The theory of island biogeography was developed by MacArthur and Wilson (1963,1967). It is an equilibrium theory that indicates that the number of species on an island is determined by the ballance between extinction rates and imigration rates. In graphical form the equilibrium number of species (S) is determined by the intersection of the immigration (I) and extinction (E) curves. The immigration and extinction functions tend to be curved. The extinction curve rises as species diversity increases because competition between species can increase extinction. The immigration rate initially decreases more rapidly because some species are superior colonizers.

Small islands support fewer species than large islands primarily because they have higher extinction rates. Islands located near to the mainland tend to have more species primarily because they tend to have higher immigration rates.

The boundary conditions of the theory of island biogeography include the following:

1. frequent random extinctions

2. extinction is a function of species richness

3. the mainland species pool>>equilibrium number.

Simberloff and Wilson (1969) tested the theory of island biogeography on four small mangrove islands in the Florida Keys. They had the islands fumigated with methyl bromide and estimated the number of species prior to defaunation.

Phytoplankton in Lakes

Maguire(1963) set beakers running from a pond in Texas. Immigration rates followed the theory. The number of species proceeded to equilibrium in 2 months.

 If the number of species on an island obeys an equilibrium model, there should be a tendency for the island to return to the equilibrium number of species after a change in the number of species. In 1883 the island of Krakatoa experienced a seeries of volcanic eruptions that eliminated its flora and fauna. A number of expeditions to the remnant island called Rakatta described the recovering flora and fauna. Bush and Wittaker (1991) found that the patterns of immigration and extinction for plants on the island of Rakatta did not follow the equilibrium theory of island biogeography. Competitive and successional processes seemed to play a larger role in controlling the number of species that accumulated on the island. The immigration and extinction curves for butterflies and land birds tended to be controlled by plant succession and not the theory of island biogeography.