Chapter 9 - Community Structure

Abstract

Communities are composed of the species occupying a site. Communities have been described and compared in terms of species diversity, food web structure, and functional group organization. Species diversity can be measured as species richness and evenness. Food web structure represents the network of pairwise interactions among the species in the community. Functional group organization reflects combination of species on the basis of their effects on ecological processes, regardless of taxonomic affiliation. Many taxa show latitudinal gradients in abundance, with species richness increasing toward the equator. However, the climate gradient thought to underlie this trend is correlated with latitudinal gradients in habitat area and productivity. Habitat area and stability, resource availability, and species interactions are major factors that affect community structure. Habitat area affects the pool of species available and the heterogeneity of habitat conditions and resources. Competition, predation, and mutualism also affect species directly and indirectly. Keystone species include predators that focus on the most abundant prey species, thereby reducing competition among prey species and maintaining more species than would coexist in the absence of the predator. Predators also may maintain trophic cascades that regulate food web structure.