Biogeographic Scale and Biodiversity of Mountain Forest Mammals of Western North America

Abstract

This paper develops a multi-scale approach to the analysis of the influence of immigration and extinction on community structure of non-volant mammals on montane forest islands of western North America. Patterns were studied at three ecological scales (species richness, community nestedness and distributions of individual species) and four spatial scales (intra-archipelago, archipelago, inter- archipelago and regional scales) to investigate the influence of scale on our ability to perceive patterns and assess causality in biogeography. Consistent with Brown's relaxation hypothesis, community structure of mammals of the Great Basin mountains appears to have been strongly influenced by extinction. Species richness and community nestedness of these mammals were significantly associated with area (P 0.30). Analyses conducted at finer scales, however, suggested that these communities may also be influenced by immigration. In the northern portion of the Great Basin, where immigration filters are less severe, richness tended to decline with isolation, and community nestedness was as strongly associated with isolation as it was with area (P=0.090). Analyses of distributions of eight species occurring on between five and thirteen montane forests in the Great Basin (entire archipelago) revealed that the distributions of three species were significantly (P<0.01) associated with both isolation and area. Distributions of four other species were associated with just area, while that of the remaining species was independent of area and isolation. Species richness and community nestedness of mammals in the American Southwest archipelago were significantly associated with isolation and with area (P<0.01). Of the seven species included in analyses of distribution patterns, three were significantly associated with both isolation and area, two with just area and two with just isolation. These results are consistent with the hypothesis that, in comparison to the Great Basin, immigration occurred more often for mammals of the American Southwest and its effects are more obvious at coarse, as well as fine ecological scales. Species richness and nestedness of mammals in the Cordillero-Madrean Region were more strongly associated with isolation than they were with area. Of six species occurring within seven to fourteen montane forests in this region, the distribution of one was significantly associated with both area (P= 0.043) and isolation (P= 0.004), while distributions of the remaining five were significantly associated with just isolation (P<0.001). These results indicate that montane forest islands of western North America are complex systems, isolated by a heterogeneous mix of immigration filters. Distributions of the component species are determined by interactions between system and-species characteristics that influence immigration and extinction. The effects of immigration and extinction should be most evident for the fine-scale ecological patterns, i.e. patterns in distributions of individual species. As the spatial scale broadens, however, the influence of these fundamental, biogeographic forces becomes evident for more species and at coarse ecological scales, involving not just patterns in insular distributions, but patterns in nestedness and richness as well.