Seasonal persistence of bumblebee populations is affected by landscape context

Bumblebee communities and their foraging resources were surveyed in south Swedish agricultural landscapes of contrasting complexity, defined by the size of arable fields and the amount of permanent grazed pastures. After the flowering of oilseed rape (OSR), simplified landscapes contained substantially less herbaceous flower resources and a lower proportion of perennials, compared to complex ones. The seasonal pattern of bumblebee abundance differed between landscape types. Initial bumblebee abundances were equal in both landscape types. However, by late July there was a sharp decline of bumblebees in simple landscapes, while the abundance continued to increase in complex landscapes. This suggests that a larger proportion of bumblebee colonies may fail to reproduce in simple landscapes despite a beneficial early season. Overall, bumblebee abundance in late July was positively related to three inter-related variables: area of permanent pasture, area of ley fields and total amount of herbaceous flowers, while early abundances (June to early July) did not relate to these variables. We suggest that in simplified landscapes of this region, bumblebee abundance is limited by floral resources mainly from midsummer and onward. Spring and early summer resources may indeed be sufficient for colony establishment and initial growth even in simplified landscapes, possibly as a result of large scale farming of OSR. The initially equal abundances of workers in simple and complex landscapes, as well as the fact that also many of the regionally rare species seem to persist in simple landscapes, suggest that rare species can survive in pockets of beneficial habitat and/or there may be an inflow of queens from nearby complex landscapes. If the latter is true, further simplification or abandonment of complex landscapes may threaten bumblebee populations also in neighbouring simple landscapes.