Turnover and Equilibria in Insular Avifaunas, with Special Reference to the California Channel Islands

Abstract

According to current biogeographical theory, the number of species comprising an insular biota is the result of a dynamic interaction between extinction and immigration rates. These rates, in turn, are thought to depend primarily on island size and distance from sources of potential colonists (MacArthur and Wilson 1963, 1967). This equilibrium model has been tested in a number of ways, including studies of recolonization of Krakatau, an island whose entire biota was destroyed by a natural catastrophe (MacArthur and Wilson 1967, after Dammerman 1948; Docters van Leeuwen 1936), tallies of the species of terrestrial arthropods present on tiny mangrove islands before and after intentional defaunation (Simberloff 1969; Simberloff and Wilson 1969, 1970; Wilson and Simberloff 1969), and through comparisons of the results of repeated surveys of insular avifaunas (Diamond 1969, 1971; Hunt and Hunt 1974; Terborgh and Faaborg 1973). It is understandable that ornithology should play an important role in the maturation of this body of theory, as it has in other areas of evolutionary biology. Compared with most other kinds of organisms, birds are relatively easy to observe and are well known taxonomically and biologically. Moreover, birds have been studied over a sufficient period in some regions so that it is possible to compare recent surveys with others going back 50 years or