AVIFAUNAL EQUILIBRIA AND SPECIES TURNOVER RATES ON THE CHANNEL ISLANDS OF CALIFORNIA

Insular species diversities, and their dependence on island size and isolation, have been postulated to represent a dynamic equilibrium between species immigration rates and species extinction rates. This interpretation has been tested by determining the land and freshwater birds breeding on the nine Channel Islands off southern California in 1968 and comparing the results with a similar survey for the years up to 1917. Most of the islands were found to be in equilibrium as to number of species, but between 17 and 62 per cent of the 1917 breeding species had disappeared by 1968, and an approximately equal number of new immigrant species had become established. Percentage turnover rates vary inversely as insular species diversities, with no effect of distance apparent.