island biogeography, theory of A theory, advanced in 1967 by R. H. MacArthur and E. O. Wilson and now shown to be too simplistic, so largely discredited, that the number of species on an island will reach a dynamic equilibrium between the continual immigration of species from a mainland source and the extinction of species already present. Once equilibrium is reached the species number will remain constant, but with a continually changing composition. The theory goes on to state that if the immigration rates and the extinction rates are known, then the species number at equilibrium can be calculated. They theory fails, however, to take account of species interaction or of habitat diversity which is, de facto, usually greater on big islands; it also makes a major and possibly erroneous assumption that immigration is independent of island size. The theory also compares topographically identical near and far islands in relation to the mainland source, maintaining that near islands would have a higher rate of immigration than far ones because the biogeographical barrier presented by the sea would be smaller. It also states that the extinction rate would be independent of island location, and therefore the resulting equilibrium point for the number of species would be higher for the nearer islands. The assumption that the extinction rate is independent of island location may be incorrect because of the rescue effect.