Patchy distribution of a Madrean Sky Island squirrel shaped by historical habitat configuration

Abstract

Sky islands, mountain-top forests isolated by surrounding lowlands, offer unique opportunities to test how past and present landscapes shape species distributions. We examined the distribution of the Arizona gray squirrel (Sciurus arizonensis) across the Madrean Archipelago to test the constraint-based dynamic island biogeography (C-DIB) model, which posits that current occupancy in the sky islands reflects historical habitat size and connectivity. Using verified specimen records, we modeled climatically suitable habitats across four time periods: the Last Glacial Maximum (LGM), Mid-Holocene (MH), Present, and Future. For each mountain, we quantified suitable habitat area and estimated least-cost dispersal distances to assess both persistence and colonization potential. Our results suggest that species presence is best explained by LGM habitat metrics, which marginally outperformed models based on current conditions. Mountains that were large or well-connected during the LGM continue to support S. arizonensis, whereas historically isolated ranges remain unoccupied despite suitable contemporary habitat. These findings indicate a legacy of Pleistocene connectivity and reveal patterns of distributional disequilibrium. Furthermore, climatically suitable habitat for S. arizonensis has shifted both elevationally and geographically through time, reflecting long-term responses to climatic change. Together, these results emphasize the importance of protecting historically connected refugia, restoring riparian corridors that facilitate dispersal, and developing mountain range-specific management strategies that account for elevational shifts and potential downslope habitat recovery under future climate scenarios.