NUMBERS OF BIRD SPECIES ON THE CALIFORNIA ISLANDS.

Abstract

Variation in bird species diversity on islands is related to island area in a simple way. Correlations between numbers and area have been found for birds of the West Indies, East Indies, Madagascar and the Comoro Islands (Preston, 1962), and for a number of other archipelagos. In reality of course the situation is more complex. Area is often correlated with environmental diversity, a complex collection of factors which frequently exerts a more direct effect on species diversity than area itself (MacArthur, 1964; Watson, 1964). It is likely also that interactions with other species, both in competition and predation, figure predominantly (Crowell, 1962; Grant, 1966; Lack, 1969). And there are distance effects on rates of immigration and extinction such that those islands that are isolated tend to have fewer species than those near a source of propagules (MacArthur and Wilson, 1967). There have been two basic approaches to the study of variation of insular diversity. One is to consider numbers of species as a dependent variable and factors such as area, habitat diversity, topography, and distance from the nearest mainland or other islands as independent variables, and by multiple regression or similar analysis determine the components which explain statistically most of the variation in numbers. This has been done for numbers of land and freshwater bird species on various islands of the Moluccas, Melanesia, Micronesia, and Polynesia (MacArthur and Wilson, 1963), the islands of the Aegean Sea (Watson, 1964),