The relationship between intraspecific variation in body size and diversity for three vertebrate taxa across North America

Benjamin Baiser, Wildlife Ecology and Conservation, University of Florida, Gainesville, FL, Quentin D. Read, SESYNC, Annapolis, MD, Sydne Record, Biology, Bryn Mawr College, Bryn Mawr, PA, Angela L. Strecker, Institute for Watershed Studies, Western Washington University, Bellingham, WA, Morgan W. Tingley, Ecology & Evolutionary Biology, University of California, Los Angeles, CA and Phoebe L. Zarnetske, Department of Integrative Biology, Michigan State University, East Lansing, MI

Background/Question/Methods
Organismal traits are a central focus of contemporary community ecology. Traits are commonly used to predict species’ responses to climatic change and habitat loss, assess how assemblages and food webs will reorganize following colonization or emigration of new species, and to unravel large-scale patterns in species distributions across geographic gradients. Recently, intraspecific trait variation (ITV) has made a resurgence in the field of community ecology advancing theories of community assembly, food web ecology, and macroecology. ITV provides a currency for assessing the roles of abiotic and biotic processes, as it reflects the mechanisms driving species occurrence and responses to change. Here, we apply a novel metric of trait overlap based on individual body size measurements to test the hypothesis that species richness decreases with body size overlap for small mammals, birds, and salamanders across North America.
Results/Conclusions
We calculated body size overlap based on individual body sizes for 23 small mammal assemblages, 946 bird assemblages, and 827 salamander assemblages. The small mammal assemblages we analyzed were located across the United States and ranged in species richness from 2 to 14 species. Small mammal richness showed a significant negative relationship with body size overlap indicating that more species can coexist at a given locale when body size niches differ among coexisting species. Salamander assemblages were located in Eastern North America and ranged in species richness from 2 to 5 species. Salamander species richness significantly increased with body size niche overlap suggesting that environmental filtering on body size can lead to greater local species richness. Bird assemblages from across North America ranged from 2 to 90 species and showed no relationship between species richness and body size overlap. Overall, body size overlap between coexisting species was related to assemblage species richness for both small mammals and salamanders, but in different directions. Our results suggest that assessing niche overlap based on body size ITV can lend insight into local diversity patterns. To better understand how niche overlap based on ITV influences species richness, we suggest considering traits beyond body size in a multivariate niche framework.