Background/Question/Methods Species are differentially affected by habitat fragmentation as a consequence of differences in mobility and area requirements. A quantitative understanding of these differences is essential not only for conservation biology but also for basic ecological theory. We used (i) a trait-based approach to examine lepidopteran responses to patch size and (ii) a quantitative theory to interpret results. Theory suggests that the density distribution of mobile species should depend on the scaling of net migration rates, whereas the distribution of less mobile species should depend on local growth. Using data from USA and from multiple localities within Europe, we calculated the slope in the relationship between patch size and density for 300+ species (both moths and butterflies). These slopes are then evaluated in relation to butterfly traits and matrix composition. As estimates of mobility we use both wing span and expert mobility rankings, and to evaluate the role of local growth we use the fact that habitat specialists have higher local growth rates than more generalist species.

Results/Conclusions Results corroborate theory. The slope in the density-area relationships for large species is zero for moths and strongly negative for butterflies, matching quantitative predictions from estimates of migration rates. Small and less mobile habitat specialists have a slope that is less negative than the slope of large and mobile specialist species, whereas the slope does not change with size for generalist species. These analyses suggest that a trait-based approach could be useful for understanding the variability in density response among butterfly species to patch size, and potentially to other aspects of spatial heterogeneity.