The field of biogeography is concerned with the distribution of species in relation both to geography and to other species. Biogeography comprises two disciplines: historical biogeography, which is concerned with the origins and evolutionary histories of species on a long time scale, and ecological biogeography, which deals with the current interactions of species with their environments and each other on a much shorter time scale.
Historical biogeographers depend heavily on evidence from other disciplines. Fossil records provide a large part of the information needed to determine distributions and past interactions. Molecular biology furnished historical biogeographers with molecular clocks, metabolic molecules whose change over time help track the relatedness of species.
Historical biogeographers also make use of a tool called an area cladogram. This diagram is made by taking a taxonomic tree, which shows various species and their relatedness, and replacing the species names with the geographic location in which those species are found. This new tree allows scientists to determine how the differences in environments have effected the evolutionary history of different species of common origin. A sample area cladogram is shown in .
Unlike historical biogeographers, ecological biogeographers make extensive use of current population information. They study the ways in which species develop and interact in the presence of other species and different environments. Many ecological biogeographers mimic Darwin: they study island communities as a type of experimental system to test hypotheses about species development.
Much of ecological biogeography is concerned with species richness, the number of different species an area supports. In specific, ecological biographers have developed the species richness equilibrium model. The model begins with an uninhabited "island" that can be either a literal island or an area of like habitats completely surrounded by unlike habitats. All species available to colonize the new area are called the "species pool." As more and more new species enter the new area, the species pool becomes smaller and smaller, and the immigration rate (the probability that any given species moving into the area will be a new species) decreases. At the same time, the island becomes more and more crowded and supplies become scarce, causing the extinction rate to increase. The point at which the extinction rate and the immigration rate balance is called the equilibrium point. The model predicts that changes in extinction and immigration rates will tend toward the equilibrium point, which is different for every island, depending on resources and degree of separation from other areas. This is shown graphically in the figure below.