Chapter Summary

• Communities vary in size and scope, and it is often impractical to define their boundaries. Ecologists often focus on a subset of species in a community based on similarities in taxonomy, resource use (guilds), function (functional groups), or trophic and energetic connections (food webs). Review Figure 56.2

• Food webs can be divided into trophic levels based on how groups of species interact and obtain energy. Food webs identify primary producers, primary consumers (herbivores), secondary consumers (carnivores), and so on. Omnivores feed on multiple trophic levels, and detritivores feed on the waste products and dead bodies of organisms. Review Figure 56.3, Activity 56.1

• Species diversity and composition are important descriptors of community structure. The Shannon index is a measure of species diversity that combines the number of species (species richness) and their relative abundances compared with other species (species evenness). Review Figure 56.4, Table 56.1, Activity 56.2

• Biodiversity is a term used to span multiple scales of diversity, from genes to species to communities. Review Figure 56.5

• The three basic factors responsible for community membership are (1) the regional species pool and dispersal ability (species supply), (2) physical and chemical environmental conditions (abiotic conditions), and (3) species interactions (biotic conditions). These three factors act as “filters” that exclude (or include) species in communities. Review Focus: Key Figure 56.6

• Direct interactions occur between two species. Indirect interactions occur when the direct relationships between two species are mediated by a third (or more) species. Review Figure 56.8

• A trophic cascade occurs when the rate of consumption at one trophic level results in a change in species abundance or composition at lower trophic levels. Review Figure 56.9

• Some species have especially strong negative effects on community structure, resulting in resource partitioning or resource-mediated coexistence. The intermediate disturbance hypothesis proposes that disturbance promotes species diversity, in part by reducing the negative effects of dominant competitive species. Review Figure 56.10

• Keystone species influence community structure disproportionally to their size or abundance, producing trophic cascades. Foundation species, by providing food and habitat for other species, have a large effect on communities as a consequence of their large size or great abundance. Ecosystem engineering species affect community structure by creating, modifying, or maintaining physical habitat for themselves and other species irrespective of their size or abundance. Review Figures 56.11, 56.12

• In communities where species interactions are more equivalent, lottery, or neutral, models propose that as long as all individuals have similar chances of obtaining resources, and no clear advantage in population growth, then their presence in the community should be maintained by chance events that free up resources for competing individuals.

• Communities constantly undergo change in species composition over time as a result of abiotic and biotic factors, a process known as succession. Disturbance and stress are two significant forms of abiotic-induced community change. A disturbance is an abiotic event that physically or chemically injures or kills individuals, whereas stress occurs when some abiotic factor reduces the growth, reproduction, or ultimately survival of individuals. Review Figure 56.14

• There are two types of succession: primary succession is the recovery of a community after a catastrophic disturbance kills all the organisms in it, and secondary succession is the reestablishment of a community when most, but not all, organisms have been destroyed. Review Figure 56.15, Animation 56.1, Investigating Life: Rising from the Ashes

• Facilitation and inhibition influence primary and secondary succession.

• The outcome of succession is not always predictable; alternative successional pathways, or regime shifts, lead to different community assemblages under similar environmental conditions. Hysteresis occurs when a community is unable to shift back to its original state even when the original conditions are restored. Review Figure 56.17

• Studies show that the relationships between species diversity and community function (measured as productivity and stability) are often positive. Stability is a measure of resistance to, or recovery from, disturbance. Review Figure 56.18

• Species diversity–community function relationships can be used in agricultural settings to increase crop productivity.