Responses of wild pollinators to honey bee colony presence and habitat restoration in calcareous grasslands

Biodiversity has been declining in the last decades, including pollinating insects. As an important ecosystem service, pollinators contribute to functioning ecosystems and sustainable crop yields. There have been growing concerns that competition for floral resources between managed and wild pollinators might contribute to the current decline of pollinators, however comprehensive field data to support these concerns are scarce, particularly from northern Europe. Here, we focus on semi-natural calcareous grasslands to assess potential competition between managed and wild pollinators within the context of habitat restoration. Based on transect walks at 30 sites across Estonian islands and coastline, most of which were restored from overgrown areas to open grasslands, we measured the abundance and species richness of pollinator species from five groups (bumblebees, butterflies and burnet moths, hoverflies, solitary bees, honey bees). We assessed the potential impact of honey bee competition by relating the number of honey bee colonies within a 2 km radius to the abundance and species richness of wild pollinators while correcting for the restoration status of the grassland (restored vs reference sites), local flower cover, and the percentage of forest within a 2 km radius. We found that honey bee colony density had no effect on the abundance or species richness of wild pollinators. Flower cover had some relevance, while the other variables revealed no effects. In conclusion, our results do not indicate detrimental effects of managed honey bees on flower-visiting wild pollinators at current colony densities in our study region.