 83 (2002): 1479–1559.
Kingsland, S. E. Modeling Nature: Episodes in the History of Population Ecology. Chicago: University of Chicago Press, 1985.
Lawton, J. H. "Are There General Laws in Ecology?" Oikos 84 (1999): 177–192.
Lindeman, R. L. "The Trophic-dynamic Aspect of Ecology." Ecology 23 (1942): 399–418.
MacArthur, R. H. "Species Packing and Competitive Equilibrium for Many Species." Theoretical Population Biology 1 (1969): 1–11.
McIntosh, R. P. The Background of Ecology: Concept and Theory. Cambridge, U.K.: Cambridge University Press, 1985.
Mitman, Gregg. The State of Nature: Ecology, Community, and American Social Thought. Chicago: University of Chicago Press, 1992.
Shrader-Frechette, K. S., and E. D. McCoy. Method in Ecology and the Logic of Case Studies. Cambridge, U.K.: Cambridge University Press, 1993.
Wali, Mohan. "Ecology Today: Beyond the Bounds of Science." Nature and Resources 35 (1999): 38–50.
Robert P. McIntosh
Protostomes are one of the most diverse and abundant groups in the animal kingdom. Their distribution, variety, and abundance are largely the result of evolutionary adaptations to climatic changes in their respective environments. At present, they inhabit a wide range of terrestrial and marine environments. Many protostomes are familiar to most people, including spiders, earthworms, snails, mussels, and squid, to mention just a few. Their lifestyles, origins and diversity all have underlying structures and functions that depend on the interactions between abiotic (nonliving) and biotic (living) components of the environment both past and present. From the fossil record, protostomes first appeared about 600 million years ago, although researchers believe that many of the early members of this group became extinct. Those few that did survive, however, evolved and radiated, or diversified, into the variety of protostomes that biologists recognize today.
Diversity of protostomes
Protostomes are presently classified into annelids, arthropods, mollusks, brachiopods and bryozoans. The annelids are thought to include about 9,000 species known to be living in marine, freshwater, or moist soil environments. Perhaps the best-known annelid genera are Hirudo (leeches), Nereis (clamworms) and Lumbricus (earthworms).
By contrast, the largest group in the animal kingdom is the arthropods, which account for almost three-quarters of all living animal species, and have adapted successfully to most terrestrial and aquatic habitats around the world. Early arthropods known as trilobites are an extinct group that have been extensively described from the fossil record. With regard to present-day species, some arthropods are free-living while others are parasitic. This extremely large group of protostomes includes the crustaceans (e.g., Astacus, crayfish; Carcinus, shore crab); the myriapods (e.g., Lulus, millipedes); the insects (e.g., Periplaneta, cockroaches; Apis, bees); and the arachnids (e.g., Scorpio, scorpions; Epeira, web-spinning spiders).
Mollusks are the second largest group in the animal kingdom, comprising around 100,000 known living species. Most are marine (e.g., Mytilus, mussels; Loligo, squid; and Octopus), although some are such well-known terrestrial animals as Helix, the land snail, and Limax, the garden slug.
Brachiopods (lampshells) and bryozoans are marine organisms that are distinguished by a feeding structure called a lophophore. Bryozoans are colony-forming animals attached at the base to the substrates on which they live. The most common bryozoans include such encrusting species as Bowerbankia and gelatinous colonies like Alcyonidium, some of which provide food and shelter to many small benthic organisms. Understanding how protostomes interact with one another and their physical environment, yet continue to survive from generation to generation is a central theme in their ecology.
Major themes in ecology
Ecology is a broad topic, yet it has a number of themes that apply to all living organisms. At its most basic level, ecology is the study of interactions between animals and the abiotic and biotic factors in their environment through the acquisition and reallocation of energy and nutrients. Ecologists also examine the cyclic transfer of these elements to sustain life processes. The major themes in ecology include limiting factors; ecosystems; population issues; ecological niches; species interactions; competition; predator and prey dynamics; feeding strategies; reproductive strategies; and biodiversity. Research related to these themes has produced some of the most complex and diverse findings in the animal kingdom when it is focused on protostomes.
The concept of limiting factors in ecology is related to the control or regulation of population growth. These factors include abiotic as well as biotic aspects of the environment. For protostomes, the availability and consumption of food is an important biotic limiting factor. During the planktonic stages of many benthic (ocean bottom) protostomes, for example, the seasonal abundance of phytoplankton in the water column will directly affect the mortality of protostomes, and hence the number or success of individuals during recruitment.
Other limiting factors are abiotic, particularly temperature, salinity and light. These factors affect the type and number
of protostomes that can exist within a given environment. For many species, the prevailing abiotic conditions are strongly associated with the characteristics of their habitat. These features are usually well defined. The spatial distribution and abundance of protostomes in either a freshwater pond or rocky shore, for example, will tend to show clear patterns defined by both physical and biological factors.
Protostomes are poikilothermic, or cold-blooded, which means that they do not regulate their body temperature; consequently, they are at the mercy of ambient conditions. Poikilothermy does have, however, a few advantages. Poikilotherms can conserve energy needed for warmth and real-locate it to such other important body functions as flight in insects; maintaining water-jet propulsion in squid and cuttlefish; and molting of the exoskeleton in crustaceans. The chief disadvantage of poikilothermy is that at low temperatures, many protostomes either reduce or restrict their activity. Temperature gradients in aquatic environments tend to determine the distribution of certain species; for example, high temperatures are better tolerated by the shore crab Necora puber when exposed to desiccation (drying out) by a retreating tide, than by such subtidal species as the masted crab Corystes cassivelaunus.
The salt concentration of sea water is critical for most marine protostomes. Most marine species tolerate only a relatively narrow salinity range (33–35 psu); under normal conditions, however, seasonal fluctuations in salinity are usually gradual. These fluctuations may be associated with changes in seawater temperature, freshwater input from estuaries, or evaporation in enclosed water bodies. High or low salinity outside the normal range of a species will affect its ability to regulate the body's osmotic pressure relative to its surroundings. Failure to regulate this pressure can result in death. In estuaries, salinity gradients are quite pronounced and can directly influence the distribution of protostomes year-round.
The length of daylight can have a profound effect on protostome behavior by modulating internally controlled rhythms, such as physiological responses to feeding or daily locomotory activity. Timely emergence of prey coincides with dawn and dusk activities, or strictly nocturnal existence. Generally, these activities are to avoid predators that use visual cues to detect prey.
In the broadest sense, an ecosystem is a functional unit made up of all the organisms or species in a particular place that interact with one another and with their environment to provide a continuous flow of energy and nutrients. The success of a species in interacting with its environment will affect the long-term success of its population as well as the functioning of the ecosystem in which it lives.
The biotic component of an ecosystem consists of autotrophic and heterotrophic organisms. All living organisms fall into one of these two categories. Autotrophic organisms (e.g., plants, many protists, and some bacteria) are essentially self-sufficient, synthesizing their own food from simple inorganic material; whereas heterotrophic organisms, including the protostomes, require the organic material produced by the autotrophs. The organisms in any ecosystem are linked by their potential to pass on energy and nutrients to others, usually in the form of waste products and either living or dead organisms. Protostomes that obtain their energy from living organisms are called consumers, of which there are two basic types, herbivores and carnivores. Herbivores consume plant material, whereas carnivores eat both herbivores and other carnivores. Detritivores obtain energy from either dead organisms or from organic compounds dispersed in the environment.
The transfer of energy from one organism to another and their feeding relationships (e.g., producers and consumers) is called a food web. The various stages of the web are called trophic levels; for example, the first level is occupied by autotrophic organisms or primary producers, and the subsequent levels are occupied in turn by heterotrophic organisms or consumers. An ecosystem constantly recycles energy and nutrients as organisms are consumed, die, and decompose, only to be assimilated by detritivores and utilized as nutrients by plants ready to produce food for consumers.
A population is defined as a group of individuals of the same species inhabiting the same area, which can be defined as a local, regional, island, continental, or marine area. In ecology, the size and nature of a population reflect the dynamic relationships among reproductive rates, survivorship, migration, and immigration. One measure of reproduction is fecundity, which is defined as the number of offspring produced by a single sexually mature female. Fecundity in protostomes is usually expressed as the average number of fertilized eggs produced in a breeding cycle or over a lifetime. Most female protostomes produce high numbers of fertilized eggs over the course of their lives. An oyster, for example, is estimated to produce on average 100 million fertilized eggs during its maturity.
Survivorship refers to the percentage of individuals that survive to reach sexual maturity; it may well have a greater impact on the size of a particular population than fecundity. In the aquatic environment, planktonic larvae are often highly vulnerable to predators; although fecundity is high in females of these species, it is offset by relatively low survivorship resulting from heavy predation. Other factors that increase mortality include starvation, diseases of various types, and cannibalism. In addition, the size of a population may increase or decrease because of migration and immigration. The migratory monarch butterfly Danaus is a well-known example of a species with a well-defined migratory pattern. A slightly less familiar example is the greenfly Aphis, which produces several generations of wingless parthenogenetic females through the summer months until autumn, when the wingless aphids produce winged females. These winged females are produced specifically for migration and dispersion. Immigration usually involves the recruitment of individuals into an existing population. In some circumstances, individuals that are displaced following the loss of a habitat may move into the habitat of a neighboring population.
Population stability is often determined by the success of reproduction and recruitment. This success will largely depend on a given species' reproductive strategy. For example, broadcast spawners rely on a high number of fertilized eggs being widely dispersed, whereas brooders produce a smaller number of eggs with limited dispersal potential. The objective of both strategies is long-term survival. Ecologists use numerical models to understand and explain the interactional dynamics of a population. At an elementary level, these models describe the growth pattern of a population in terms of the number of individuals surviving over time. When the number of individuals in a population or its growth rate neither increases nor decreases, the population is said to be stable. Several factors may be responsible for maintaining this stability, as individuals will continue to grow and reproduce. Depleted food supplies or increased predation are common examples of factors limiting population growth. The rate at which a species reproduces in order to maintain its population size is related to its population strategy.
There are two basic strategies for achieving population maintenance: r-strategy and K-strategy. R-strategy refers to a rapid exponential growth in population followed by overuse of resources and an equally rapid population decline. Species that rely on r-strategy tend to be short-lived organisms that mature quickly and often die shortly after they reproduce. They have many young with little or no investment in rearing them; few defensive strategies; and an opportunistic tendency to invade new habitats. Most pest species exhibit r-strategy reproduction. Species that rely on K-strategy, by
contrast, tend to grow slowly, to have relatively long life spans, and to produce very few young. They usually invest care in the rearing of their young. Most endangered species fall into this second category. The lesser octopus Eledone, for example, attaches its egg masses to rocks, where the adults give the eggs a certain amount of parental care until they hatch; this species is an example of an animal that exhibits the K-strategy. Protostomes have many reproductive strategies that are intermediate between the extremes of the r- and K-strategy.
Individuals of every species must survive long enough to reproduce successfully. Reproduction takes additional energy, and so species that can efficiently capture and ingest their food resource are most likely to reproduce at their optimal level and leave more descendants. This is a fundamental characteristic of the concept of the ecological niche.
The concept of an ecological niche is fundamental to understanding the ways in which a species interacts with the abiotic and biotic factors in its environment. From a broad
perspective, the ecological niche of a species is its relative position within the community in which it lives, often referred to as its habitat. More specifically, the ecological niche also includes the ability of a species to successfully employ life history strategies that allow it to produce the maximum number of offspring. Life history strategies are behavioral responses that enable members of a species to adapt to and make use of their habitat; forage successfully for food; avoid and defend themselves against predators; and find mates and reproduce. Successful breeding will produce descendants or offspring that will carry the genetic makeup of the individual into the next generation. The second generation will continue to interact with the environment frequently repeating the life cycle of its parents.
A species represents the lowest taxonomic group that can be clearly defined by characteristics that separate it from another at the same taxonomic level. For example, members of a species must be capable of breeding among themselves to produce offspring; possess a genetically similar makeup; and have unique structural and functional characteristics that equip the species to survive. Some species exhibit distinctive variations in structure and function among their members. Honeybees are perhaps the best known example of morphological and functional differentiation, with individuals classified as drones, workers and queens. While all of these individuals are both members of a single colony and members of the same species, they display slight variations in their individual genetic composition, which creates major differences in their functional roles. All individual bees are adapted to performing specific tasks that ensure the survival of the colony as a whole.
There are two basic types of interactions among individual animals, namely intraspecific and interspecific. "Intraspecific" refers to interactions among animals of the same species. It incorporates the concepts of completion (for food, shelter, territory, and breeding partners) and social organization (e.g., the interactions among individuals within a colony of insects). "Interspecific" refers to interactions among members of different species, including predator and prey relationships; competition for food, shelter, and space; and such different associations as parasitism. Parasites have negative effects on their hosts, from which they obtain food, protection, and optimal conditions for survival. They often cause disease and deprive their hosts of nutrients. For example, tapeworms live inside the gut of their host, which provides them with nutrients that allow the tapeworms to grow by adding segments to their body; each segment is essentially a factory for the production of more offspring.
Competition develops when two or more species seek to acquire the same resources within a given habitat. For example, field experiments have shown that two species of barnacle, Chthamalus stellatus and Balanus balanoides, compete directly for space, substrates (surfaces to live on), and elevation within the intertidal zone of a rocky shoreline. The competition between these two species is so effective that C. stellatus is usually found only on the upper shore, where it is better adapted to surviving such conditions, while B. balanoides is generally confined to the lower shore, where it outcompetes C. stellatus for space. Ecologists refer to this type of interaction as competitive exclusion, or Gause's principle, named for the Russian biologist C. F. Gause, who first discovered the occurence in 1934. Gause used two species of paramecia in a series of laboratory experiments. Since then the principle of competitive exclusion, which states that only one species in a given community can occupy a given ecological niche at any one time, has been extensively documented in both laboratory and field investigations.
The concept of competition is closely linked to the concept of the ecological niche. When a given ecological niche is filled, there may be competition among species for that niche. The chances of a species becoming established depends on many factors, including migration; availability of food; the animal's ability to find suitable food; and its ability to defend itself or compete for the ecological niche. The more specialized a species, the lower its chances of finding itself in direct competition with another species. This general rule is related to the concept of resource partitioning, which refers to the sharing of available resources among different species to reduce opportunities of competition and ensure a more stable community.
The stages in the life cycles of some species appear to have evolved in order to minimize competition and thus maximize survival rates by enabling members of a species to occupy completely different habitats at different points in their life cycle. Many benthic protostomes, for example, begin their relatively brief early life as planktonic larvae feeding on other small planktonic organisms in the water column before settling to the seabed, where they undergo metamorphosis into a juvenile form. A number of insects have a short adult life span of only a few hours, following a much longer period of 2–3 years as nymphs living on weeds or decaying matter. These strategies may serve to reduce competition for resources between adults and juveniles (or adults and larvae).
Predator and prey dynamics
Predation is the flow of energy through a food web; it is an important factor in the ecology of populations, the mortality of prey, and the birth of new predators. Predation is an important evolutionary force in that the process of natural selection tends to favor predators that are more effective and prey that is more evasive. Some researchers use the term "arms race" to describe the evolutionary adaptations found in some predator-prey relationships. Certain snails, for example, have developed heavily armored shells, while their predator crabs have evolved powerful crushing claws. Prey defenses can have a stabilizing effect in predator-prey interactions when the predator serves as a strong selective agent to favor better defensive adaptations in the prey. Easily captured animals are eliminated, while others with more effective defenses may rapidly dominate a local population. Other examples of prey defenses include the camouflage of the peppered moth, and behavior such as the nocturnal activity of prey to avoid being seen by predators.
The feeding strategies of protostomes have evolved to locate, capture, and handle different types of food. These strategies can be broadly classified into four groups: 1) Suspension or filter feeding. Organisms in this group obtain their food by filtering organic material from the water column directly above the sea floor, river bed, or lake bottom. 2) Deposit feeding. These protostomes consume particles of food found on the surface of the sediment. 3) Scavenging. Scavengers are organisms that eat carrion, or dead and decaying animal matter. 4) Predation. Predators capture and ingest prey species.
Suspension feeding in mollusks, for example, involves drawing organic particles into the mantle cavity in respiratory currents and trapping them on their ciliated gills. This type of feeding is usually associated with sessile protostomes, many of which have feather-like structures adapted for collecting material from the passing water currents. Brachiopods and bryozoans are suspension feeders with a specialized lophophore, which is a ring of ciliated tentacles that is used to gather food. In lowland river beds, the larval forms of the insect Diptera (blackflies) feed by attaching themselves to plants with hooks anchored in secreted pads of silk. The larvae trap organic particles using paired head fans.
Deposit feeders ingest detritus and other microscopic organic particles. These animals are commonly associated with muddy sediments and selectively pluck organic particles from the sediment surface, although some are less selective. The semaphore crab Heloecius cordiformis has a specialized feeding claw shaped like a spoon for digging through the surface layers of muddy sediments. The crab's mouthparts then sift through the sediment and extract the organic matter.
Scavengers are unselective feeders that eat when the opportunity arises. Many amphipods scavenge for animal and plant debris on the sea floor. When leeches are not sucking the blood of a host, they often feed on detritus or decaying plant and animal material. Such scavengers as octopus, shrimp, and isopods often swarm in large numbers to a piece of flesh that has fallen to the sea bed.
Predators often have highly sophisticated structures to help them locate, capture, and restrain prey whether mobile or sedentary. Often these structures include mandibles (modified jaws) that are able to crush and hold prey items. The larvae of Dobson flies are predatory; they feed on aquatic macroinvertebrates in rivers and streams. In reef habitats, such cone shells as Conus geographus are predators that capture their prey using a harpoon-shaped radula, which is a flexible tongue or ribbon lined with rows of teeth present in most mollusks. When the prey comes within striking range, the radula shoots out and penetrates any exposed tissue. The cone shell then releases a deadly venom known as conotoxin that paralyzes its prey (such as fish or marine polychaete worms).
Biodiversity is a measurement of the total variety of life forms and their interactions within a designated geographical area. Ecologists use the concept to evaluate genetic diversity, species diversity and ecosystem diversity. Areas high in biological diversity are strongly associated with habitat complexity, for the simple reason that complex habitats provide numerous hiding places for prey, opportunities for predators to ambush their prey, and a range of substrates suitable for sessile organisms. Moreover, habitats high in biodiversity are thought to be more stable and less vulnerable to environmental change. For these reasons, conservation of biologically diverse areas is considered important to maintaining the longevity and health of an ecosystem. An instructive example of a complex habitat that provides shelter and feeds a broad range of species (including protostomes), and yet is threatened at the same time, is the coral reef.
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Steven Mark Freeman, PhD
Ecology, the study of an organism's relationship to its surroundings, consists of several distinct areas, all of which have their own specific approaches and methods. Ecophysiology deals with physiological mechanisms, evolutionary ecology is concerned with life history-related, fitness-relevant, and population-genetic aspects, behavioral ecology looks at how the animal deals with its surroundings, and community ecology asks how groups of species can live together. This text will approach how mammals are able to adapt to extreme conditions and will also consider spatial and temporal distribution, predator-prey relationships, and relationships between species forming similar niches.
Mammals are endotherms. This means that they have to put a great deal of energy into regulating their body temperatures. In a cold environment, there are several strategies to deal with those challenges: mammals, contrary to reptiles, are often capable of developing a rather thick isolatory tissue (sub-cutaneous fat) plus thick fur. This option is not open to reptiles due to the fact that they need to retain the high thermal conductancy of their integument in order to heat themselves by exposure to sun rays. Small mammals, however, have this capability in a lesser extent than larger ones. An additional option for larger species is migration.
Another mammalian adaptation is known as non-shivering thermogenesis (NST), the burning of so-called brown fat, a special tissue rich in mitochondria and often deposited around the neck or between the shoulder blades. The most effective way of dealing with the challenge of a cold environment is torpor, the reduction of one's body temperature and basal metabolic rate, in some species to around or even slightly below 32°F (0°C). For example, the Arctic ground squirrel (Spermophilus parryii) goes down to a startling body temperature of 28°F (-2°C). Daily torpor, or larger periods of hibernation, can be found in members of at least five placental and two marsupial orders. The largest species found with "real torpor," lowering their body temperatures by at least 50°F (10°C), are badgers (both the American and the Eurasian species—in the latter case it was found in an individual of 27 lb [13 kg] body weight). Bears also become dormant in winter, but their body temperature is lowered only by about 41°F (5°C), and their physiological mechanisms are different from those of the smaller species.
How do mammals deal with desert conditions? Deserts are not only characterized by extreme temperatures (hot and cold, many small species thus also exhibit daily torpor), but also by arid conditions and a low biodiversity. One strategy for smaller mammals to deal with this low productivity again is heterothermy, to reduce basal metabolism and thus economize on one's energy demand. Large mammals such as camels can also store heat quite effectively in their large bodies; camels can increase their body temperatures under heat-stress up to 106°F (41°C) during the day, and lower it to around 93°F (34°C) at night. This can save about 12,000 kJ and 1.3 gal (5 l) of sweat. Water balance in many species of desert-dwelling mammals is improved by a counter-current system in their nasal conchae, and by recycling water in the kidneys. Kangaroo rats (Dipodomys) are among those species that regularly live without the need to drink open water; they are able to survive on the water content of their food (seeds), and from oxidation. Grazing mammals can improve their water balance by feeding at night, because grass species are rather rich in tissue fluid at that time. Carnivores extract water from vertebrate prey—even fennecs can keep their water balance simply by feeding on mice. The effectiveness of desert mammals is increased by behavioral adaptations, such as regularly retreating into burrows or shady areas where it's not only cooler but also more humid.
An even more challenging habitat for mammals is extreme high altitude. While desert means cold plus dry plus food scarcity, high altitude means desert plus low oxygen. Thus, physiological adaptations found in mammals at high altitudes include all those just discussed, plus specific ones to improve gas exchange (lung tissue and blood capillaries becoming more intricate), better oxygen transport in the blood, and better oxygen-dissociative capabilities from hemoglobin to body tissues. Also, smaller mammals in arid or mountainous habitats often retreat into underground burrows.
Living underground continually, or at least for a large part of the animal's daily activities, is as challenging an environment as the one they might escape from. Members of at least 11 families (marsupials, insectivores, and rodents) have adopted a subterranean lifestyle with two different methods of digging: hand digging, as performed by moles and the marsupial mole, mostly in loose soils, or tooth digging, performed by rodents in hard substrates. The environment in an underground tunnel has several specific
characteristics. It is often hot, rather humid, and carries more carbon dioxide and less oxygen than fresh air does. Most subterranean species thus are rather small (the largest reaching a few kilograms only), sparsely furred, have a low resting metabolic rate, and have a more effective cardiac and respiratory system (for example, myoglobin density and capillarization in muscle tissue is higher, hemoglobin has a higher oxygen affinity, hearts are bigger and more effective, and the animals are more tolerant of hypoxia than related, non-burrowing species). Another way of dealing with high carbon dioxide concentrations seems to be excreting bicarbonates via urine, which is achieved by a high concentration of calcium-ion (Ca2+) and magnesium-ion (Mg2+) in the urine. Subterranean mammals also encounter a unique sensory environment. Their preferred mode of communication, even in small species, is low-frequency acoustics, as they are rather insensitive to high frequencies and vibratory communication. Some mole rats (Cryptomys hottentotus, Spalax ehrenbergi) are also capable of magnetic field orientation.
There are, especially for smaller mammals, other ecological factors at least as important (if not even more so) that also determine which activity patterns (being diurnal, nocturnal or crepuscular, being active in short or long bouts, etc.) are most adaptive under given situations. One of these factors is predation. Even larger species, such as kangaroos, tend be active at times when their predators are less likely to attack. In small mammals, predator-prey relationships are perhaps even more decisive. This holds true not only for prey species but also for the predators. Small mammalian predators such as weasels, mongooses, or small dasyurids are potential prey to other raptors and larger mammals themselves. On the other hand, being of small body size means that the energy demands and constraints are particularly severe. Thus, they have to term their activity patterns much more carefully than larger species. Being potential prey puts a heavy ecological load on all smaller mammals. Being active at times of low predator activity (dusk and dawn, when most diurnal raptors are no longer active and many owls and mammals not yet active) is one possibility of escape. Being active in a synchronized way provides safety in numbers (the dilution effect), and predation stress can explain ecologically the often dramatic suddenness in the onset of activity. It is also interesting to note that the onset of activity, in most species, is more fixed by internal factors—termination, however, is more variable.
Besides predation, inter- and possibly intra-specific competition also must be considered as influencing activity. Being active at different times of day or using different parts of a habitat at different times can raise the possibility of niche separation, as has been shown in communities of Gerbillus as well as heteromyid species. The behaviorally or ecologically
dominant species in these communities regularly excludes the others from the "best" temporal niche. Similarly, intra-specific competition often drives subordinate individuals into other temporal niches and is often the first step in excluding an individual from a group or litter, long before agonistic expulsion is to be seen.
In general, there is a clear relationship between body size and activity patterns of mammals: the smaller the species, the more likely it is to be nocturnal. This is confirmed for both herbivores and carnivores. Being nocturnal offers better protection from being detected by predators as well as competitors (which is a more important factor when belonging to a small species). In small carnivores, additionally, the effect of finding more prey during the night further enhances this preference. There are, however, exceptions to this rule. Microtine rodents are characterized by very short, ultradian activity patterns, which are very adaptive to the present ecological conditions. Insectivorous and gregarious small mongooses are diurnal, and tree squirrels are all diurnal.
Predators and prey—are they influencing each other's population biology? The answer to this question is as variable as the species and ecosystems studied to answer it: population cycles of voles, lemmings, and snowshoe hares, mostly in a 3–5 year period, have long been suggested to be driven by specialist predators. However, controlled removal of weasels (Mustela nivalis) in one study did not prevent population crashes of field voles, nor did it influence population dynamics at any other stage of the cycle. Also, for several populations of snowshoe hares, both cyclic and non-cyclic ones, predators (weasels, mink, bobcats, lynx, coyotes, and several birds of prey) were the most frequent cause of death for radio-collared hares, and hare population cycles did heavily influence the reproduction, mortality, and movement of the predators. Nevertheless, at or near peak densities, predator activity appeared to have almost no influence on hare density. At lowest density, no influence was evident either, provided that enough cover was available for the hares to retreat into. Survival of hares was directly related to good cover and good feeding conditions. Juvenile and malnourished individuals were more at risk not only due to predators but also due to hard winters.
In a large, comparative, long-term study of many species of predators and prey (wolf and lynx, weasels and stoats, and raptors and owls, and prey from European bison and moose down to amphibians and shrews), it was found that the largest species were barely influenced by predators at all, that amphibians were mostly influenced in their population densities by weather, and that predators in general could neither influence prey densities nor fluctuations. There were, however, a few exceptions: lynx were able to limit roe deer densities below carrying capacity, and both wolves and lynx were obviously able to influence population densities of roe and red deer. The reason might be that, contrary to most other predatorprey systems, both predators are smaller and have a higher reproductive rate than their prey. In those cases, predators might be able to react (numerically, by means of litter size and survival) more rapidly to changing conditions than their prey does. In many ecosystems, both temperate and tropical, large species of prey migrate and thus leave the areas with highest predator activity. Both migrating gnu and caribou, for example, have been shown to lower predation risk by this strategy. Comparison of migrating and nonmigrating ungulates in the Serengeti, following a severe decline of buffalo (the largest nonmigrating herbivore there) and large predators to poaching, led to astonishing results: topi (Damaliscus lunatus), impala (Aepyceros melampus), Thomson's gazelle (Gazella thomsonii), and warthog (Phacochoerus aethiopicus) seem to be predator-controlled. The red hartebeest (Alcelaphus buselaphus), a close relative of topi (but one with a different feeding style) seem to be regulated by intra-specific competition, and giraffe (Giraffa camelopardalis) and waterbuck (Kobus ellipsiprymnus) declined due to poaching. At least in Thomson's gazelle, but probably also others, the reason for the influence of predators on population performance seem to be more complex than simple mortality. Vigilance and flight increase, whenever predator pressure increases, and this of course affects all individuals, not only the unlucky ones being killed. These costs of anti-predator behavior (avoiding potentially dangerous feeding habitats, spending time alert or on the move, etc.) have to be carried by all members of the population, and they do not bring any benefit to the predators (contrary to the "direct costs" of killed animals). This example demonstrates again the complexity of the whole issue.
How can communities, groups of several or even many species, live together? The ecological term "guild" defines a group of species that use the same resources in a comparable way. Thus we would expect them to compete for these resources, and either ecological displacement or niche separation, at least along one or a few niche axes, should occur. In many guilds of species, a recurring phenomenon known as character displacement exists. This means that in areas where two or more competing species occur (sympatric occurrence), at least one trait should differ more pronouncedly than between populations of the same species in non-overlapping (allopatric) habitats. One example: the ermine, a small weasel, is smaller in Ireland than in Great Britain, where an even smaller species, the least weasel (M. nivalis) occurs sympatrically. Guilds of carnivores have been studied in many countries, and in many cases character displacement is evident. In areas of sympatry between two small cat species of South America, the margay (Leopardus wiedii) and the jaguarundi (Herpailurus yaguarondi), the margay is more aboreal. Degree of arboreality is also a frequent pattern in separate primate species, both in guilds of guenons and between sympatric lorisids such as the angwantibo (Arctocebus) and the potto (Perodicticus) in tropical Africa. In the case of the lorisids, one is a smaller, more slender-built species using thinner branches and the upper canopy, while the other is larger and more stoutly built, using the lower, thicker branches.
Carnivore communities are special in that direct, aggressive competition can be observed. Nevertheless, there are many axes along which species can separate. One is body size, which directly relates to prey size. In African savannas, lions, leopards, hyenas, hunting dogs, cheetahs, and jackals all coexist, and direct competition for similar-sized prey is almost fully restricted to cheetahs versus hunting dogs. This relatively peaceful coexistence is due to many factors: social hunting allows some species to take prey of much larger size than their own; the leopard is more arboreal than the other predators; and some predator species migrate to follow their prey while others don't. In sympatric carnivore species of similar body size, gape size (the ability to open one's mouth more or less widely) often acts as a separating axis (as in the guild of cats in South America, mentioned above). Comparative studies of carnivore guilds in Israel (13 species, 4 families), the British Isles (5 species of mustelids), and East Africa (3 species of jackals) found that there is regular separation, apart from body size, in terms of degree of cursorial locomotion, in diameter or shape of canine teeth, and in skull length. Direct overlap of all these niche parameters mostly occurred when a newly introduced species such as American mink (Mustela vison) or a recently immigrated one, such as the striped jackal (Canis adustus) in East Africa, was present. In those cases, character divergence and niche shifts or niche compression did become evident, and mostly led to displacement in one species (European mink [Mustela lutreola], silverback jackal [Canis mesomelas]).
A guild of granivorous (seed-eating) rodents was studied in the Sonora Desert of Arizona. One cricetid and four heteromyid species of different sizes did occur in this community, and all appeared to eat seeds of the same plant species (except for one large species eating a somewhat larger number of insects and one small species eating more seeds of one particular bush). A remarkable difference, however, was found in the spatial arrangement of their feeding places. The large species, a kangaroo rat, mostly exploited patches rich in seeds, such as near rocks or in depressions in the soil. The small species, a pocket mouse, also collected seeds from patches but only in about 6.6% of observed feeding bouts. For the rest, seeds were collected in a more systematic way, while searching in a "sauntering" manner. Both species used olfactory cues to search for food. However, as the kangaroo rats move bipedally in a rapid hop, and thus can easily move from one patch to the other, the smaller species walk quadrupedally. These differences in locomotion not only carry different energetic costs but also allow the animals to make use of olfactory gradients (gradual increases/decreases of concentration) differently: the faster an animal moves, the easier it can detect an olfactory gradient when a larger patch of seeds exudes some stronger smell. The slow-moving pocket mouse, on the other hand, can sniff out individual grains.
The most obvious and oft-cited guilds of larger mammals certainly are ungulate communities. Many studies have been made on groups of herbivore species both in temperate and tropical areas. Despite the fact that up to at least six species of native ungulates can coexist in temperate climes and more than 20 in some African savannas, it is surprising that most studies do not find obvious competition effects. Contrary to carnivores, where inter-species killing and direct competition over food are regular features, there is practically no evidence of direct aggressive competition. Even indirect, long-term effects on population density caused by the changing number of another species is mostly absent. Things only change as soon as newly introduced species come into the community. Thus it was found that feral muntjacs (Muntiacus) in Great Britain severely competed with roe deer (Capreolus). When cattle were introduced into an area where black-tailed deer (Odocoileus) were numerous, the deer retreated into other habitats, which cattle avoided. Competition between species of deer was documented in communities of deer in New Zealand, where all three species (red [cervus elaphus], fallow [Dama dama], and white-tailed deer [Odocoileus virginianus]) had been introduced. Thus it seems that guilds of ungulates having a long (co-) evolutionary history together can coexist, probably because there are enough dimensions along which to separate. The most famous example is the grazer-browser division or, more accurately, the division into concentrate selectors, bulk-feeders, and intermediate feeders. Apart from selecting leaves, grass, or something in between, there are subdivisions in each set of species. Height preference is also a separating criterion for grazers: some species, such as zebra, feed on high and dry or lignified grass, while others feed only on lower, mostly fresh plants. Another separating axis is bite size, which is determined by jaw/snout size and tooth rows. Animals with a broader mouth are less selective in feeding. Body and gut size are also important criteria in deciding what to forage and how to digest. The smaller species have a higher energy demand and specialize on high quality leaves; larger species feed on lower quality grass such as stems or leaf sheaths, or older plants. The reason for this is that larger species, with larger fermentation chambers in their guts, can digest cell walls more effectively and need less energy per unit of body mass. Equids are better able to extract energy from large amounts of fiber-rich food, and ruminants do better with restricted food mass. Equids have a lower reproductive rate than ruminants but are better able to defend themselves against predators due to their sociality. Often in savannas there is a succession of different ungulate species foraging on the same spot, one after the other: zebras (Equus) start by eating
the high, old, dry grass; wildebeest (Connochaetes) follow and eat the lower grass plants, but still select on the level of whole plants; Thomson's gazelle, with their narrow snouts, select only the freshest parts in the middle of grass plants; and kongoni feed on the long, lignified stalks that remain. Migrating versus non-migrating is another axis of niche separation. This is the famous Bell-Jarman principle first described for the Serengeti and other East African ecosystems, which allows up to eight species of grazers and about 20 species of ruminants in total to coexist.
Dayan, T., and D. Simborloff. "Patterns of Size Separation in Carnivore Communities." In: Carnivore Behavior, Ecology and Evolution, vol. 2, edited by J. C. Gittleman, 243–266. Chicago: University of Chicago Press, 1996.
Halle, S., and N. C. Stenseth, eds. Activity Patterns in Small Mammals. Berlin: Springer, 2000.
Jedrzejewska, B., and W. Jedrzejewski. Predation in Vertebrate Communities. The Bialowieza Primeval Forest as a Case Study. Berlin: Springer, 1998.
Keith, L. B. "Dynamics of Snowshoe Hare Population." In Current Mammalogy, vol. 2, edited by H. H. Genoways, 119–196. New York/London: Plenuum, 1990.
Nevo, E. Mosaic Evolution of Subterraneous Mammals. Oxford: Oxford University Press, 1999.
Putman, R. J. Competition and Resource Partitioning in Temperate Ungulate Assemblies. London: Chapman & Hall, 1996.
Reichman, O. J. "Factors Influencing Foraging in Desert Rodents." In Foraging Behavior, edited by A. C. Kamil and T. D. Sargent, 195–214. New York: Garland, 1981.
Sinclair, A. R. E., and P. Arcese, eds. Serengeti II. Chicago: University of Chicago Press, 1995.
Duncan, P. "Competition and Coexistence Between Species in Ungulate Communities of African Savanna Ecosystems." Advances in Ethology 35 (2000): 3–6.
Oli, M. K. "Population Cycles of Small Rodents Are Caused by Specialist Predators: Or Are They? TREE 18 (2003): 105–107.
Udo Gansloßer, PhD
Ecology is the study of organisms and their relationship to the environment. The field was born in 1866 when German biologist and philosopher Ernst Haeckel (1834-1919) created the precursor to the modern word "ecology" by combining the Greek words oikos, meaning "home," and logos, meaning "study," to create the word "oecology." Haeckel used this word to summarize the concept of natural selection and the struggle for existence that English naturalist Charles Darwin (1809-1882) had outlined in his ground-breaking work on evolution, On the Origin of Species.
In the early twentieth century, even before the modern word ecology had been invented, interest in what is now called plant ecology began to grow. American botanist and ecologist Frederic Clements (1874-1945) and others conceived the idea that plants would develop in an orderly succession of formations from pioneer species to a well-defined and stable group of species called a climax community. Clements believed that plant formations were like intact organisms with a predictable pattern of birth, growth, and death. Clements's ideas were quickly challenged. American botanist and plant ecologist Henry Allan Gleason (1882-1975) argued that the distribution of plants was the result of random events in the environment that combine to form an individual and possibly unique plant community. Partially in response to the rigid classification developed by Clements, British ecologist Arthur Tansley (1871-1955) in 1935 coined the word ecosystem to describe what he called a quasi organism. Tansley's concept of the ecosystem as a single physical unit containing both organisms and their environment is essentially the same to this day.
The concept of ecology may seem fairly simple, but in practice it is very complex. As the field developed, scientists soon found themselves unable to master the entire discipline, and even within the already narrowed field of plant ecology, subfields rapidly developed. Today, there are six major fields of plant ecology:
- Population ecologists study the relationship of individuals of one species in a given area to each other and to their environment. A population ecologist might be interested in what environmental conditions limit the northern range of black spruce trees in the Canadian boreal forest.
- Community ecologists study the distribution and abundance of groups of species and how they are influenced by biological and environmental factors. Community ecologists have studied the major associations of deciduous forests in the eastern United States and how the environment, in terms of climate, soils, and topography, controls this association.
- Ecosystem ecologists study energy and matter transport through organisms (see below). This includes studies of how nutrients, energy, and biomass are cycled through ecosystems. The study area for ecosystem ecologists depends on the defined ecosystem and can vary from small ponds or tiny forest plots to the entire globe. Ecosystem ecologists are today conducting politically and economically important research on the global carbon cycle.
- Physiological ecologists study how environmental factors such as light, temperature, and humidity influence the biochemical functioning of individual organisms. Physiological ecology and ecosystem ecology are very complementary; often ecologists have a hard time deciding if they are one or the other.
- Landscape ecologists study the biological and environmental factors that influence vegetation patterns observed in a landscape. Landscape ecologists may study the factors controlling the boundary between forests and grasslands.
- Human ecologists study the influence of human activity, both currently and historically, in controlling the distribution and abundance of organisms. Human ecology also examines the social and cultural factors that control the way humans exploit, alter, and manage the environment. Most ecological research has focused too much on natural ecosystems while pretending that humans do not exist. For example, an ecologist coming across the deciduous forest in New England today might assume that the forest always looked that way. In fact, the present pattern of forest distribution is the result of extensive human modifications by Native Americans, European colonists, and foresters.
In the 1950s the idea of the food web began to emerge in ecosystem ecology. Food webs and the related topics of trophic levels and energy flow are some of the most critical ecological concepts because they illustrate the connections between organisms that are required to maintain healthy ecosystems.
Energy flow refers to the way that energy is transformed through a food chain containing a series of levels, including plants, consumers, predators, and decomposers. Each step in the food chain is called a trophic level (from the Greek word trophikos, meaning "nutrition"). Primary producers (plants, algae, and photosynthetic microbes) are the base of food chains and are the lowest trophic level. They transform energy from the Sun into sugars. Primary producers thus make their own food and are called autotrophs ; all other organisms ultimately use the energy produced by autotrophs and are called heterotrophs . At the next trophic level, primary consumers (herbivores ) eat some of the sugars produced by primary producers. Secondary consumers (predators) consume primary consumers and other secondary consumers. Decomposers such as earthworms, maggots, fungi, and bacteria break down the carcasses of dead primary and secondary consumers and un-eaten primary producers.
The amount of living material (biomass) and energy in food chains has a specific ordering between trophic levels. Consider a simple example of an African savanna ecosystem consisting of trees and grasses (primary producers), gazelles and zebras (primary consumers), and lions (secondary consumers). If we check the trophic levels, we will find that primary producers have the most biomass, followed by primary consumers, and then secondary consumers. The amount of energy at each trophic level will follow the same pattern. This ordering of trophic levels forms a pyramid with primary producers at the bottom followed by primary consumers in the middle and secondary consumers on the top. More energy is required at the lower levels of the pyramid because during the transfer between trophic levels energy is lost through heat and waste products.
Most ecosystems on land follow the pyramid pattern. In the ocean or other aquatic systems, the opposite pattern may at times be true: at any one time, the biomass and energy of the primary and secondary consumers may exceed those of the primary producers. This is because photosynthetic algae have a very short life span. Even though they may have a low biomass at any one time, their biomass measured over the whole year will be larger than the biomass of the consumers.
The pyramid concept of trophic levels is consistent across many terrestrial ecosystems, but in reality the interactions among organisms are much more complex than in the African example. A food web is a network of connected food chains and is used to describe community interactions. Consider a food chain in the Rocky Mountains. Small aquatic plants are primary consumers in a stream ecosystem. Arthropods and fly larvae feed on the plants and are in turn consumed by trout. Bears eat the trout. But each part of this food chain is also connected to other food chains. Birds feed on plants and fish, while bears will also feed on roots, tubers, and rodents. The complete network of these connections forms an ecosystem food web.
Food webs are usually more complex in ecosystems that have not been disturbed for a long time. Food webs in coral reefs and tropical forests have thousands of highly specific food chains. In these ecosystems, many animals are adapted to feed on one or only a few food sources. Disruption of a few elements can have serious consequences for the entire food web. By contrast, the tundra ecosystem was covered in ice until about 8000 B. C. E. In this ecosystem, there has been less time to evolve complex and specific food webs. Species tend to be interchangeable. Removal of one species or interaction does not usually seriously damage the health of the entire food web.
Advances in Ecological Research
Advances in the ecology field happen frequently. The following four examples from the late twentieth century show the breadth of the field as well as the need for ecologists to reach across disciplines.
There are thousands of kinds of leaves, ranging from tiny evergreen needles to enormous tropical leaves more than fifty centimeters wide. In spite of great diversity, leaves follow a strict set of rules. Long-lived leaves, such as ten-year-old spruce needles, have a low nitrogen concentration (this means low rates of photosynthesis) and thick, dense leaves that are highly resistant to herbivores. Short-lived leaves, such as blades of grass lasting only weeks or a few months, have a very high nitrogen concentration and thin, light leaves. Almost all leaves follow this pattern and are either long-lived with low rates of photosynthesis and a high resistance to herbivores, or short-lived with high rates of photosynthesis and herbivory. Intermediate levels of all three traits are also possible. This finding, drawn from hundreds of plants all over the world, helps to explain the appearance and physiology of leaves and is one of the most important ecological findings in recent years.
Ecosystem Carbon Storage.
Many ecologists wanted to know the total amount of carbon released or stored by ecosystems, but until recently, there was no way to accomplish this. Experimental meteorologists devised a method called eddy covariance to measure the amount of carbon dioxide entering or leaving an ecosystem. By adding up these numbers over the course of a day or year, ecologists can now determine if an ecosystem is storing more carbon through photosynthesis or releasing more carbon through respiration. They found that many forests are storing carbon, but that some, especially in the boreal forest, can release carbon due to slight changes in climate. This research is critical for understanding the carbon cycle and the potential for global climate change.
Impacts of Rising Carbon Dioxide.
Scientists have published hundreds of research articles on the response of plants in greenhouses or special enclosures to increased carbon dioxide (CO2) levels, but there had been no way to test the response of real ecosystems. Scientists at the Brookhaven National Laboratories developed the Free-Air CO2 Enrichment (FACE) system. FACE uses a circle of instruments that pump CO2 into the atmosphere to artificially increase the CO2 levels of a real ecosystem. The increased CO2 increased photosynthesis, supporting earlier greenhouse results showing that plants would respond to higher CO2.
Ecology and Natural Resource Management
Beginning in the early twentieth century, ecological theories began to be seriously considered in natural resource management. Unfortunately, results were not always good. In an application of Darwinian theory, U.S. Forest Service managers believed that by clearing old, unproductive forests and replacing them with young, vigorously growing forests they would increase forest health and productivity. Instead, throughout much of the dry inland Rocky Mountains foresters created dense thickets of fire- and insect-susceptible forests. Today, guided by modern ecological research, this policy is changing to include a focus on returning fire to the ecosystem and managing forests for the health of the entire ecosystem, not just human economics. This is called ecosystem management. In large part, it was the legal, political, and social pressures exerted by nonscientist citizen activists that caused this shift in natural resource management policy.
Ecological research has been used in many other ways to improve natural resource management. Due to ecological research showing the catastrophic effects of cyanide on river ecosystems, cyanide heap leach gold mining is now being restricted. Ecologists showed how DDT, an insecticide common in much of the world during the mid-1900s, was transferred through trophic levels until it reached toxic levels in secondary consumers. Millions of birds were killed before DDT was banned in most of the world. Ecologists found that large, interconnected populations of grizzly bears were required to ensure long-term breeding success of the species and natural resource managers are now designing migration corridors to link the remaining bear populations.
In short, there are very few areas of natural resource management that are unaffected by ecology. Critical developments include:
- ecosystem management for recreation, water quality, and protection of endangered species, not just economic development
- an increased awareness of public health consequences
- attempts to reintroduce elements of ecosystems that had been removed by humans
- consideration of the complex and sometimes fragile nature of food webs before making resource management decisions.
Role of Computer Modeling
Politicians, scientists, and natural resource managers are becoming more and more interested in complicated ecological questions over large regions. For example, the economically critical and politically sensitive issue of the global carbon cycle is being answered mostly by ecosystem ecologists. Clearly, it is impossible to measure the entire Earth. Another solution is required, and computer models have filled this need.
A computer model is a system of mathematical equations that ecologists use to represent the ecosystem or problem being assessed. Models do not duplicate reality; they are simplified systems that attempt to represent the most critical processes while ignoring all the details that are impossible to measure or extremely difficult to represent with mathematics. Ecological models range from detailed treatments of gas exchange for a single leaf to carbon cycle models for the entire globe. Developing a good model of the global carbon cycle is like trying to make the simplest possible car: you strip away everything you possibly can until the car stops running. Just as in a car you could probably remove the windows and the passenger seat but not the transmission or the engine, in a global carbon model, you can probably ignore individual species and hour-to-hour weather changes but not vegetation and climate.
Computer models have an extremely significant role in ecology. In fact, because so much in ecology is so difficult to measure except for the smallest plot, models are common in every field of ecology. Models are highly useful for testing scenarios. What will happen to stream flow and fish populations if 50 percent of trees are cut in a watershed? How will elk populations change if wolves are reintroduced to a particular area? How will the introduction of small controlled fires affect the potential for larger, highly destructive fires? How will forests respond if carbon dioxide levels double in the next one hundred years? These are just some of the ways that ecological models are used.
see also Biome; Carbon Cycle; Clements, Frederic; Ecology, Energy Flow; Ecology, Fire; Ecology, History of; Global Warming; Interactions, Plant-Fungal; Interactions, Plant-Insect; Interactions, Plant-Plant; Interactions, Plant-Vertebrate; Plant Community Processes; Savanna; Tundra.
Michael A. White
Bailey, Robert G. Ecosystem Geography. New York: Springer-Verlag, 1996.
Colinvaux, Paul. Ecology 2. New York: John Wiley & Sons, 1993.
Crawley, Michael J., ed. Plant Ecology, 2nd ed. Cambridge, UK: Blackwell Science 1997.
Daily, Gretchen C., ed. Nature's Services: Societal Dependence on Natural Ecosystems. Washington, DC: Island Press, 1997.
Dickinson, Gordon, and Kevin Murphy. Ecosystems: A Functional Approach. New York: Routledge, 1998.
Dodson, Stanley. Ecology. New York: Oxford University Press, 1998.
Kohm, Kathryn A., and Jerry F. Franklin, eds. Creating a Forestry for the 21st Century: The Science of Ecosystem Management. Washington, DC: Island Press, 1997.
Morgan, Sally. Ecology and Environment: The Cycles of Life. New York: Oxford University Press, 1995.
Real, Leslie A., and James H. Brown, eds. Foundations of Ecology: Classic Papers with Commentaries. Chicago: University of Chicago Press, 1991.
Waring, Richard H., and Steven W. Running. Forest Ecosystems: Analysis at Multiple Scales, 2nd ed. San Diego: Academic Press, 1998.
The term ecology is, etymologically, the logic of living creatures in their homes, a word suggestively related to ecumenical, with common roots in the Greek oikos, the inhabited world. Named in 1866 by German biologist Ernst Haeckel (1834–1919), ecology is a biological science like molecular biology or evolutionary theory, though often thought to be less mature. Ecosystems are complicated; experiments are difficult on these open systems, often large, that resist analysis. Ecology has nevertheless been thrust into the public arena, with the advent of the ecological crisis. Ecology has also become increasingly global, and still more complex, as when planetary carbon dioxide cycles affect climate change.
Ethics, policy, theology, and ecology
Ecology mixes with ethics, an ecological (or environmental) ethics urging that humans ought to find a lifestyle more respectful of, or harmonious with, nature. Ethics, which seeks a satisfactory fit for humans in their communities, has traditionally dwelt on justice, fairness, love, rights, or peace, settling disputes of right and wrong that arise among humans. Ethics now also concerns the troubled planet, its fauna, flora, species, and ecosystems.
American forester Aldo Leopold urged a new commandment in "The Land Ethic," a chapter in his 1968 book A Sand County Almanac : "A thing is right when it tends to preserve the integrity, stability, and beauty of the biotic community. It is wrong when it tends otherwise" (pp. 224–225). Since the United Nations Conference on Environment and Development, held in 1992 in Rio de Janeiro, Brazil, the focus of environmental policy, often referred to as ecosystem management, has been a sustainable economy based on a sustainable biosphere.
Theologians have argued that religion needs to pay more attention to ecology, and perhaps also vice versa. Partly this is in response to allegations that Christians view humans as having God-given dominion over nature; they dominate nature and are responsible for the ecological crisis. An ecological theology may hope to find norms directly in ecological science, but often an ecological perspective rather freely borrows and adapts various goods thought to be found in ecology into human social affairs, such as wholeness, interrelatedness, balance, harmony, efficiency, embodiment, dynamism, naturalness, and sustainability.
Leading concepts in ecological science
Leading concepts in ecology involve ecosystems, succession of communities rejuvenated by disturbances, energy flow, niches and habitats, food chains and webs, carrying capacity, populations and survival rates, diversity, and stability. A main claim is that every organism is what it is where it is because its place is essential to its being; the "skinout" environment is as vital as the "skin-in" metabolisms. Early ecologists favored ideas such as homeostasis and equilibrium. Contemporary ecologists emphasize a greater role for contingency or even chaos. Others emphasize self-organizing systems (autopoiesis ), also an ancient idea: "The earth produces of itself [Greek: automatically]" (Luke 4:28). Some find that natural selection on the edge of chaos offers the greatest possibility for self-organization and survival in changing environments, often also passing over to self-transformation.
The stability of ecosystems is dynamic, not a frozen sameness, and may differ with particular systems and depend on the level of analysis. There are perennial processes—wind, rain, soil, photosynthesis, competition, predation, symbiosis, trophic pyramids or food chains, and networks. Ecosystems may wander or be stable within bounds. When unusual disturbances come, ecosystems can be displaced beyond recovery of their former patterns. Then they settle into new equilibria. Ecosystems are always on historical trajectory, a dynamism of chaos and order entwined.
Michael E. Soulé and Gary Lease have demonstrated in their 1995 book, Reinventing Nature? Responses to Postmodern Deconstruction, that ecology as a science has not proven immune from postmodernist and deconstructionist claims that science in all its forms—astrophysics to ecology—is a cultural construct of the Enlightenment West. Science is pragmatic and enables scientific cultures to get what they want out of nature; science is not descriptive of what nature is really like, apart from humans and their biases and preferences. According to this view, humans should make no pretensions to know what nature is like without them, but can choose what it is like to interact with nature, living harmoniously with it, which will result in a higher quality life. This fits well with a bioregional perspective. Environmental ethics is as much applied geography as it is pure ecology.
Some interpreters, such as Mark Sagoff, conclude that human environmental policy cannot be drawn from nature. Ecology, a piecemeal science in their estimation, can, at best, offer generalizations of regional or local scope, and supply various tools (such as eutrophication of lakes, keystone species, nutrient recycling, niches, succession) for whatever the particular circumstances at hand. Humans ought to step in with our management objectives and reshape the ecosystems we inhabit consonant with our cultural goals.
Other interpreters, such as David Pimentel, Laura Westra, and Reed Noss, argue that human life does and ought to include nature and culture entwined, humans as part of, rather than apart from, their ecosystems. Ecosystems are dependable life support systems. There is a kind of order that arises spontaneously and systematically when many self-actualizing units interactively pursue their own programs, each doing its own thing and forced into informed co-action with other units.
In culture, the logic of language or the integrated connections of the market are examples of such co-action. We legitimately respect cultural heritages, such as Judaism or Christianity, or democracy or science, none of which are centrally controlled processes, all of which mix elements of integrity and dependability with dynamic change, even surprise and unpredictability. We might wish for "integrity, stability, and beauty" in democracy or science, without denying the elements of pluralism, dynamism, contingency, and historical development.
Ecosystems, though likewise complex, open, and decentralized, are orderly and predictable enough to make ecological science possible—and also to make possible an ethics respecting these dynamic, creative, vital processes. The fauna and flora originally in place, independently of humans, will with high probability be species naturally selected for their adaptive fits, as evolutionary and ecological theory both teach. Misfits go extinct and unstable ecosystems collapse and are replaced by more stable or resilient ones (perhaps rejuvenated by chaos or upset by catastrophe).
This ecosystemic nature, once flourishing independently and for millennia continuing along with humans, has in the last one hundred years come under increasing jeopardy—variously described as a threat to ecosystem health, integrity, or quality.
Since the 1990s, emphasis has been ecosystem management. This approach appeals alike to scientists, who see the need for understanding ecosystems objectively and for applied technologies, and also to humanists, who find that humans are cultural animals who rebuild their environments and who desire benefits for people. The combined ecosystem/management policy promises to operate at system-wide levels, presumably to manage for indefinite sustainability, alike of ecosystems and their outputs. Such management connects with the idea of nature as "natural resources" at the same time that it has a "respect nature" dimension. Christian ethicists note that the secular word "manage" is a stand-in for the earlier theological word "steward." Adam was placed in the garden "to till and keep it" (Gen. 2:15).
Pristine natural systems no longer exist anywhere on Earth (the insecticide DDT has been found in penguins in Antarctica). Perhaps 95 percent of a landscape will be rebuilt for culture, considering lands plowed and grazed, forests managed, rivers dammed, and so on. Still, only about 25 percent of the land, in most nations, is under permanent agriculture; a large percentage is more or less rural, still with some processes of wild nature taking place. The twenty-first century promises an escalation of development that threatens both the sustainability of landscapes supporting culture as well as their intrinsic integrity.
Scientists and ethicists alike have traditionally divided their disciplines into realms of the "is" and the "ought." No study of nature can tell humans what ought to happen. This neat division has been challenged by ecologists and their philosophical and theological interpreters. The analysis here first distinguishes between interhuman ethics and environmental ethics. The claim that nature ought sometimes to be taken as norm within environmental ethics is not to be confused with a different claim, that nature teaches us how we ought to behave toward each other. Nature as moral tutor has always been, and remains, doubtful ethics. Compassion and charity, justice and honesty, are not virtues found in wild nature. There is no way to derive any of the familiar moral maxims from nature: "One ought to keep promises." "Do to others as you would have them do to you." "Do not cause needless suffering." No natural decalogue endorses the Ten Commandments.
But, continuing the analysis, there may be goods (values) in nature with which humans ought to conform. Animals, plants, and species, integrated into ecosystems, may embody values that, though nonmoral, count morally when moral agents encounter these. To grant that morality emerges in human beings out of nonmoral nature does not settle the question whether we, who are moral, should sometimes orient our conduct in accord with value there. Theologians will add that God bade Earth bring forth its swarming kinds and found this genesis very good. Palestine was a promised land; Earth is a promising planet, but only if its ecologies globally form a biosphere.
Environmental science can inform environmental ethics in subtle ways. Scientists describe the "order," "dynamic stability," and "diversity" in these biotic "communities." They describe "interdependence," or speak of "health" or "integrity," perhaps of their "resilience" or "efficiency." Scientists describe the "adapted fit" that organisms have in their niches. They describe an ecosystem as "flourishing," as "self-organizing." Strictly interpreted, these are only descriptive terms; and yet often they are already quasi-evaluative terms, perhaps not always so but often enough that by the time the descriptions of ecosystems are in, some values are already there. In this sense, ecology is rather like medical science, with therapeutic purpose, seeking such flourishing health.
Ecology in classical religions
Is there ecological wisdom in the classical religions? Religion and science have to be carefully delineated, each in its own domain. One makes a mistake to ask about technical ecology in the Bible (such as the Lotka-Volterra equations, dealing with population size and carrying capacity). But ecology is a science at native range. Residents on landscapes live immersed in their local ecology. At the pragmatic ranges of the sower who sows, waits for the seed to grow, and reaps the harvest, the Hebrews knew their landscape. Abraham and Lot, and later Jacob and Esau, dispersed their flocks and herds because "the land could not support both of them dwelling together" (Gen. 13:2-13; 36:6-8). There were too many sheep and goats eating the sparse grasses and shrubs of their semi-arid landscape, and these nomads recognized this. They were exceeding the carrying capacity, ecologists now say.
Here academic ecologists can learn a great deal from people indigenous to a landscape for centuries. Such ecological wisdom might be as readily found with the Arunta in Australia, or with the Navajos in the American Southwest on their landscapes. This would be indigenous wisdom rather than divine revelation. Such wisdom is often supported more by mythology than by science. Such wisdom is also frequently mixed with error and misunderstanding.
Christian (and other) ethicists can with considerable plausibility make the claim that neither conservation, nor a sustainable biosphere, nor sustainable development, nor any other harmony between humans and nature can be gained until persons learn to use the earth both justly and charitably. Those twin concepts are not found either in wild nature or in any science that studies nature. They must be grounded in some ethical authority, and this has classically been religious.
One needs human ecology, humane ecology, and this requires insight more into human nature than into wild nature. True, humans cannot know the right way to act if they are ignorant of the causal outcomes in the natural systems they modify—for example, the carrying capacity of the Bethel-Ai rangeland in the hill country of Judaea. But there must be more. The Hebrews were convinced that they were given a blessing with a mandate. The land flows with milk and honey (assuming good land husbandry) if and only if there is obedience to Torah. Abraham said to Lot, "Let there be no strife between me and you, and between your herdsmen and my herdsmen" (Gen. 13:8), and they partitioned the common good equitably among themselves. The Hebrews also include the fauna within their covenant. "Behold I establish my covenant with you and your descendants after you, and with every living creature that is with you, the birds, the cattle, and every beast of the earth with you" (Gen. 9:5). In modern terms, the covenant was both ecumenical and ecological.
See also Animal Rights; Autopoiesis; Chaos Theory; Deep Ecology; Ecofeminism; Ecology, Ethics of; Ecology, Religious and Philosophical Aspects; Ecology, Science of; Ecotheology; Feminism and Science; Feminist Cosmology; Feminist Theology; Gaia Hypothesis; Womanist Theology
golley, frank. a primer for ecological literacy. new haven, conn.: yale university press, 1998.
gumbine, r. edward. "what is ecosystem management?" conservation biology 8 (1994): 27-38.
leopold, aldo. "the land ethic." in a sand county almanac. new york: oxford university press, 1968.
northcott, michael s. the environment and christian ethics. cambridge, uk: cambridge university press, 1996.
pimentel, david; westra, laura; and noss, reed f., eds. ecological integrity: integrating environment, conservation, and health. washington, d.c.: island press, 2000.
rolston, holmes, iii. "the bible and ecology." interpretation: journal of bible and theology 50 (1996): 16–26.
sagoff, mark. "ethics, ecology, and the environment: integrating science and law." tennessee law review 56 (1988): 77-229.
soulé, michael e., and lease, gary, eds. reinventing nature? responses to postmodern deconstruction. washington, d.c.: island press, 1995.
holmes rolston, iii
The word ecology was coined in 1870 by the German zoologist Ernst Haeckel from the Greek words oikos (house) and logos (logic or knowledge) to describe the scientific study of the relationships among organisms and their environment . Biologists began referring to themselves as ecologists at the end of the nineteenth century and shortly thereafter the first ecological societies and journals appeared. Since that time ecology has become a major branch of biological science. The contextual, historical understanding of organisms as well as the systems basis of ecology set it apart from the reductionist, experimental approach prevalent in many other areas of science.
This broad ecological view is gaining significance today as modern resource-intensive lifestyles consume much of nature's supplies. Although intuitive ecology has always been a part of some cultures, current environmental crises make a systematic, scientific understanding of ecological principles especially important.
For many ecologists the basic structural units of ecological organization are species and populations. A biological species consists of all the organisms potentially able to interbreed under natural conditions and to produce fertile offspring. A population consists of all the members of a single species occupying a common geographical area at the same time. An ecological community is composed of a number of populations that live and interact in a specific region.
This population-community view of ecology is grounded in natural history—the study of where and how organisms live—and the Darwinian theory of natural selection and evolution . Proponents of this approach generally view ecological systems primarily as networks of interacting organisms. Abiotic forces such as weather, soils, and topography are often regarded as external factors that influence but are apart from the central living core of the system.
In the past three decades the emphasis on species, populations, and communities in ecology has been replaced by a more quantitative, thermodynamic analysis of the processes through which energy flows and the cycling of nutrients and toxins are carried out in ecosystems. This process-functional approach is concerned more with the ecosystem as a whole than the particular species or populations that make it up. In this perspective, both the living organisms and the abiotic physical components of the environment are equal members of the system.
The feeding relationships among different species in a community are a key to understanding ecosystem function. Who eats whom, where, how, and when determine how energy and materials move through the system. They also influence natural selection, evolution, and species adaptation to a particular set of environmental conditions. Ecosystems are open systems, insofar as energy and materials flow through them. Nutrients, however, are often recycled extremely efficiently so that the annual losses to sediments or through surface water runoff are relatively small in many mature ecosystems. In undisturbed tropical rain forests, for instance, nearly 100% of leaves and detritus are decomposed and recycled within a few days after they fall to the forest floor.
Because of thermodynamic losses every time energy is exchanged between organisms or converted from one form to another, an external energy source is an indispensable component of every ecological system. Green plants capture solar energy through photosynthesis and convert it into energy-rich organic compounds that are the basis for all other life in the community. This energy capture is referred to as "primary productivity." These green plants form the first trophic (or feeding) level of most communities.
Herbivores (animals that eat plants) make up the next trophic level , while carnivores (animals that eat other animals) add to the complexity and diversity of the community. Detritivores (such as beetles and earthworms) and decomposers (generally bacteria and fungi ) convert dead organisms or waste products to inorganic chemicals . The nutrient recycling they perform is essential to the continuation of life. Together, all these interacting organisms form a food chain/web through which energy flows and nutrients and toxins are recycled. Due to intrinsic inefficiencies in transferring material and energy between organisms, the energy content in successive trophic levels is usually represented as a pyramid in which primary producers form the base and the top consumers occupy the apex.
This introduces the problem of persistent contaminants in the food chain. Because they tend not to be broken and metabolized in each step in the food chain in the way that other compounds are, persistent contaminants such as pesticides and heavy metals tend to accumulate in top carnivores, often reaching toxic levels many times higher than original environmental concentrations. This biomagnification is an important issue in pollution control policies. In many lakes and rivers, for instance, game fish have accumulated dangerously high levels of mercury and chlorinated hydrocarbons that present a health threat to humans and other fish-eating species.
Diversity, in ecological terms, is a measure of the number of different species in a community, while abundance is the total number of individuals. Tropical rain forests, although they occupy only about five% of the earth's land area, are thought to contain somewhere around half of all terrestrial plant and animals species, while coral reefs and estuaries are generally the most productive and diverse aquatic communities. Community complexity refers to the number of species at each trophic level as well as the total number of trophic levels and ecological niches in a community.
Structure describes the patterns of organization, both spatial and functional, in a community. In a tropical rain forest , for instance, distinctly different groups of organisms live on the surface, at mid-levels in the trees, and in the canopy, giving the forest vertical structure. A patchy mosaic of tree species, each of which may have a unique community of associated animals and smaller plants living in its branches, gives the forest horizontal structure as well.
For every physical factor in the environment there are both maximum and minimum tolerable limits beyond which a given species cannot survive. The factor closest to the tolerance limit for a particular species at a particular time is the critical factor that will determine the abundance and distribution of that species in that ecosystem. Natural selection is the process by which environmental pressures—including biotic factors such as predation, competition , and disease, as well as physical factors such as temperature, moisture, soil type, and space—affect survival and reproduction of organisms. Over a very long time, given a large enough number of organisms, natural selection works on the randomly occurring variation in a population to allow evolution of species and adaptation of the population to a particular set of environmental conditions.
Habitat describes the place or set of environmental conditions in which an organism lives; niche describes the role an organism plays. A yard and garden, for instance, may provide habitat for a family of cottontail rabbits. Their niche is being primary consumers (eating vegetables and herbs).
Organisms interact within communities in many ways. Symbiosis is the intimate living together of two species; commensalism describes a relationship in which one species benefits while the other is neither helped nor harmed. Lichens , the thin crusty plants often seen on exposed rocks, are an obligate symbiotic association of a fungus and an alga. Neither can survive without the other. Some orchids and bromeliads (air plants), on the other hand, live commensally on the branches of tropical trees. The orchid benefits by having a place to live but the tree is neither helped nor hurt by the presence of the orchid.
Predation—feeding on another organism—can involve pathogens, parasites , and herbivores as well as carnivorous predators. Competition is another kind of antagonistic relationship in which organisms vie for space, food, or other resources. Predation, competition, and natural selection often lead to niche specialization and resource partitioning that reduce competition between species. The principle of competitive exclusion states that no two species will remain in direct competition for very long in the same habitat because natural selection and adaptation will cause organisms to specialize in when, where, or how they live to minimize conflict over resources. This can contribute to the evolution of a given species into new forms over time.
It is also possible, on the other hand, for species to coevolve, meaning that each changes gradually in response to the other to form an intimate and often highly dependent relationship either as predator and prey or for mutual aid. Because individuals of a particular species may be widely dispersed in tropical forests, many plants have become dependent on insects, birds, or mammals to carry pollen from one flower to another. Some amazing examples of coevolution and mutual dependence have resulted.
Ecological succession , the process of ecosystem development, describes the changes through which whole communities progress as different species colonize an area and change its environment. A typical successional series starts with pioneer species such as grasses or fireweed that colonize bare ground after a disturbance. Organic material from these pioneers helps build soil and hold moisture, allowing shrubs and then tree seedlings to become established. Gradual changes in shade, temperature, nutrient availability, wind protection, and living space favor different animal communities as one type of plant replaces its predecessors. Primary succession starts with a previously unoccupied site. Secondary succession occurs on a site that has been disturbed by external forces such as fires, storms, or humans. In many cases, succession proceeds until a mature "climax" community is established. Introduction of new species by natural processes, such as opening of a land bridge, or by human intervention can upset the natural relationships in a community and cause catastrophic changes for indigenous species.
Biomes consist of broad regional groups of related communities. Their distribution is determined primarily by climate , topography, and soils. Often similar niches are occupied by different but similar species (called ecological equivalents) in geographically separated biomes. Some of the major biomes of the world are deserts, grasslands , wetlands , forests of various types, and tundra .
The relationship between diversity and stability in ecosystems is a controversial topic in ecology. F. E. Clements, an early biogeographer, championed the concept of climax communities: stable, predictable associations towards which ecological systems tend to progress if allowed to follow natural tendencies. Deciduous, broad-leaved forests are climax communities in moist, temperate regions of the eastern United States according to Clements, while grasslands are characteristic of the dryer western plains. In this view, homeostasis (a dynamic steady-state equilibrium), complexity, and stability are endpoints in ecological succession. Ecological processes, if allowed to operate without external interference, tend to create a natural balance between organisms and their environment.
H. A. Gleason , another pioneer biogeographer and contemporary of Clements, argued that ecological systems are much more dynamic and variable than the climax theory proposes. Gleason saw communities as temporary or even accidental combinations of continually changing biota rather than predictable associations. Ecosystems may or may not be stable, balanced, and efficient; change, in this view, is thought to be more characteristic than constancy. Diversity may or may not be associated with stability. Some communities such as salt marshes that have only a few plant species may be highly resilient and stable while species-rich communities such as coral reefs may be highly sensitive to disturbance.
Although many ecologists now tend to agree with the process-functional view of Gleason rather than the population-community view of Clements, some retain a belief in the balance of nature and the tendency for undisturbed ecosystems to reach an ideal state if left undisturbed. The efficacy and ethics of human intervention in natural systems may be interpreted very differently in these divergent understandings of ecology. Those who see stability and constancy in nature often call for policies that attempt to maintain historic conditions and associations. Those who see greater variability and individuality in communities may favor more activist management and be willing to accept change as inevitable.
In spite of some uncertainty, however, about how to explain ecological processes and the communities they create, we have learned a great deal about the world around us through scientific ecological studies in the past century. This important field of study remains a crucial component in our ability to manage resources sustainably and to avoid or repair environmental damage caused by human actions.
[William P. Cunningham Ph.D. ]
Ricklefs, R. E. Ecology. 3rd ed. New York: W. H. Freeman, 1990.