SRA

Biodiversity loss, as often found in intensively managed agricultural landscapes, correlates with reduced ecosystem functioning, e.g. pollination by insects. Land-use also alters the composition, diversity and abundance of plant communities. How this change in floral resource diversity affects pollen foraging, dietary composition and nutritional quality as well as reproductive fitness in solitary wild bee species is still largely unclear. To better understand the impact of land-use intensity on interactions between wild bees and plants we investigated communities of trap nesting solitary bees, their pollen foraging, reproductive fitness and the nutritional quality of larval food composed along a land-use intensity gradient across three bioregions in Germany.We found that, across bioregions, bee species diversity decreased with increasing land-use intensity despite bioregion specific community compositions and interaction networks. Land-use intensity also affected the diversity and composition of plant species visited for pollen collection by different species. These differences in plants and thereby food availability may explain the absence of several bee species at sites of high land-use intensity. For the only species present throughout land-use gradients and bioregions, Osmia bicornis, the diversity of collected pollen and the number of brood cells produced by females also decreased with increasing land-use intensity. Moreover, O. bicornis pollen provisions composed at sites of high land-use intensity showed comparatively low protein, i.e. total amino acid, and lipid, i.e. total fatty acid, content. Likewise, concentrations of several fatty acids and most essential amino acids decreased with increasing land-use intensity, while the protein to lipid (P:L) ration, known to be critical for offspring production, remained constant. The observed changes in wild bee-plant pollen interaction patterns indicate that flexible generalists, such as O. bicornis, can partly compensate alterations in the floral resource landscape by shifting to alternative plant sources, which enable them to maintain stable nutrient ratios despite losses in overall nutrient intake. In contrast, other, less flexible, bee species may disappear. Intensified land-use consequently appears to act as a filter favoring those species that are highly flexible in their resource use and thus able to adjust foraging patterns to maintain diets of a nutritional quality that is sufficient to rear offspring.