Chapter 55: Community Ecology What are ecological communities? The species that live and interact in an area constitute an ecological community. Ecological communities are loose assemblages of organisms. The organisms in a community can be divided into trophic levels based on the source of their energy. Primary producers get their energy from sunlight. Herbivores that get their energy by eating primary producers are primary consumers; organisms that get their energy by eating herbivores are secondary consumers; and so on. A food chain diagrams who eats whom. A food web shows how food chains are interconnected in an ecological community. Other kinds of diagrams show how energy decreases as it flows from lower to higher trophic levels and the biomass of organisms present at each trophic level. Most of the energy ingested by organisms that is converted to biomass is eventually consumed by decomposers. What processes influence community structure? Species interactions fall into five categories: predation or parasitism benefits the predator while harming the prey; competition is harmful to all participants; mutualisms are associations that benefit all participants; commensal interactions benefit one party while neither harming nor benefiting the other; amensal interactions have no effect on one party but harm another. Predator–prey interactions typically undergo oscillations. As predator populations grow, they may eat most of their prey; the predator population then crashes. Predators may restrict the ways in which prey species make use of a habitat and may restrict the geographic distributions of their prey. Mimicry is an adaptation by prey to predation. In Batesian mimicry, a palatable species mimics an unpalatable species. In Müllerian mimicry, two or more unpalatable species converge to resemble one another. Microparasite populations can persist only if, on average, each infected host individual transmits the infection to one other individual. Competition may restrict the abundances and ranges of species. Interference competition makes foraging more difficult; exploitation competition depresses prey populations. In competitive exclusion, one species prevents all members of another species from using a habitat. How do species interactions cause trophic cascades? A predator, by reducing populations of its prey, may cause a trophic cascade of indirect effects across successively lower trophic levels. Trophic cascades may project across habitats. Ecosystem engineers are organisms that build structures that create environments for other species. A keystone species affects the entire community out of proportion to its abundance. How do disturbances affect ecological communities? A disturbance is an event that changes the survival rate of one or more species in a community. Ecological succession is a change in community composition following a disturbance. Primary succession begins on sites that lack living organisms. Secondary succession begins on sites where some organisms have survived the most recent disturbance. The intermediate disturbance hypothesis explains why communities with intermediate levels of disturbance often have more species than communities with very low or high levels of disturbance. Species that have already become established may facilitate or inhibit colonization by other species. What determines species richness in ecological communities? The number of species living in a community is its species richness. More species of most clades are found in low-latitude than in high-latitude regions. Species richness often increases with productivity, but only up to a point. Species-rich ecosystems tend to vary less in both productivity and species composition than species-poor ecosystems.