Communities (zoology)

An ecological community is the assemblage of species found in a given time and place. The populations that form a community interact through the processes of competition, predation, parasitism, and mutualism. The structure and function of communities are determined by the nature and strength of the population interactions within them, but these interactions are affected by the environment in which a community exists. An ecological community, together with its physical environment, is called an ecosystem. No ecological system can be studied apart from its physical environment; the structure and function of every community are determined in part by its interactions with its environment.

The species constituting a community occupy different functional roles. The most common way to characterize a community functionally is by describing the flow of energy through it. Correspondingly, communities usually contain three groups of species: those that obtain energy through photosynthesis (called producers), those that obtain energy by consuming other organisms (consumers), and those that decompose dead organisms (decomposers). The pathway through which energy travels from producer through one or more consumers and finally to decomposer is called a food chain. Each link in a food chain is called a trophic level. Interconnected food chains in a community constitute a food web. Food webs have no analogy in populations.

Very few communities are so simple that they can be readily described by a food web. Most communities are compartmentalized: A given set of producers tends to be consumed by a limited number of consumers, which, in turn, are preyed upon by only a few predators, and so on. Alternatively, consumers may obtain energy by specializing in one part of their prey (for example, some birds may eat only seeds of plants) but utilize a wide range of prey species. Compartmentalization is an important feature of community structure; it influences the formation, organization, and persistence of a community.

Dominant and Keystone Species

Some species, called dominant species, can exert powerful control over the abundance of other species because of the dominant species’ large size, extended life span, or ability to monopolize energy or other resources. Communities are named according to their dominant species: for example, oak-hickory forest, redwood forest, sagebrush desert, and tallgrass prairie. Some species, called keystone species, have a disproportionately large effect on community structure by preventing dominant species from monopolizing the community. Keystone species usually exert their effects through predation, while dominant species are good competitors (that is, better at obtaining and holding resources than other species). Other keystone species provide the foundation for the particular ecological niche where they live, such as bees that pollinate plants so they can reproduce or coral that provides a feeding and hiding ground for marine life.

The species that make up a community are seldom distributed uniformly across the landscape; rather, some degree of patchiness is characteristic of virtually all species. There has been conflicting evidence as to the nature of this patchiness. As one moves across an environmental gradient (for example, from wet to dry conditions or from low to high elevations), there is a corresponding change in species observed and in the type of community present. Some studies have suggested that changes in species composition usually occur along relatively sharp boundaries and that these boundaries mark the boundaries between adjacent communities. Other studies have indicated that species tend to respond individually to environmental gradients and that community boundaries are not sharply defined; rather, most communities broadly integrate into one another.

The Nature of Community

These conflicting results have fueled a continuing debate as to the underlying nature of the community. Some communities behave in a coordinated manner; for example, if a prairie is consumed by fire, it regenerates in a predictable sequence, ultimately returning to the same structure and composition it had before the fire. This coordinated response is to be expected if the species in a community have evolved together with one another. In this case, the community behaves analogously to an organism, maintaining its structure and function in the face of environmental disturbances and fluctuations (as long as the disturbances and fluctuations are not too extreme). The existence of relatively sharp boundaries between adjacent communities supports this explanation of the nature of the community.

In other communities, it appears that the response to environmental fluctuation or disturbance is determined by the evolved adaptations of the species available. There is no coordinated community response, but rather a coincidental assembly of community structure over time. Some sets of species interact together so strongly that they enter a community together, but there is no evidence of an evolved community tendency to resist or accommodate environmental change. Data support this explanation of the community as an entity formed primarily of species that happen to share similar environmental requirements.

Mechanisms of Community Structure

Disagreement about the underlying nature of communities usually reflects disagreement on the relative importance of the underlying mechanisms that determine community structure. Interspecific competition has long been invoked as the primary agent structuring communities. Competition is certainly important in some communities, but there is insufficient evidence to indicate how widespread and important it is in determining community structure. Much of the difficulty occurs because ecologists must infer the existence of past competition from present patterns in communities. It appears that competition has been important in many vertebrate communities and in communities dominated by sessile organisms, such as plants; it does not appear to have been important in structuring communities of plant-eating insects. Furthermore, the effects of competition typically affect individuals that use identical resources so that only a small percentage of species in a community may be experiencing significant competition at any time.

The effects of predation on community structure depend on the nature of the predation. Keystone species usually exert their influence by selectively preying on species that is competitively dominant. Predators that do not specialize on one or a few species may also have a major effect on community structure if they attack prey in proportion to their abundance; this frequency-dependent predation prevents any prey species from achieving dominance. If a predator is too efficient, it can drive its prey to extinction, which may cause a selective predator to become extinct as well. Predation appears to be most important in determining community structure in environments that are predictable or unchanging.

Natural Disturbances

A variable or unpredictable environment influences the structure of a community. No environment is completely uniform; longer-term or seasonal environmental fluctuations affect community structure by limiting opportunities for colonization, causing direct mortality, and hindering or exacerbating the effects of competition and predation. Furthermore, all communities experience at least occasional disturbance: unpredictable, seemingly random environmental changes that may be quite severe. It is useful in this regard to distinguish between disasters and catastrophes. A disaster is an event that occurs so frequently in the life of a population that adaptation is possible; for example, a fire occurs so often in tallgrass prairies that most of the plant species have become fire-adapted—they have become efficient at acquiring nutrients left in the ash and at sprouting or germinating quickly following a fire. In comparison, a catastrophe is so intense, widespread, or infrequent that a population cannot adapt to it; the eruption of Mount St. Helens in 1980, for example, was so violent and so unpredictable that the species affected could not evolve adequate responses to it.

Natural disturbances occur at a variety of scales. Small-scale disturbances may simply create small openings in a community that are filled in by other species suited to thrive in such spaces. Large disturbances are qualitatively different from small disturbances in that large portions of a community may be destroyed, including some of the ability to recover from the disturbance. Early ecologists almost always saw disturbances as destructive and disruptive for communities. Under this assumption, most mathematical models portrayed communities as generally being in some stable state, at equilibrium; if a disturbance occurred, the community inevitably returned to the same (or some alternative) equilibrium. It later became clear, however, that natural disturbance is a part of almost all natural communities. Ecologists now recognize that few communities exhibit a stable equilibrium; instead, communities are dynamic, always responding to the last disturbance, and always adjusting to the most recent environmental fluctuation. In the twenty-first century, communities were impacted by global warming, changing their habits and habitats.

The Long-Term Dynamics of Communities

The evidence suggests that three conclusions can be drawn with regard to the long-term dynamics of communities. First, it can no longer be assumed that communities remain at equilibrium until changed by outside forces. Disturbances are so common; they occur at so many different scales and frequencies, and they so readily affect the processes of competition and predation that the community must be viewed as an entity that is constantly changing as its constituent species readjust to disturbance and to one another.

Second, communities exhibit several types of stability in the face of disturbance. A community may absorb disturbance without markedly changing until it reaches a threshold and suddenly and rapidly shifts to a new state, called resistance stability. Alternatively, a community may change easily when disturbed but quickly return to its former state; this characteristic is called resilience stability. Resilience stability may occur over a wide range of conditions and scales of disturbance; such a system is said to be dynamically robust. On the other hand, a community that exhibits resilience only within a narrow range of conditions is said to be dynamically fragile.

Finally, there is no simple way to predict the stability of a community. At the end of the 1970s, it appeared that complex communities were generally more stable than simple communities. It appeared that stability was conferred by more intricate food webs, by more structural complexity, and by higher species diversity. On the basis of numerous field studies and theoretical models, ecologists now conclude that no such relationship exists. Both very complex communities, such as tropical rainforests, and very simple communities, such as Arctic tundra, may be very fragile when disturbed.

Complex Systems

Most communities consist of thousands of species, and their complexity makes them very difficult to study. Most community ecologists specialize in taxonomically restricted subsets of communities (such as plant communities, bird communities, insect communities, or moss communities) or in functionally restricted subsets of communities (such as soil communities, tree-hole communities, pond communities, or detrivore communities).

The type of community under investigation and the questions of interest determine the appropriate methods of study. The central questions in most community studies are how many species are present and what the abundance of each is. The answers to these questions can be estimated using mark-recapture methods or any other enumeration method.

Often the aim is to compare communities (or to compare the same community at different times). A specialized parameter called similarity is used to compare and classify communities; more than two dozen measures of similarity are available. Measures of similarity are typically subjected to cluster analysis, a set of techniques that groups communities on the basis of their similarity.

Many multivariate techniques are used to search for patterns in community data. Direct gradient analysis is the simplest of these techniques; it is used to study the distribution of species along an environmental gradient. Ordination includes several methods for collapsing community data for many species in many communities along several environmental gradients onto a single graph that summarizes their relationships and patterns.

Patterns of Community Responses to Disturbance

At the most basic level, destruction of a community eliminates the species comprising the community. If the community is restricted in its extent, and if its constituent species are found nowhere else, those species become extinct. If the community covers a large area or is found in several areas, local extinction of species may occur without causing global extinction.

Destruction of a community can cause unexpected changes in environmental conditions that were modified by the intact community. Even partial destruction of an extensive community can eliminate species. For example, the checkerboard pattern of clear-cutting in Douglas fir forests of the Pacific Northwest threatens the survival of the northern spotted owl, the marbled murrelet, Vaux’s swift, and the red tree vole, even though fragments of the community remain. Many fragments are simply too small to support these species. A Douglas fir forest is regenerated following cutting, but this young, even-aged stand is so different from an old, mixed-age forest that it functions as a different type of community.

Altering the population of one species can affect others in a community. The black-footed ferret, sometimes called the American polecat, was once found widely throughout central North America as a predator of prairie dogs and the continent’s only native ferret species. As prairie dogs were poisoned, drowned, and shot throughout their range, the number of black-footed ferrets declined. In 1989, fewer than one hundred black-footed ferrets were in a captive breeding program in Wyoming in a final attempt to preserve the species. However, they remain among the most endangered mammals in the United States in the twenty-first century because reintroduction efforts continue to be limited by habitat destruction, disease, and the depletion of prairie dogs, which the ferret depends on for food. Despite successfully breeding the ferrets in captivity for decades, their community’s ecosystem, including the grasslands, their prey, and their shelter, continues to be too damaged to support the native species. Scientisst note that their population’s recovery in the wild to 3,000 black-footed ferrets will signify the healing of the grasslands.

Introducing a new species into a community can severely alter the interactions in the community. The introduction of the European rabbit into Australia led to a population explosion of rabbits, excessive predation on vegetation, and resulting declines in many native marsupials.

Finally, many communities exhibit stability thresholds, and if a community is disturbed beyond its threshold, its structure is permanently changed. For example, acid deposition in lakes is initially buffered by natural processes. As acid deposition exceeds the buffering capacity of a lake, it causes insoluble aluminum in the lake bottom to become soluble, and this soluble aluminum kills aquatic organisms directly or by making them more susceptible to disease. The lesson is clear: It is far easier to disrupt or destroy natural systems (even accidentally) than it is to restore or reconstruct them.

Principal Terms

Ecosystem: A community together with its physical environment

Food Chain: A pathway through which energy travels in a community

Food Web: The interconnections among all food chains in a community

Frequency-Dependent Predation: Predation on whichever species is most common in a community

Global Extinction: The loss of all members of a species

Keystone Species: A species that determines the structure of a community, usually by predation on its dominant competitor

Local Extinction: The loss of one or more populations of a species, but with at least one population of the species remaining

Resilience Stability: Stability exhibited by a community that changes its structure when disturbed but returns to its original structure when the disturbance ends

Resistance Stability: Stability exhibited by a community that absorbs the effects of a disturbance until it can no longer do so; then, it typically shifts permanently to an alternate structure

Trophic Level: A single link in a food chain; all species that obtain energy in the same way are said to be at the same trophic level

Bibliography

Aber, John D., and Jerry M. Melillo. Terrestrial Ecosystems. 2nd ed. San Diego, Academic Press, 2001.

Begon, Michael, and Colin R. Townsend. Ecology from Individuals to Ecosystems. 5th ed., Hoboken, Wiley, 2021.

Bucko, Katherine. "Animal Communities are Being Reshuffled by Climate Change." Earth.com, 6 June 2022, www.earth.com/news/animal-communities-are-being-reshuffled-by-climate-change. Accessed 10 July 2023.

"The Importance of Community—A Human and Animal Perspective." Two Oceans Aquarium, Apr. 2020, www.aquarium.co.za/news/the-importance-of-community-a-human-and-animal-perspective. Accessed 10 Sept. 2024.

Krebs, Charles J. Ecological Methodology. Reading, Addison Wesley Longman, 2007.

Krebs, Charles J. Ecology: The Experimental Analysis of Distribution and Abundance. 6th ed. Menlo Park, Benjamin/Cummings, 2016.

Krebs, Charles J., et al. “Population and Community Ecology: Past Progress and Future Directions.” Integrative Zoology, 2024, doi.org/10.1111/1749-4877.12863.

Pickett, S. T. A., and P. S. White, eds. The Ecology of Natural Disturbance and Patch Dynamics. London, Elsevier Science, 2012.

Pielou, E. C. The Interpretation of Ecological Data: A Primer on Classification and Ordination. Hoboken, John Wiley & Sons, 1984.