DOT Project Number:  90-00-LRTF-832

Fiscal Year:  1998

Award:  $12,500

Principal Investigator:  Diane Debinski, Department of Ecology, Evolution and Organismal Biology, Iowa State University, debinski@iastate.edu

Other Project Participants:  L. Ries and M.L. Wieland

Summary Report:

THE CONSERVATION VALUE OF ROADSIDE PRAIRIE RESTORATION TO BUTTERFLY POPULATIONS

Restoration of roadsides to native habitat has been suggested to benefit wildlife in two ways: by adding habitat and restoring connectivity between fragmented reserves. In Iowa, which has one of the highest road densities in the United States, roadside vegetation has traditionally been managed to maintain a monoculture of exotic grass. Recently, several Iowa counties have begun integrated roadside vegetation management, a program which both restores roadsides to native prairie vegetation and restricts the use of herbicides and mowing. We evaluated the effect of this management regime along central Iowa roadsides. We surveyed 12 separate prairie roadside areas and compared abundance and species richness of disturbance-tolerant and habitat-sensitive butterflies in roadside prairies with nearby roadsides dominated by either weeds (primarily nonnative legumes) or nonnative grasses.

We found that management of roadsides profoundly affects the butterfly community. Species richness of habitat-sensitive butterflies showed a two-fold increase compared with grassy and weedy roadsides (p < 0.0001); abundance increased almost five-fold for prairie compared to grassy roadsides (p < 0.02). Species richness of disturbance-tolerant butterflies showed no response to roadside management, although abundance was slightly higher in weedy and prairie roadsides compared with grassy (p < 0.01). Of all habitat variables explored, the species richness of plants in flower showed the strongest effect on mean richness and abundance of both disturbance-tolerant and habitat-sensitive species. Although there were higher concentrations of road-killed butterflies near weedy and prairie roadsides compared with grassy roadsides, relative numbers indicated mortality risk was more than twice as high in grassy roadsides (p < 0.0001). Tracking studies found butterflies were less likely to exit prairie roadsides, indicating mortality rates may be lower and offering preliminary evidence that roadsides have the potential to be used as corridors. Overall, our results indicate that roadside restoration benefits butterfly populations. More detailed demographic data are necessary, however, to explore the possibility that roadsides are acting as sink habitat for some habitat-restricted species.