UC Irvine

With the study of the equilibrium theory of island biogeography pioneered by MacArthur and Wilson ( 1963, 1967), increasing attention has been paid to the study of turnover rates (Mayr 1965; Diamond 1969, 1971; Power 1972; Terborgh and Faaberg 1973). The failure of bird species turnover rates to show the predicted negative correlation with island size in the California Channel Islands (Diamond 1969) raises questions as to what factors other than island size may be involved in determining species turnover. · Previous studies of avifaunal turnover have treated faunas as a whole and have not differentiated turnover rates for rare species from those of common species. Since very large birds and species at high levels on the food chain often exist at relatively low densities, these organisms would be expected to have a greater probability of extinction on small islands than those species which maintain larger populations (Brown 1971). A theoretical basis for this hypothesis can be derived from Gilpin ( 197 4). He has shown ( pers. comm.) that extinction rates should be a negative exponential function of K, and thus should be very sensitive to small population size.