Uncovering historical small mammal biodiversity among the Madrean Sky Islands

The Madrean Sky Island Archipelago is a system of 54 mountains with isolated woodland habitat above 1,600 meters, primarily in the Sonoran Desert. These mountains harbor a wide variety of native small mammals spanning 11 families of bats, rodents and shrews. Improved understanding of Madrean Sky Island biodiversity will potentially advance studies of biogeography, phylogenetic relationships, host-symbiont interactions, and processes of community assembly in this ecoregion. However, which species are found in each sky island and how their populations are genetically related remain open questions. To establish the current knowledge baseline, we used voucher specimen geocoordinates and elevations to summarize the extent and timing of past collecting efforts for small mammals in woodland habitats across the Madrean Sky Islands. In total, 97 species of small mammals (39 bats, 54 rodents, 4 shrews) from 9,541 specimens were collected from 1884 to 2023. Of these historical specimens, 79% come from five sky islands (Chiricahuas, Pinalenos, Huachucas, Animas, and Santa Catalinas) and only 25 sky islands in the Madrean system have any recorded specimens. Mexico's 25 sky islands are mostly unsampled (only the San Luis, Sierra dos Ajos, and Sierra La Mariquita have any specimens) and several of Arizona's larger sky islands have fewer than 40 specimens (Galiuros, Canelo Hills, Santa Teresas, Mules, and Dragoons). A large majority of small mammal specimens (87%) were collected prior to 1980, meaning they were collected without DNA/RNA preservation as a priority. This distributional summary is the current basis for all derived biodiversity knowledge of Madrean Sky Island small mammals, illustrating clear gaps regarding most species of woodland-dwelling bats, rodents, and shrews. This work lays the foundation for future fieldwork and voucher specimen preservation in the Madrean Sky Islands, especially from undersampled mountains where biodiversity assumptions are unconfirmed.