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Individual study: The effect of roadside prairie restoration on butterfly communities, central Iowa, USA

Published source details

Ries L., Debinski D.M. & Wieland M.L. (2001) Conservation value of roadside prairie restoration to butterfly communities. Conservation Biology, 15, 401-411

Summary

There is an estimated 8 million hectares of vegetated roadsides in the USA. Roadside vegetation has been suggested as a potential resource to increase wildlife habitat, such as nature reserve buffer strips, and to increase connectivity between fragmented habitats. Some suggest however that such habitat is of low value, as they act as population sinks because of the high levels of mortality sustained by some taxa. Recently, many US states have instigated policies to restore roadside edges to native vegetation, primarily to control weeds and prevent erosion, and this might therefore have additional costs or benefits for roadside wildlife attracyed to the strips of roadside habitat. In the USA, prairie grassland is considered the most endangered ecosystem, and in Iowa (where less than 0.01% of original tall grass prairie remains) 21 of its 44 threatened butterfly species are prairie specialists. In this study, the effect of restoring Iowa roadside edges to prairie on the roadside butterfly communities is investigated.

Study site: All restored (n = 8) or native (never ploughed, n = 4) prairie roadsides (>0.5 km long) were selected in Hamilton, Hardin, Story, and Polk Counties of central Iowa. Restored roadsides were seeded with native plants, and mowing and herbicide use was restricted. In 160 m sections, extending 1.6 km each side of the 'roadside prairie', vegetation was classified as weedy (>20% non-native legumes), grassy (<5% forbs cover and dominated by non-native grasses, in particular Bromus spp.), or prairie (dominated by native prairie grasses and forbs).

Experimental design: Three experimental plots (50 x 5 m) were established within each roadside class (grass, weedy and prairie) within each area. Grass and weedy plots were separated by at least 50 m and they were separated from prairie plots by over 500 m. Plots were not positioned in wetland vegetation, and all were bordered by row crops. Additionally, to compare restored roadside prairie with nonroadside native prairie reserve fragments, three randomly positioned plots were placed in each of four prairie remnants (2-16 ha).

Butterfly & flower surveys: Three rounds of surveys were conducted between 22 June and 22 August 1998, and during each a plot was visited on three occasions, giving nine sample periods. Each sample was conducted between 10:00 and 18:30 hrs when the temperature was between 21° and 35°C and sustained wind below 16 km/hour. Walking a 50 m line transect down the centre of the plot, every individual encountered of over-wintering (non-migratory) butterfly species was recorded.

Also, the percentage cover of flowers in every plot was estimated once during each round. Six 5 x 1 m transects were placed perpendicular to the 50 m transect at 10 m intervals. Each 5 x 1 m transect was divided into five 1 x 1 m quadrats, in which a visual estimate of the percentage cover of flowers was made. The 30 values were averaged over the entire plot.

Butterfly roadkill mortality: During the last two rounds of butterfly surveys, roadkill censuses were also conducted. Adjacent to the 50 m transect, both road edges were walked where the vegetation met the road surface and the number of each over-wintering butterfly species found dead was recorded.

Restored versus native prairie: There was no significant differences in butterfly species richness or overall abundance for both habitat-sensitive and disturbance-tolerant species between restored and native roadsides. This allowed the authors to combine restored and native prairie roadsides for other comparisons. When comparing roadside prairie and nonroadside prairie, there was no difference in butterfly species richness or abundance for habitat-sensitive species. However, disturbance tolerant species were twice as abundant (3.3 vs 1.8) and had higher species richness (2.3 vs 1.6) in roadside prairie compared with nonroadside prairie.

Butterfly relative abundance: There was a higher abundance of habitat-sensitive butterflies in prairie (2.3, estimated from original figure) than weedy (1.4) or grass (0.5) plots. In contrast, there was no significant difference in the species richness of disturbance-tolerant butterflies in prairie (2.3), weedy (2.1), and grassy (1.9) plots.
Butterfly species richness: Similarly to relative abundance, there was a higher species richness of habitat-sensitive butterflies in prairie (1.7, estimated from original figure) than weedy (1.0) or grass (0.8) plots. There was no significant difference in the abundance of disturbance-tolerant butterflies in prairie (3.2) and weedy (3.4) plots, but both were higher than grassy (2.1) plots.

Butterfly roadkill mortality: The number of butterflies found dead was divided by the number seen on roadside surveys. There were more butterflies found dead next to grassy (2.0) than to weedy (0.9) and prairie (0.5) plots

Conclusion: Roadside prairie restoration has proved very successful, with butterfly species richness and abundance increasing to levels consistent with native roadside and nonroadside prairies. There also does not seem to be sink dynamics occurring, with a small decrease in butterfly mortality compared to non-native grassy and weedy roadsides.


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