Learning Objectives

18.3.1 Graph and explain the species-area relationship and know why it differs between islands and mainland areas.

18.3.2 Explain regional species diversity for islands and island-like areas using the equilibrium theory of island biogeography.

An important thread that runs through this chapter, and through biogeography generally, is the relationship between species richness and geographic area. We saw in the Case Study that large fragments of Amazon rainforest had greater species richness than smaller fragments. In our global tour of the world's forests, we saw that species diversity was greatest in the tropics (see Table 18.1), the climate zone whose geographic area is largest (see Figure 18.15A). This so- called species-area relationship, in which species richness increases with the area sampled, has been documented at a variety of spatial scales, from small ponds to whole continents. Most studies of species-area relationships have been targeted at regional spatial scales, where these relationships tend to be good predictors of differences in species richness.