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 m, 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 advance studies of biogeography, phylogeny, host-symbiont interactions, and community assembly in this ecoregion. However, which species reside 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 in woodland habitats across the Madrean Sky Islands. In total, at least 88 species (35 bats, 50 rodents, and 3 shrews) from 9,540 specimens were collected from 1884 to 2023. Of these specimens, 79% come from 5 sky islands (Chiricahuas, Pinaleños, Huachucas, Animas, and Santa Catalinas). 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 confirmed specimens), and several of Arizona's larger sky islands have fewer than 40 specimens (Galiuros, Canelo Hills, Santa Teresas, Mules, 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 adds tentative sampling for another 6 sky islands and 6 species, but georeferencing confirmation is needed, and major patterns are unchanged. This summary forms the 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. Future fieldwork and voucher specimen preservation (including of flash-frozen tissues) are critically needed in this region, both to monitor changes compared to historical sampling and to test biodiversity assumptions on unsampled mountains.