Island biogeography is the study of plant and distributions on islands. Studies of this field boomed soon after the publication of MacArthur and Wilson's seminal "Theory of island biogeography" in 1967. Their theory is simple: the number of kinds of plants and animals on an undisturbed island — that is, a natural island unaffected by man or other calamity—is determined by two processes, immigration and extinction. In other words, the number of species on an island is the sum of the species that arrive, breed and live there successfully, minus the number of species that arrive but fall to breed or that eventually become extinct. If a new island starting with zero kinds of birds lies near a mainland that has 100 kinds of birds, then a certain percent of those mainland species are eventually going to find their way to the island. When the first species arrives and establishes itself, the potential number of immigrants decreases by one, since there are now only 99 immigrant species available from the mainland. At the same time, the potential for extinctions increases by one, because with the arrival of the first species, there is now also one species that could become extinct, where at first there was one. You may have heard of Krakatoa, which was famously all but destroyed by a volcanic explosion, exterminating every living thing on it., back in 1883. Well, between 1883 and 1933, 34 species of birds became established there, but 5 of them also became extinct. As equilibrium approaches, turnover continues, but the total number of different species levels odd. When the over all bird population finally reaches a mature equilibrium—when the arrival rate of new species balances the extinction rate of unsuccessful species—the island may host anywhere from only a few to almost all of those 100 mainland species, depending on the island's overall receptivity.