Great Basin Naturalist Memoirs


The present paper compares the distribution of boreal birds and mammals among the isolated mountain ranges of the Great Basin and relates those patterns to the developing theory of insular biogeography. The results indicate that the distribution of permanent resident bird species represents an approximate equilibrium between contemporary rates of colonization and extinction. A shallow slope of the species-area curve (Z = 0.165), no significant reduction in numbers of species as a function of insular isolation (distance to nearest continent), and a strong dependence of species diversity on habitat diversity all suggest that immigration rates of boreal birds are sufficiently high to maintain populations on almost all islands where there are appropriate habitats. In contrast, the insular faunas of boreal mammals represent relictual populations that receive no significant contemporary immigration. The insular mammal faunas have been derived by extinction from a set of species that colonized the islands when habitat bridges connected them to the continents in the late Pleistocene. A relatively steep species-area curve (Z = 0.326), no effect of isolation on species diversity, and the absence of appropriate species from large areas of apparently suitable habitat all support this conclusion. Measures of habitat diversity that are closely correlated with bird species diversity do not account for much of the variation in number of mammal species among islands. Insular area is the single variable that accounts for most of the variability in both bird and mammal species diversity; this supports the approach of using standard parameters such as area in comparative empirical analyses and general biogeographic theory. The results of this study suggest that extremes of vagility among taxa and a recent history of paleoclimatic and geological changes make it unlikely that equilibrial distributions, of the sort MacArthur and Wilson (1967) propose for the biotas of oceanic islands, are characteristic of the insular distributions of terrestrial and freshwater vertebrates of western North America.