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 should advance studies of biogeography, phylogeny, host-symbiont interactions, and 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 georeferenced voucher specimens to summarize the extent and timing of past collecting efforts for small mammals in woodland habitats across the Madrean Sky Islands. In total, at least 88 species (35 bats, 50 rodents, 3 shrews) from 9,540 specimens were collected from 1884-2023. Of these specimens, 79% come from 5 sky islands (Chiricahuas, Pinalenos, Huachucas, Animas, and Santa Catalinas) and at least 23 sky islands in the Madrean system have no recorded specimens. Mexico's 25 sky islands are mostly unsampled (only the San Luis, Sierra dos Ajos, and Sierra La Mariquita have any georeferenced specimens) and several of Arizona's larger sky islands have fewer than 40 specimens (Galiuros, Canelo Hills, Santa Teresas, Mules, and Dragoons). Most small mammal specimens (88%) were collected in 1980 or earlier without prioritizing frozen tissues for DNA/RNA preservation. Including 3,946 non-georeferenced specimens may add sampling for another 6 sky islands and 6 species, but georeferencing confirmation is needed and major patterns are unchanged. 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. There is a critical need for future fieldwork and voucher specimen preservation (including of flash-frozen tissues) in this region, both to monitor changes compared to historical sampling and to test biodiversity assumptions on unsampled mountains.