Do body size and diet breadth affect partitioning of species diversity? A test with forest Lepidoptera

Two of the major themes resulting from recent macroecological research are the central roles that body size and niche breadth may play as determinants of species geographical distribution. Unanswered questions, however, linger regarding how similarities in body size or niche breadth affect the allocation of alpha- and beta-diversity across spatial scales. Using data on moth diversity in the eastern deciduous forest of North America, we tested the predictions that smaller-bodied and diet-restricted species would have lower levels of alpha-diversity within forest stands and greater beta-diversity at higher sampling scales compared to larger or more generalist species. Moths were sampled using a nested sampling design consisting of three hierarchical levels: 20 forest stands, 5 sites and 3 ecoregions. Body size for 492 species was estimated as mean forewing length, and diet breadth was assessed from the published literature. Moth species were then classified according to body size (small or large) or diet breadth (generalist or restricted), and partitioning was conducted on each group. Diversity partitions for large- and small-bodied species yielded similar patterns. When observed diversity components differed from those derived from our null model, a consistent pattern was observed: alpha-diversity was greater than expected, beta-diversity among forest stands was less than expected, and beta-diversity among sites and ecoregions was higher than expected. In contrast, diet-restricted moths contributed significantly less to stand-level alpha-diversity than generalist feeders. Furthermore, specialists contributed to a greater proportion of beta-diversity across scales compared to generalist moths. Because absolute measures of beta-diversity among stands were greater for generalists than for restricted feeders, we suggest that regional beta-diversity of forest moths may be influenced by several possible factors: intraspecific aggregation of diet-restricted species, local fluctuations in population size of eruptive generalists and small geographical distributions of generalist moths than predicted by the geographical extent of putative host plants.