Island Biogeograhy of Montane Forest Mammals in the American Southwest

Information from packrat (Neotoma spp.) middens and other paleoenvironmental indicators was used to reconstruct Pleistocene distributions of macrohabitats in the American Southwest (Arizona, New Mexico, and southern Utah and Colorado). This revealed a system of southern montane forests that were isolated during the Pleistocene (primarily by woodlands) as well as today by grasslands, chaparral, and desertscrub). Based on the presence of at least five species of nonvolant forest mammals on these montane islands, we inferred that these mammals immigrated across woodlands during the Pleistocene. Because the great majority of montane forests in the American Southwest are now isolated by woodlands, but not grasslands, chaparral, or desertscrub, we hypothesized that post—Pleistocene immigrations may influence the structure of mammalian communities on these montane islands. This hypothesis was supported by the highly significant correlation of mammalian species richness with current isolation as well as area (P < .01 for simple correlations and for partial correlations after removing effects of latitude and area or isolation: 26 species, 27 islands). When isolation was partitioned into distance to be traveled across woodland vs. grassland—chaparral habitats, species richness was significantly correlated with the latter measure of isolation (P < .01) but not with the former, indicating that woodlands do not represent major barriers to immigration. Seventeen of 26 species considered are reported with at least 10% of their record locations in woodlands and other low—elevation habitats. Analyses of patterns of occurrence of seven species of forest mammals inhabiting between 7 and 16 islands revealed that insular distribution was significantly (P < .01) affected by area in two species, by area and isolation in three species, and by isolation alone in the remaining two species. Based on our reconstruction on Pleistocene macrohabitat distributions, analyses of community— and species—level patterns, and evidence on the ability of forest mammals to inhabit and disperse across woodlands and other low—elevation habitats, we conclude that community structure of montane forest mammals in this region of the American Southwest is influenced by post—Pleistocene immigrations as well as extinctions.