Body size is a fundamental organismal trait which is known to influence space and resource use, population density and growth rate, and interspecific interactions. The idea that body size must therefore be important in structuring ecological communities is not new, but has primarily been investigated using the species size distribution (SSD). The SSD may be misleading, however, because not all species are equally represented within a community, and hence the modal species body size may not coincide with the modal size of an individual. On the other hand, the individual size distribution (ISD) is a more accurate characterization of how resources are partitioned among coexisting individuals within an assemblage. Here, we utilize data from over 3,500 North American breeding and wintering bird assemblages to examine the structure of ISDs along environmental gradients of temperature, productivity, and habitat structure. We also conduct Mantel tests to examine how much of the similarity in ISDs between two sites can be explained by the difference in environmental conditions and geographic distance between them.  

Results/Conclusions

ISDs are complex, multimodal distributions and are thus not easily characterized by simple moments. A principal components analysis finds that >74% of the variation in ISD structure is explained by the first four axes, and that these axes respond differentially to environmental gradients. While measures of productivity and habitat structure were most important for explaining variation in breeding assemblage ISDs (R² ranging from 8 - 17%, p < .001), temperature was strongly correlated with the ISDs of winter assemblages (10 - 28%, p < .001). During both seasons, pairwise similarity between the ISDs of any two assemblages was a function of both the spatial and environmental distances between them. Importantly, we observed a strong effect of biome-level habitat complexity, with some habitat types failing to support modes commonly observed elsewhere. Our results are consistent with the textural discontinuity hypothesis, which suggests that ecological and environmental processes operating at multiple spatial and temporal scales result in scale-dependent resource availability, and hence favor organisms of particular sizes.