A major goal of ecology is to uncover the processes responsible for community assembly and local patterns of species diversity. Both biotic interactions and temperature are frequently cited as major drivers of community assembly, but their influence is debated. The recent growth of trait-based approaches to ecology now permits stronger tests of classic theories of coexistence and assembly. In particular, analysis of intraspecific trait variation (ITV) facilitates detection of competition and other assembly processes; however, this approach has largely been restricted to plant communities and has rarely been tested on animals. Here, we use new data from the National Ecological Observatory Network (NEON) to assess the role of ITV in community assembly in mammals. NEON is a a continental-scale observation network with standardized sampling protocols that aims to document ecological change. We quantify within- and among-species variation in body size in 23 North American rodent communities that span deserts to forests. We present an improved method for calculating community-level trait overlap among species which accounts for individual-level variation and can uncover the signatures of environmental filtering, biogeography, and competitive avoidance in shaping species' trait distributions.
We find that body size segregation in rodents shows strong signatures of competition and niche partitioning, with body sizes more finely partitioned among species than the null expectation in 17 of 23 rodent communities. This result reflects the role of body size in governing dietary preferences, microhabitat selection, and life history tradeoffs. Further, body size overlap among species decreases with mean annual temperature and species richness. Together, temperature and species richness explain over 60% of variation in body size partitioning, with each driver acting independently. These findings support theoretical predictions that competition promotes niche partitioning in communities with higher mean annual temperature and higher species richness. Our results have implications for understanding the mechanisms underlying the latitudinal diversity gradient, supporting the prediction of classical niche theory that tighter packing of species into trait space enables maintenance of higher diversity in warmer areas. Our approach also highlights how a trait-based approach that properly accounts for intraspecific trait variation can reveal local drivers of community assembly.