Changes in Small Mammal Communities Over the Last 25,000 Years Show a Complex Relationship Between Composition, Traits, and Aridity

Many ecosystems today face increasingly frequent and extreme droughts. Small mammals can be harbingers of larger ecological changes, making them critical components for conservation. We use the fossil record to explore how small mammal communities respond to aridity changes. Recent, short-term droughts caused small mammals to increase in evenness, as dominant generalist species suffer severe population fluctuations. It has also been hypothesized that with increasing aridity, herbivore tooth crown heights increase to combat wear. Here, we examine community-level changes, including evenness, hypsodonty, and diet across a series of arid-semiarid cycles. We compare two caves: Natural Trap Cave (NTC), which is open and arid, and Samwell Cave (SC), which is closed and forested. Evenness decreased at both caves from the Late Pleistocene to the Late Holocene. At NTC, dietary generalists were more common in the Pleistocene (61%) with herbivores dominating in the Middle (62%) and Late Holocene (57%). However, at SC generalist taxa increased into the Holocene. The Pleistocene community at NTC had the highest percentage of taxa with low tooth crown heights (42%) halving into the Holocene. Changes in hypsodonty and a shift from generalist to herbivorous taxa at NTC do not align with anticipated responses to aridity, which decreased from the Pleistocene to the recent. Functional relationships of these communities to aridity may be more nuanced than previously thought. Bioavailability of water and local vegetation types at NTC can help parse out these trends. Pleistocene precipitation mostly accumulated during the winter, not aiding plant growth. Thus, generalist mammals who could survive on scant, arid-adapted plants and invertebrates dominated. Precipitation through the Holocene shifted to summer accumulation, allowing plants, like grasses, to colonize and herbivorous taxa to increase in abundance with corresponding shifts to high crowned communities.