Colonization of the Krakatau Islands by vertebrates: equilibrium, succession, and possible delayed extinction

MacArthur and Wilson [MacArthur, R. H. & Wilson, E. O. (1967) The Theory of Island Biogeography (Princeton Univ. Press, Princeton, NJ)] used the rate of recolonization of the Krakatau Islands (sterilized in the cataclysmic 1883 eruption) by birds and vascular plants to test their equilibrium model of immigration and extinction processes on islands. Working with data only up to 1933, they concluded that the number of resident land bird species had approached equilibrium by 1908-1919, 25-36 years after the eruption, when the number of vascular plant species was still increasing. Recent surveys of vertebrates on the archipelago show continuing colonization, as well as species turnover. Nonmigrant nonmarine birds and reptiles are only now approaching equilibrium species numbers; archipelago acquisition rates for these groups are lower than in the 40 years following the 1883 eruption. Apparent (minimal) annual extinction rates are an order of magnitude lower than those calculated for equilibrium by MacArthur and Wilson. The active volcano Anak Krakatau emerged in 1930 and suffered an eruption in 1952 that destroyed the vegetation. It is still at an early stage of succession and on the threshold of major vegetational change. This island and a physically dynamic, ever-young spit on Sertung I, also held at an early successional stage, may provide (i) ecological refuges for some species whose optimal habitat on the older islands is being extirpated by vegetational succession and (ii) ecological "windows" through which such species may still establish from the mainland, thus postponing their extinction on the archipelago.