Floral resources in the surrounding landscape matrix augment plant species richness of bumblebee pollen loads in small, fragmented calcareous grasslands

Pollinator populations have shown substantial declines worldwide due to habitat loss and fragmentation, with bees experiencing particularly sharp reductions in both species richness and abundance. While local management measures in semi-natural grasslands and agri-environmental schemes (AES) intend to improve farmland biodiversity, the extent to which these measures affect foraging behavior and pollen resource use of bees in fragmented landscapes remains poorly understood. In this study, we tested the hypothesis that landscape and grassland management affected floral resource use of two bumblebee species (Bombus lapidarius and B. pascuorum) in fragmented calcareous grasslands in Central Germany. Pollen metabarcoding was used to assess the richness and community composition of pollen loads from both bumblebee species. Moreover, we compared the interactions detected by pollen analysis to those observed in transect walks. The results showed that both species foraged in the broader landscape and collected pollen from plant species found in AES. The percentage of land within 1 km allocated to non-productive AES (e.g., flower strips) was associated with higher pollen species richness in both B. lapidarius and B. pascuorum. In the latter species, this relationship was particularly strong in smaller sites. Landscape and site management variables affected the species composition of pollen assemblages collected from both bumblebee species. Our study showed that bumblebees utilize floral resources from the broader landscape, especially from non-productive AES, illustrating that these schemes can support local pollinator communities of semi-natural grasslands in fragmented agricultural landscapes.