Community Ecology

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Community ecology

In biological ecology , the concept of community has been defined in various ways, each definition offering particular advantages and creating its own set of problems. As the authors of one article in a 1987 British Ecological Society symposium on community ecology noted, "Community ecology may be unique amongst the branches of science in lacking a consensus definition of the entity with which it is principally concerned." Daniel Botkin provides a summary of the major alternatives: an ecological community is "either (1) a set of interacting populations of different species found in an area, meaning that the community is the living part of an ecosystem ; or (2) all of the species found in a local area, whether or not they actually interact; or (3) all of the species of the same kind found in a local area, as in a 'plant community' or 'animal community'." Peter Taylor describes an ecological community as consisting "of the populations of different species co-inhabiting a site—a lake, the leaf litter layer in a forest, a dung pat, and so on."

A definition of community is significant because so many different parts of the environment are investigated under that rubric in ecology: animal, plant, insect, primate, forest, even herbaceous plant communities. Often what is described is a component of a community rather than a whole community made up of many different species of plants and animals. Such a whole assemblage in community ecology has no scale, but it does denote a certain level of organization and is different from components of a community, guilds, or taxons.

Community ecology thus focuses on the living part of ecosystems, mostly on communities of interacting populations, however circumscribed. While a community can include species thrown together by locale that do not necessarily interact, community ecology generally emphasizes the wide diversity of species interactions that exist within the area.

Community ecologists investigate interactions under numerous labels and categories, including on-going studies of traditional topics such as predation, competition , trophic exchanges, and others. Borrowing concepts from other disciplines, such as physics, ecologists are also beginning to look closely at the linkages and assemblages that emerge from strong versus weak interactions, or from positive compared to negative interactions. Researchers continue to investigate the relative importance of intra-species and inter-species interactions in terms of their importance to community composition and rates of succession . One recent study, for example, analyzed the importance of interspecific interactions in the structuring of the geographical distribution and abundance of a tropical stream fish community.

The importance of trophic relationships and food web dynamics have always been recognized by ecologists, but a recent article in the journal Ecology can still suggest that "a step toward understanding the dynamics of communities is to describe the pathways along which feeding interactions occur." Ecologists are beginning to quantify feeding interactions, and are moving far beyond the old, linear food chain studies to recognize the complex, multiple linkages created by the feeding relationships among large assemblages of species. Current research also recognizes the composite effects that trophic interactions have on the whole community.

One example of the significance of feeding relationships is the continuing role of predation in community ecology. A study might conclude that "Alaskan kelp forests are broadly dependent on sea otter predation for protection against destructive grazing." Another might look at the timing of predation, and conclude that "temporal variability in predation by whelks can have distinctive effects on prey, create distinctive community compositions, and affect successional paths in [an] intertidal community."

Community ecologists still emphasize species interactions that have been prominent through all the decades of field research. Ecologists still claim, for example, that "assumptions about competition lie at the heart of several models of plant community organization." Other articles in the same journal Ecology reinforce this emphasis, stating e.g., "that competition is pervasive in the [Chihuahuan desert rodent] community." Another example, in the Journal of Ecology indicates that "interspecific competition may limit the growth, branching, survival and berry production by Vaccinium dwarf shrubs in a subarctic plant community."

The degree of pattern or randomness of community structure has long been an issue in community ecology. Natural communities are immensely complex and it is difficult to simplify this complexity down to useable, predictive models. Researchers continue to investigate the extent to which species' interactions can result in a unit organized enough to be considered a coherent community. The mechanics of community assembly depend heavily on invasions, rates of succession, and on changes in the physical environment, as well as a diversity of co-evolutionary patterns.

A debate still continues among community ecologists about the importance of complexity, the role of species diversity and richness in the maintenance of a community. Community structure patterns might be dependent, in part, on factors as seemingly trivial as seed weight. Accuracy in estimations of the number of species in a given community is difficult, including the large number of microorganisms as yet unidentified and unnamed. Statistical inference can help to dilute this issue somewhat.

Disturbances and perturbations are important factors in the composition and character of communities. One study may focus on the role of fire in tall tussock grasslands . Another recent study of old-growth forests in British Columbia concluded that "small-scale natural disturbances involving the death of one to a few trees and creating gaps in the forest canopy are key processes in the...community ecology of many forests." In the recent "landscape view" emerging in ecology, some research indicates that natural communities may persist in patches, as long as they remain interconnected. This underlines again the importance of context in any attempt to understand community ecology.

The most significant source of disturbance and change in natural communities is human activity. Recent studies in community ecology have documented the impact of human removal and then reintroduction of a keystone species such as largemouth bass into a lake; have questioned the impact of "importing" an alien seaweed from Japan to expand Brit tan's kelp farms; and have indicated that "human fishing pressure has influenced Caribbean coral reef community structure by affecting predator-herbivore relationships." Humans can also have positive effects through deliberate attempts to offset destructive impacts. A recent study published in the journal Nature described the deliberate modification of pH levels in lakes by adding phosphate fertilizer , which reversed acidification without "drastically altering the community structure." Such manipulations, modifications, alterations, and disruptions by humans could obviously be expanded into a very long list, but the idea should be clear.

One other type of human impact, however, is receiving a lot of attention: evidence is mounting that climate change is increasingly impacting natural communities. For example, one recent study suggested that "increased warming could change the dominant vegetation of a widespread meadow habitat , changing the competitive relationships between sagebrush and cinquefoil." Another indicated that "regional climatic warming may be altering the species composition of Alaskan arctic tundra" through changes in light, temperature and nutrients that affect community and ecosystem processes.

The information gained in community ecology studies is becoming increasingly important to achieving conservation objectives and establishing guidelines to management of the natural systems on which all humans depend. For example, clearer understanding of linkages established in communities through trophic exchanges can help predict the impacts of concentrations of toxins and pollutants. Better understanding of organismic interactions in a community context can help in comprehending the processes that lead to extinction of species, information critical to attempts to slow the loss of biological diversity. Research into community dynamics can result in better decisions about establishing preserves and refuges, and can create sustainable harvesting strategies.

The professional ecology journals do not publish research on the importance of community for the human species. Yet, few concepts are more significant in human affairs than the role of different kinds of community in establishing ties and linkages among human individuals and between humans and the locales in which they live. Equally important is the lack of community, the looseness of ties and the alienation from locale and from others that stem from failures to establish a sense of community. As in natural communities, interactions among human individuals create community and are in turn shaped by community.

[Gerald L. Young Ph.D. ]