Functional perspectives on the dynamics of desert lizard assemblages

Ecology has long sought whether there are general assembly rules that underlie patterns of community organization. However, new perspectives of community disassembly and reassembly have emerged as environments depart from baseline conditions due to agents of global change. A trait-based approach can provide insights as to how changes affect performance of a species and their subsequent loss, gain, or persistence in an assemblage. We quantified the spatiotemporal dynamics in species and functional diversity from systematic surveys of local lizard assemblages distributed along an elevation gradient from three surveys in a 50-year period (1950s-2000s) in Big Bend National Park (BBNP), USA. BBNP was subjected to and then released from intensive grazing, which has resulted in changes to vegetative composition and cover. We examine changes in local assemblages in the context lizard functional traits, including thermal niche breadth. We hypothesized that temperature specialists were more likely to colonize or be lost from local sites. Species richness was lowest in the 1950s (N = 3 species), and subsequent surveys revealed that species richness increased in both the 1960s and 2000s (N = 6 and N = 8 species, respectively), but not all sites responded uniformly. We found that functional diversity in this system was not affected by this drastic increase species richness, indicative of functional redundancy in species traits. Lizards that were added to sites tended to be smaller bodied with lower thermal tolerances. Lizards with high preferred body temperatures were locally extirpated from high elevation sites that experienced high shrub encroachment, likely due to a reduction in the preferred thermal gradients these species. These results reveal that long-term desert lizard functional diversity was maintained, but the consequences of changes to functional diversity at the local scale need to be examined further.