Native and agricultural grassland use by stable and declining bumble bees in Midwestern North America

Bumble bees are important pollinators in both natural and anthropogenic systems. In recent years, some species have suffered declines, including in the American Midwest. While loss of floral resources and grassland habitat may contribute to these declines, little is known about how native versus agricultural grasslands affect bumble bee populations. We conducted 3 years of bumble bee population and floral use surveys across restored tallgrass prairie, bison pasture, and cattle pasture in Northern Illinois. We found 1.75-3.5 times more bumble bees in native plant dominant tallgrass prairie than in exotic-dominant pasture sites. In addition, bumble bees were two times more abundant in bison pasture than in cattle pasture. Bumble bee species richness was approximately 1.5-2 times greater in prairie, while richness was comparable in the two pasture sites. Habitat-dependent abundance and richness were driven by individual bumble bee species habitat preferences. Across habitats, nativeMonarda fistulosawas highly favoured by foraging bumble bees, particularly by the regionally decliningB. auricomus. In the absence of native species, exoticTrifolium pratenseandDaucus carotawere important forage sources in pastures. These results indicate that grassland management practises to shift grassland plant communities towards native-dominant over exotic-dominant communities may strengthen bumble bee populations, although these effects will likely beBombusspecies-dependent. Our findings highlight the need to understand the impacts of habitat on specific bumble bee species, as this will be critical in developing conservation plans for declining populations now and in the future.