One characteristic of a community is species richness, the number of species in the community. A related measure is species evenness, which is the relative abundance of each species. These two measures provide slightly different information. Species richness is a simple count of the species in the community. Each species contributes one count to the total regardless of whether the species' population is one or 1 million.

In contrast, species evenness takes into account how common each species is in the community. To calculate the species evenness of a community, ecologists must measure or estimate the population size of all species in the community. In general, ecologists study both species richness and species evenness when they investigate communities.

Latitude and Species Richness

Species richness varies with latitude (distance from the equator). As a general rule, the closer a community is to the equator, the more species it will contain. Species richness is greatest in the tropical rain forests. For example, entomologists Edward O. Wilson and Terry Erwin identified nearly as many species of ants in a single tree in Peru as can be found in the entire British Isles.

Why do the Tropics contain more species than the temperate zones do? One hypothesis is that temperate habitats, having formed since the last Ice Age, are younger. Therefore, tropical habitats were not disturbed by the ice ages, but habitats closer to the poles were disturbed. Also, the climate is more stable in the Tropics. This stability allows species to specialize to a greater degree than they could in temperate regions, where the climate is more variable.

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