Bumblebee nest density and the scale of available forage in arable landscapes

Combining the needs of agricultural production with enhancing biodiversity requires a landscape-scale approach since the geographic scale at which most non-farmed species operate is unconstrained by farm boundaries. Bumblebees are a key component of farmland biodiversity as pollinators of both crops and wild flora. However, the factors determining their densities in such landscapes remain poorly understood. Using a combination of remote-sensed landscape data and molecular markers, we quantify the effects of land use (oilseed rape, field beans and non-cropped areas, all of which provide suitable bumblebee forage), at various spatial scales to find the best predictor of colony density for the bumblebee Bombus pascuorum Scopoli in an arable landscape. Estimated colony density was positively correlated with the area of all habitat categories within 1000 m of the sample site. No significant relationships were found for greater or lesser distances. This concurs with earlier estimates of the foraging range of this species. We found no evidence that nest sizes increased with forage availability, although our data do not allow us to categorically exclude this possibility. It has long been suspected that forage availability limits bee abundance in agricultural landscapes but there is little direct evidence for this. Here we report a direct relationship between floral abundance and bumblebee nest density within a notionally fixed area. Importantly, we suggest that the forage availability within the previously published estimated foraging distance for this species at this location is a good predictor of the scale of impact of forage provision on nesting density.