Effects of experimental habitat fragmentation on patch use by butterflies and skippers (Lepidoptera)

We conducted an experimental study to assess the effects of habitat fragmen tation on patch use by butterflies and skippers. Five fragmentation treatme nts (20%, 40%. 60%, 80%. and 100% habitat remaining) were replicated five t imes by selectively mowing 15 X 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 richne ss or abundance declines linearly with habitat area, (2) the patch heteroge neity hypothesis, in which richness or abundance peaks at intermediate leve ls of fragmentation, (3) the critical threshold hypothesis, in which a nonl inear decrease in richness or abundance occurs below some critical level of habitat loss. and (4) the patch quality hypothesis, in which declines in r ichness or abundance with increasing habitat loss are partially offset by t he resource value of the remnant habitat. We found that fragmentation treatment significantly affected species richne ss and total number of patch visits for the overall community, as well as t he numbers of patch visits by individual species. Community and species res ponses were linearly related to the amount of habitat remaining, as predict ed by the proportional area model of fragmentation. We also found that plot s with a higher cover of flowering forbs received greater patch use. Thus, the effects of habitat quantity (patch area) and habitat quality (flower co ver) were independent and additive, suggesting that moderately sized patche s of high resource quality may be equivalent to large patches of lower qual ity. Therefore, to maintain local population densities of Lepidoptera, it m ay be possible to compensate for losses in habitat area with improvements: in habitat quality. Species did not uniformly respond to fragmentation. Rare species were dispr oportionately affected by fragmentation and did not occur in patches with < 40% habitat remaining. Several common species appeared unaffected by fragm entation: however, other common species were also significantly influenced by fragmentation treatment. These findings support the notion that rare spe cies may be especially vulnerable to fragmentation, but it could be quite d ifficult to predict a priori how more common species will respond to habita t loss.