Diets maintained in a changing world: Does land‐use intensification alter wild bee communities by selecting for flexible generalists?

Biodiversity loss, as often found in intensively managed agricultural landscapes, correlates with reduced ecosystem functioning, for example, pollination by insects, and with altered plant composition, diversity, and abundance. But how does this change in floral resource diversity and composition relate to occurrence and resource use patterns of trap‐nesting solitary bees? To better understand the impact of land‐use intensification on communities of trap‐nesting solitary bees in managed grasslands, we investigated their pollen foraging, reproductive fitness, and the nutritional quality of larval food along a land‐use intensity gradient in Germany. We found bee species diversity to decrease with increasing land‐use intensity irrespective of region‐specific community compositions and interaction networks. Land use also strongly affected the diversity and composition of pollen collected by bees. Lack of suitable pollen sources likely explains the absence of several bee species at sites of high land‐use intensity. The only species present throughout, Osmia bicornis (red mason bee), foraged on largely different pollen sources across sites. In doing so, it maintained a relatively stable, albeit variable nutritional quality of larval diets (i.e., protein to lipid (P:L) ratio). The observed changes in bee–plant pollen interaction patterns indicate that only the flexible generalists, such as O. bicornis, may be able to compensate the strong alterations in floral resource landscapes and to obtain food of sufficient quality through readily shifting to alternative plant sources. In contrast, other, less flexible, bee species disappear. Our study provides new insight into the importance of land‐use induced changes for pollinator communities. It combines extensive field observations, pollen metabarcoding and comprehensive chemical‐nutritional analyses. We show that solitary bee species diversity decreased with increasing land‐use intensity and decreasing floral diversity with one bee species (Osmia bicornis) occurring over the entire gradient. Our detailed analyses reveal that this species is characterized by extreme flexibility in larval diets and nutrient intake ratios. We discuss the hypothesis that intensified land‐use, as found in modern landscapes, acts as a filter for flexible generalists, but excludes less flexible species.