Ecology

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Ecology

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In the past three decades the emphasis on species, populations, and communities in ecology has been replaced by a more quantitative, thermodynamic analysis of the processes through which energy flows and the cycling of nutrients and toxins are carried out in ecosystems.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. ]