COLONIZATION OF RAKATA (KRAKATAU IS) BY NONMIGRANT LAND BIRDS FROM 1883 TO 1992 AND IMPLICATIONS FOR THE VALUE OF ISLAND EQUILIBRIUM-THEORY

Colonization curves and changes in immigration and extinction rates of non-migrant land birds since the 1883 eruption are provided for the recolonization of Rakata (Krakatau Is.), including results of surveys made in 1990 and 1992. The contention by Bush & Whittaker (1991) that non-monotonic changes in observed immigration and extinction rates of birds and butterflies cast doubt on the ability of the equilibrium theory of island biogeography (MacArthur & Wilson, 1967) to provide the basis for a predictive recolonization model is examined in the light of our recent data. We conclude that rates of immigration and extinction are highly sensitive to decisions as to immigrant status or extinction of species, particularly where intersurvey intervals are short and turnover numbers are low. We note that for no group of organisms on the island is there an equilibrium in number of species, most of the observed turnover having been successional. Contrary to Bush & Whittaker (1991), the number of non-migrant land bird species has not declined since 1951 and numbers are still rising, albeit more slowly than in the early decades of this century. The colonization curve is flattening, and immigration rate is falling towards extinction rate, in general although not precise accord with the model. On comparing the colonization dynamics of birds with those of plants and butterflies and mindful of the qualifications MacArthur & Wilson (1967) placed on the applicability of their model to cases where succession is important in the early stages of colonization, we conclude that it is premature to discard their dynamic approach as a theoretical framework for the study of animal colonization of the Krakataus.