The patterns of biogeography play out over global, regional, landscape, and local spatial scales (Focus: Key Figure 53.12). Spatial scales are interconnected in a hierarchical way, with patterns at one scale setting the conditions for patterns at other scales.

At the global scale, species are isolated from one another by continents or oceans, and for long periods of time. Thus the rates of three processes—speciation, extinction, and dispersal—help determine differences in biogeography across the globe (see Figure 53.12A). At the regional scale, large-scale climate conditions are relatively uniform and species are limited by their ability to disperse, or move between locations (see Figure 53.12B). All the species that are limited to a region are part of the regional species pool (also called gamma diversity). At the landscape scale, there are multiple community types—for example, lakes, rivers, forests, and prairies—arranged in a spatially heterogeneous way across the region (see Figure 53.12C). The change in the number of species from one community to another across the landscape is known as beta diversity. Finally, the local scale (also called alpha diversity) typically encompasses a single community—a group of species assembled together as the result of the physical environment, biotic interactions with other species, and the regional species pool (see Figure 53.12D).

In the remainder of this chapter we will consider the distributional and diversity patterns of organisms at global and regional biogeographic scales. Chapter 56 will consider the causes and consequences of diversity at community and local scales.