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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 …