Summary of Chapter Concepts

Communities can have distinct or gradual boundaries. Communities frequently exist in zones that reflect changes in biotic and abiotic conditions. Based on the aggregations of species, ecologists categorize communities in terms of the dominant biological or physical conditions. Although some communities have distinct boundaries as a result of sharp changes in environmental conditions, most communities contain species with geographic ranges that are independent of each other, which produces gradual changes in the composition of communities as one moves across the landscape.

The diversity of a community incorporates both the number and relative abundance of species. Species richness is the number of species in a community whereas species evenness is the similarity of relative abundance among species in a community. Rank-abundance curves are graphical representations of richness and evenness. When we plot categories of individual abundance against the number of species that fall within each category, communities commonly exhibit a log-normal distribution, which means that few species have either few or many individuals and that most species have an intermediate number of individuals.

Species diversity is affected by resources, habitat diversity, keystone species, and disturbance. Increases in the abundance of resources and in the number of different resources can alter species diversity. Increases in habitat diversity offer a greater diversity of niches, which favors higher species diversity. Keystone species can alter the composition of species in a community because they have large effects even though they are not particularly numerous. Disturbances also affect the species diversity of communities, with the greatest diversity occurring in communities experiencing disturbances that are intermediate in frequency or intensity.

Communities are organized into food webs. Food chains are linear representations of how different species in a community feed on each other, whereas food webs are more complex and realistic representations. Species in a food web can be categorized in trophic levels and guilds according to how they obtain their energy: producers, primary consumers, secondary consumers, and tertiary consumers. Because species exist within food webs, the abundance of each species is affected not only by direct effects but also by density- and trait-mediated indirect effects. Because of these indirect effects, communities can experience trophic cascades from the top of the food chain down, and from the bottom up.

Communities respond to disturbances with resistance, resilience, or switching among alternative stable states. Resistant communities show little or no response to a disturbance. Resilient communities can be affected by a disturbance but they bounce back to their original states relatively quickly. Some communities respond to large disturbances by moving to alternative stable states in which the community persists with a different composition of species for a relatively long time.