Ecological integrity

Ecological integrity is a relatively new concept that is being actively discussed by ecologists. However, a consensus has not yet emerged as to the definition of ecological integrity. Clearly, human activities result in many environmental changes that enhance some species , ecosystems, and ecological processes, while at the same time causing important damage to others. The challenge for the concept of ecological integrity is to provide a means of distinguishing between responses that represent improvements in the quality of ecosystems, and those that are degradations.

The notion of ecological integrity is analogous to that of health. A healthy individual is relatively vigorous in his or her physical and mental capacities, and is uninfluenced by disease . Health is indicated by diagnostic symptoms that are bounded by ranges considered to be normal, and by attributes that are regarded as desirable. Unhealthy conditions are indicated by the opposite, and may require treatment to prevent further deterioration. However, the metaphor of human and ecosystem health is imperfect in some important respects, and has been criticized by ecologists. This is mostly because health refers to individual organisms, while ecological contexts are much more complex, involving many individuals of numerous species, and both living and nonliving attributes of ecosystems.

Environmental stress is a challenge to ecological integrity

Environmental stress refers to physical, chemical, and biological constraints on the productivity of species and the development of ecosystems. When they increase or decrease in intensity, stressors elicit ecological responses. Stressors can be natural environmental factors, or they can be associated with the activities of humans. Some environmental stressors are relatively local in their influence, while others are regional or global in scope. Stressors are challenges to ecological integrity.

Species and ecosystems have some capacity to tolerate changes in the intensity of environmental stressors, an attribute known as resistance. However, there are limits to resistance, which represent thresholds of tolerance. When these thresholds are exceeded, substantial ecological changes occur in response to further increases in the intensity of environmental stress.

Environmental stressors can be categorized as follows:

Physical stress

Physical stress refers to brief but intense events of kinetic energy . Because of its acute, episodic nature, this is a type of disturbance. Examples include volcanic eruptions, windstorms, and explosions.

Wildfire

Wildfire is another disturbance, during which much of the biomass of an ecosystem combusts, and the dominant species may be killed.

Pollution

Pollution occurs when chemicals occur in concentrations large enough to affect organisms, and thereby cause ecological change to occur. Toxic pollution can be caused by gases such as sulfur dioxide and ozone , elements such as mercury and arsenic, and pesticides . Nutrients such as phosphate and nitrate can distort ecological processes such as productivity, causing a type of pollution known as eutrophication .

Thermal stress

Thermal stress occurs when releases of heat cause ecological responses, as occurs near natural, hot water vents in the ocean , or with industrial discharges of heated water.

Radiation stress

Radiation stress is associated with excessive loads of ionizing energy. This can be important on mountain-tops, where there are intense exposures to ultraviolet radiation, and in places where there are uncontrolled exposures to radioactive waste .

Climatic stress

Climatic stress is caused by excessive or insufficient regimes of temperature , moisture, solar radiation, or combinations of these. Tundra and deserts are climatically stressed ecosystems, while tropical rainforest occurs in places where climate is relatively benign.

Biological stress

Biological stress is associated with the complex interactions that occur among organisms of the same or different species. Biological stress can result from competition , herbivory, predation, parasitism, and disease. The harvesting and management of species and ecosystems by humans is a type of biological stress.

Large changes in the intensity of environmental stress result in various types of ecological responses. For example, when an ecosystem is disrupted by an intense disturbance, there may be substantial mortality of its species and other damage, followed by recovery through succession . In contrast, a longer-term intensification of environmental stress, possibly associated with chronic pollution or climate change, causes more permanent ecological adjustments to occur. Relatively vulnerable species are reduced in abundance or eliminated from sites that are stressed over the longer term, and their modified niches are assumed by organisms that are more tolerant. Other common responses include a simplification of species richness, and decreased rates of productivity, decomposition , and nutrient cycling. These changes represent an ecological conversion, or a longer-term change in the character of the ecosystem.

Components of ecological integrity

Many studies have been made of the ecological responses to disturbance and to longer-term changes in the intensity of environmental stress. These studies have examined stressors associated with, for example, pollution, the harvesting of species from ecosystems, and the conversion of natural ecosystems into managed agroecosystems. The commonly observed patterns of change in these sorts of stressed ecosystems are considered to represent some of the key elements of ecological integrity. Such observations can be used to develop indicators of ecological integrity, which are useful in determining whether this condition is improving or being degraded over time. It has been suggested that greater ecological integrity is displayed by systems with the following characteristics:

Resiliency and resistance

Ecosystems with greater ecological integrity are, in a relative sense, more resilient and resistant to changes in the intensity of environmental stress. In the ecological context, resistance refers to the capacity of organisms, populations, and communities to tolerate increases in stress without exhibiting significant responses. Resistance is manifest in thresholds of tolerance. Resilience refers to the ability to recover from disturbance.

Biodiversity

In its simplest interpretation, biodiversity refers to the number of species occurring in some ecological community or in a designated area, such as a park or a country. However, biodiversity is better defined as the total richness of biological variation, including genetic variation within populations and species, the numbers of species in communities, and the patterns and dynamics of these over large areas.

Complexity of structure and function

The structural and functional complexity of ecosystems is limited by natural environmental stresses associated with climate, soil , chemistry , and other factors, and by stressors associated with human activities. As the overall intensity of stress increases or decreases, structural and functional complexity responds accordingly. Under any particular environmental regime, older ecosystems will generally be more complex than younger ecosystems.

Presence of large species

The largest, naturally occurring species in any ecosystem generally appropriate relatively large amounts of resources, occupy a great deal of space , and require large areas to sustain their populations. In addition, large species are usually long-lived, and therefore integrate the effects of stressors over an extended time. Consequently, ecosystems that are subject to an intense regime of environmental stress cannot support relatively large species. In contrast, mature ecosystems of relatively benign environments are dominated by large, long-lived species.

Presence of higher-order predators

Because top predators are dependent on a broad base of ecological productivity , they can only be sustained by relatively extensive and/or productive ecosystems.

Controlled nutrient cycling

Recently disturbed ecosystems temporarily lose some of their capability to exert biological control over nutrient cycling, and they often export large quantities of nutrients dissolved or suspended in streamwater. Systems that are not "leaky" of their nutrient capital in this way are considered to have greater ecological integrity.

Efficient energy use and transfer

Large increases in environmental stress commonly result in community respiration exceeding productivity, so that the standing crop of biomass decreases. Ecosystems that are not degrading in their capital of biomass are considered to have greater integrity than those in which biomass is decreasing over time.

Ability to maintain natural ecological values

Ecosystems that can naturally maintain their species, communities, and other important characteristics, without interventions by humans through management, have greater ecological integrity. For example, if a rare species of animal can only be sustained through intensive management of its habitat by humans, or by management of its demographics, possibly by a captive-breeding and release program, then its populations and ecosystem are lacking in ecological integrity.

Components of a "natural" community

Ecosystems that are dominated by non-native, introduced species are considered to have less ecological integrity than those composed of native species.

The last two indicators involve judgements about "naturalness" and the role of humans in ecosystems, which are philosophically controversial topics. However, most ecologists would consider that self-organizing, unmanaged ecosystems have greater ecological integrity than those that are strongly influenced by human activities. Examples of the latter include agroecosystems, forestry plantations, and urban and suburban ecosystems. None of these systems can maintain themselves in the absence of large inputs of energy, nutrients, and physical management by humans.

Indicators of ecological integrity

Indicators of ecological integrity vary widely in their scale, complexity, and intent. For example, certain metabolic indicators can suggest the responses by individual organisms and populations to toxic stress, as is the case of assays of detoxifying enzyme systems that respond to exposure to persistent chlorinated hydrocarbons , such as DDT and PCBs. Indicators related to populations of endangered species are relevant to the viability of those species, as well as the integrity of their natural communities. There are also indicators relevant to processes occurring at the level of landscape. There are even global indicators, for example, relevant to climate change, depletion of stratospheric ozone, and deforestation .

Sometimes, relatively simple indicators can be used to integrate the ecological integrity of a large and complex ecosystem. In the western United States, for instance, the viability of populations of spotted owls (Strix occidentalis) is considered to be an indicator of the integrity of the types of old-growth forest in which this endangered bird breeds. If plans to harvest and manage those forests are judged to pose a threat to the viability of a population of spotted owls or the species, this would indicate a significant challenge to the integrity of the entire old-growth forest ecosystem.

Ecologists are also beginning to develop holistic indicators of ecological integrity. These are designed as composites of various indicators, analogous to certain economic indices such as the Dow-Jones Index of the stock market, the Consumer Price Index, and gross domestic product indices of economies. Composite economic indicators like these are relatively simple to design because all of the input data are measured in a common way, for example, in dollars. However, in ecology there is no common currency among the various indicators of ecological integrity, and it is therefore difficult to develop composite indicators that people will agree upon.

In spite of the difficulties, ecologists are making progress in their development of indicators of ecological integrity. This is an important activity for ecologists, because people and their larger society need objective information about changes in the integrity of species and ecosystems so that actions can be taken to prevent unacceptable degradations. It is being increasingly recognized that human economies can only be sustained over the longer term by ecosystems with integrity. These must be capable of supplying continuous flows of renewable resources, such as trees, fish , agricultural products, and clean air and water. There are also important concerns about the intrinsic value of native species and their natural ecosystems, all of which must be sustained along with humans. A truly sustainable economy can only be based on ecosystems with integrity.

See also Indicator species; Stress, ecological.

Resources

books

Babaev, Agadzhan, and Agajan G. Babaev, eds. Desert Problems and Desertification in Central Asia: The Researches of the Desert Institute. Berlin: Springer Verlag, 1999.

Freedman, B. Environmental Ecology. 2nd ed. San Diego: Academic Press, 1995.

Hamblin, W. K., and E.H. Christiansen. Earth's Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001.

Woodley, S., J. Kay, and G. Francis, eds. Ecological Integrity and the Management of Ecosystems. Boca Raton, FL: St. Lucie Press, 1993.

periodicals

Caballero, A., and M. A. Toro. "Interrelations Between Effective Population Size and Other Pedigree Tools for the Management of Conserved Populations." Genetical Research 75, no. 3 (June 2000): 331-43.

Karr, J. "Defining and Assessing Ecological Integrity: Beyond Water Quality." Environmental Toxicology and Chemistry 12 (1993): 1521-1531.

Bill Freedman

KEY TERMS

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Stress

—Environmental constraints that cause ecological disruptions (that is, disturbance) or that limit the potential productivity of species or the development of ecosystems. Environmental stress is a challenge to ecological integrity.