Biogeography
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 Biogeography
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
.
Figure2.1: A sample of an area cladogram
Ecological Biogeography
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.
Figure 2.2: rates for arrival of new species v. extinction of existing species. The
intersection of these two lines (S) gives the equilibrium number of species
