Nonequilibrial 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. It is suggested that 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. Note added in proof.-The turnover rates on tropical islands have been discussed recently by Slud (1976). He points out that the small and far Cocos Island has neither lost nor gained a species in 72 years. During a similar period the much closer and larger Tres Marias Islands have remained constant in species number and composition (Grant and Cowan 1964), or at most gained two immigrants (tables 1 and 2). Cocos and one of the Tres Marias (Cleofas) have not been disturbed by man, and the early censuses were thorough, so natural rates of change appear to be very low. Where habitats have been altered and/or early censuses were incomplete (Mona and Karkar), calculated rates of change are probably artificially high (Lynch and Johnson 1974; Slud 1976).