Section 2

Patterns in Communities

• Species richness is the number of species in a community. Species evenness is the relative abundance of each species.

• In general, species richness is greatest near the equator, and larger areas support more species. Species interactions such as predation can promote species richness.

• Disturbances can alter a community by eliminating or removing organisms or altering resource availability.

• Species richness may improve a community's stability. Areas of low species richness may be less stable in the event of an ecological disturbance.

• Ecological succession is a change in the species composition of a community over time. Primary succession is the assembly of a community on newly created habitat. Secondary succession is the change in an existing community following a disturbance.

• Primary succession occurs in areas that have been recently exposed to the elements and lack soil. Primary succession typically proceeds from lichens and mosses to a climax community.

• Secondary succession occurs in areas where the original ecosystem has been cleared by a disturbance. Secondary succession typically proceeds from weeds to a climax community.

Vocabulary species richness (p. 405) species evenness (p. 405) species-area effect (p. 406)

disturbance (p. 407) stability (p. 407) ecological succession (p. 408)

primary succession (p. 408) secondary succession (p. 408) pioneer species (p. 408)

climax community (p. 410)

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