Aridity drives the loss of dung beetle taxonomic and functional diversity in three contrasting deserts

Aim Aridity gradients are of great interest for understanding the responses of biodiversity to water availability and water stress. However, little is known about the responses of many animal groups, which are crucial for assessing the effects of climate change. Here, we study the effects of aridity on dung beetle communities, a group with well-known responses to large-scale environmental gradients. Location Sahara, Kalahari and Chihuahuan deserts. Taxa Dung beetles of the family Scarabaeidae. Methods We conducted standardized surveys along approximately 400 km aridity gradients in each of the three deserts, and measured species richness, abundance, evenness and three aspects of trait-based functional diversity (functional richness, functional evenness and functional dispersion). By using randomization tests and linear mixed models, we compared observed with expected values for functional diversity indices from null models that hierarchically incorporate additional assembly constraints. Results Overall, we found a decrease of both taxonomic richness and functional dispersion along the three aridity gradients. Also, aridity seems to have mild effects on functional richness and functional evenness. Besides these general trends, we identified differences between deserts in the responses of both taxonomic and functional diversity. Main conclusions Aridity shows greater importance than competition and other processes of limiting similarity or stochastic processes in community assembly. Also, the functional hypervolume of dung beetle desert communities decreases with aridity not only due to species loss, but also because of selection of a few distinct phenotypes under harsh environmental conditions. Last, we observed that the different regional pools respond to aridity in different ways. Therefore, understanding future responses of dung beetle communities to the progressive decreases in water availability driven by climate change requires determining how the characteristics of the species in the regional pool interact with aridity-driven assembly processes.