Island area and diet predict diversity and distribution of bats in a Pacific Northwest archipelago

Abstract The island biogeography theory predicts that species richness in islands and island-like systems is the ultimate result of island isolation and area. Species with high dispersal capabilities are predicted to be less affected by these factors because of their capacity to move more efficiently between islands or habitats, and here we test this idea in bats, the only mammals capable of flight. We conducted mist net and acoustic surveys across 21 islands in the San Juan Archipelago (Washington State, United States) and adjacent northwest mainland to: (i) investigate the effects of island area, distance from mainland, and habitat on bat diversity; and (ii) evaluate whether differences in morphological (body mass, forearm length, wing loading) and ecological (dietary niche breadth, foraging guild) traits among species influence their prevalence across islands. We found that island size strongly influenced patterns of species richness, with larger islands having a greater number of bat species. However, neither island distance from mainland nor any measure of habitat availability was a significant predictor of species richness at the scale of this study. Additionally, we found that dietary niche breadth, as opposed to any morphological trait, best predicted the prevalence of species across the islands. Our results suggest that species with more specialized diets may be more vulnerable to habitat fragmentation, and provide insight into how geographic and ecological factors affect the diversity of insular bat communities, adding to growing knowledge about the role of species traits as mediators of their responses to large-scale landscape structure. As the only flying mammals, bats are a particularly exciting system to test predictions from island biogeography theory, including the effects island characteristics on species diversity. We surveyed bat communities across 21 islands in a near-shore archipelago in Washington State, United States, and report that island size—rather than distance from mainland or habitat quality—best explains bat species diversity. We also find that dietary specialization within an insectivorous niche influences the prevalence of bat species among the islands. Our results provide new evidence underscoring the role of ecological traits in mediating mammal species distributions in fragmented landscapes.