The impact of traffic, management and landscape on flower-visiting insects and plants in road verges

Road verges can provide habitat for flower-visiting insects that depend on open grassland habitats for feeding and nesting resources. However, road verges can also be harmful or unattractive as habitat due to impacts from traffic or management. In this thesis, I contribute to understanding how traffic intensity, vegetation management and historical landscape context shape the diversity of wild bees, butterflies, plants and briefly solitary wasps in road verges. My results show that a considerable proportion of a region's wild bee and butterfly species can occur in road verges, including red-listed species. Older road verges exhibited a higher plant species richness than younger verges, and I detected a generally positive relationship between flower-visiting insect diversity and the availability of feeding resources. However, I show that high traffic can limit species abundance and richness of wild bees and butterflies, the attractiveness of potential nesting sites, as well as population growth and reproductive success of cavity-nesting insects. High traffic even limited the abundance of flower visitors in the surrounding landscape, although this did not influence pollination success of wild plants. Wide road verges buffered some of the negative effects of traffic on species abundance and richness. Vegetation management also played a significant role in determining habitat quality for insects. Road verges that were mown only once per year in late summer provided more floral resources and exhibited greater insect diversity, though this effect could vary depending on the specific conditions of the verge. Although road verges have the potential to support flower-visiting insects, I conclude that not all verges are equally suitable habitats. Furthermore, some species are particularly sensitive to the effects of traffic intensity, road verge width and management. Wide road verges along roads with little traffic offer the best opportunities for conservation actions. My findings can provide a roadmap for optimizing management to support the diversity of flower-visiting insects and plants along roads.