Factors affecting road mortality and the suitability of road verges for butterflies

Little is known of the impact of roads on insect mortality. This is a significant gap, because road verges are regarded as an important tool for insect conservation. In this study, we investigated which factors affect the number of roadkills in grassland butterflies and, simultaneously, the species composition and abundance on road verges. We established sixty transects, two hundred metres long, on roads in farmland areas and with differing traffic volume. Each transect consisted of two parallel lines, one on either side of the road. Ordination methods showed that the species composition of the butterflies killed on the roads was primarily explained by the species composition of those living on the road verges. At least 6.8% of the butterflies were estimated to end up roadkilled. Also, the number of species and abundance of butterflies killed on the roads were positively dependent on both the abundance of butterflies on the road verges and on the traffic volume, but negatively correlated with the richness of plant species on the road verges. However, the proportion of individuals killed was negatively linked with the abundance of butterflies on the road verges, the richness of the plant species and the share of grassland in the landscape. There was a statistically significant tendency for small-bodied species to be overrepresented in the roadkill samples. Our results indicate that the verges which were of high conservation value for butterflies suffered the least from road mortality. The sowing of plant species, less frequent mowing and maintaining a high grassland cover in the vicinity of roads are recommended conservation actions for the improved conservation value of road verges for butterflies.
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