Evaluating the effects of floral resource specialisation and of nitrogen regulation on the vulnerability of social bees in agricultural landscapes

Abstract

The abundance and the diversity of bumblebees have been declining for the past decades. While some species remain abundant and widespread, others have experienced vast declines. We tested if diet breadth and exclusiveness of floral resource use affected abundance and land-use response of bumblebees and honeybees in heterogeneous agricultural landscapes. Based on an ecological niche modelling approach, we discovered that three of 13 species (12 Bombus spp. and A. mellifera) responded negatively to land use, while no species responded positively. Overall abundance as well as land-use response of different bee species was unrelated to resource use. The generalist forager B. lapidarius, the slightly more specialised forager B. terrestris/lucorum species complex and the specialist forager B. wurflenii declined most with land use. Moreover, we found that honeybees and two bumblebee species contained higher levels of nitrogen than other non-predatory Hymenoptera and performed almost perfect homeostasis despite variable nitrogen contents of floral resources. We conclude that at least common species of social bees have good nitrogen homeostasis capacity. Generally, our data do not support the notion that species with more specialised floral resource use are more likely to suffer from land use.