The model begins with an uninhabited "island" (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 probability that any given species that moves into the area
will be a new species, or the immigration rate, decreases. At the same time, the
island is becoming 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 are balanced 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.
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