NESTED FAUNAS AND EXTINCTION IN FRAGMENTED HABITATS

The faunas of habitat islands, such as those produced by fragmentation of formerly continuous habitats, are commonly made up of nonrandom subsets of the total available species pool. Faunas within an archipelago may form a nested series, with depauperate faunas made up of subsets of more species-rich faunas. The pattern is seldom perfect: wide-spread species may be absent from otherwise rich faunas (holes), and uncommon species may occur in depauperate faunas (outliers). The "nestedness" of an assemblage of faunas (its fit to the "nested subset model"; Patterson & Atmar 1986) can be measured by summing the holes and outliers. The distributions of boreal mammals and birds among mountain ranges in the Great Basin of western North America were analyzed by this method. Despite differences in their derivation (mammalian faunas are thought to be relicts, isolated since the Pleistocene; bird faunas have probably experienced recurrent colonization throughout their history) both groups show an approximately equal (and highly significant) fit to the nested subset model. They differ, however, in the relative numbers of holes and outliers in their patterns. The mammalian pattern is hole-rich; the bird pattern is outlier-rich. The present-day composition of the mammalian faunas is the result of selective extinction of species of originally richer faunas. The existence of nested subsets in these faunas suggests that extinction is a highly deterministic process: extinctions occurred in approximately the same sequence throughout the region, despite wide variation in extinction rates. Extinction sequence has, in fact, been less variable than extinction rates.