NON-EQUILIBRIAL BIRD FAUNAS ON ISLANDS

The breeding land bird faunas of Australia, New Zealand, and 14 offshore islands are small relative to those of their source areas. This impoverishment is examined in terms of the MacArthur-Wilson equilibrium model. The number of nonpasserine species on the Australian/New Zealand islands appears to be at equilibrium, but the number of passerines is not and is generally increasing. Furthermore, the passerines do not support the postulate of the model which states that immigration rate to islands should decrease with distance from source. Replacement of species does not always match the simple expectation that, when a fauna is close to equilibrium, one competitor replaces another. The excess of immigrations of passerines over extinctions cannot be attributed solely to sampling error or to human influences. The lack of equilibrium is surprising because there has been time for equilibrium to be reached since the Pleistocene. At high latitudes islands such as these may not have fixed equilibria because as climatic conditions fluctuate irregularly, so do faunal equilibrium sizes. Very large annual changes in extinction and immigration rates of the avifauna of Skokholm Island off Wales support this suggestion.