Community Structure

Community structure refers to the organization of a biological community based on numbers of individuals within different taxonomic groups and functional roles, and the underlying processes that maintain that organization. Explorations of this topic center around a few core concepts: that communities are nonrandom assemblages whose interactions result in predictable and repeatable patterns, that existing communities are stable and resist change when challenged by the normal range of environmental conditions and invasion by members of the existing species pool, and that structure emerges from a combination of habitat matching and species interactions. Variation in the composition of the regional species pool and local environmental conditions over space and time result in much variability in natural communities, but the composition of particular communities nonetheless is governed by a small number of underlying principles (Begon et al. 2005). The countering view states that assemblages are an unstructured sample of whichever species are able to survive and reproduce under local environmental conditions, changing as conditions change and by chance. A more formalized version, the neutral model of Hubbell (2001), considers all species to be essentially interchangeable, and so random replacement following stochastic colonization and extinction determines the momentary composition of short-lived assemblages.

The discussion of community structure and the rules that might govern community assembly has generated a rich literature in ecology. To enter into this topic, it is useful to distinguish some key ideas, keeping in mind that they are not fully independent. Niche-based models focus on the interplay between biotic interactions (usually predation, herbivory, and competition) and abiotic forces (primarily habitat and disturbance) that determine the suitability of a place for a particular species. The habitat template model (Southwood 1988) emphasizes the association of species with habitat features, such that individual species occur where they are best suited and more species are found where habitat conditions are most diverse. In this long-favored explanation of stream community structure, the physiological, morphological, behavioral, and life history attributes of individual species determine which will successfully colonize and maintain populations in a particular environment. Both abiotic and biotic factors can be visualized as a series of filters that determine the subset of the regional species pool that is most likely to successfully colonize and maintain populations.

Disturbance models emphasize the interplay between species interactions and variation in flow, temperature, and other environmental fac tors that periodically reduce the abundance of some or all species in an assemblage. Because predation, competition, and herbivory can potentially eliminate local populations, disturbance can limit the effectiveness of strongly interacting species, and facilitate or prevent recolonization by displaced species. A focus on disturbance seems appropriate to fluvial ecosystems because they appear to be highly variable and occasionally harsh environments. In addition, benthic invertebrates and algae are patchily distributed, and this suggests that disturbance, biotic interactions, and recolonization may combine to govern population dynamics at the local scale (Townsend 1989). Because dispersal ability varies among species, individual mobility, propensity to drift, and aerial flight ability all are important traits that may permit the persistence of weak competitors and vulnerable prey in environments where they might otherwise lose out.

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