Effects of Experimental Habitat Fragmentation on Patch Use by Butterflies and Skippers (Lepidoptera)

We conducted an experimental study to assess the effects of habitat fragmentation on patch use by butterflies and skippers. Five fragmentation treatments (20%, 40%, 60%, 80%, and 100% habitat remaining) were replicated five times by selectively mowing 15 × 15 m patches of an old field. Community and species responses to fragmentation were tested against several alternative predictions: (1) the proportional area hypothesis, in which species richness or abundance declines linearly with habitat area, (2) the patch heterogeneity hypothesis, in which richness or abundance peaks at intermediate levels of fragmentation, (3) the critical threshold hypothesis, in which a nonlinear decrease in richness or abundance occurs below some critical level of habitat loss, and (4) the patch quality hypothesis, in which declines in richness or abundance with increasing habitat loss are partially offset by the resource value of the remnant habitat. We found that fragmentation treatment significantly affected species richness and total number of patch visits for the overall community, as well as the numbers of patch visits by individual species. Community and species responses were linearly related to the amount of habitat remaining, as predicted by the proportional area model of fragmentation. We also found that plots with a higher cover of flowering forbs received greater patch use. Thus, the effects of habitat quantity (patch area) and habitat quality (flower cover) were independent and additive, suggesting that moderately sized patches of high resource quality may be equivalent to large patches of lower quality. Therefore, to maintain local population densities of Lepidoptera, it may be possible to compensate for losses in habitat area with improvements in habitat quality. Species did not uniformly respond to fragmentation. Rare species were disproportionately affected by fragmentation and did not occur in patches with <40% habitat remaining. Several common species appeared unaffected by fragmentation; however, other common species were also significantly influenced by fragmentation treatment. These findings support the notion that rare species may be especially vulnerable to fragmentation, but it could be quite difficult to predict a priori how more common species will respond to habitat loss.