Grazing supports more oligolectic bees than hay fields – functional differences in pollinator communities among pasture landscapes

Agriculture relies heavily on the ecosystem services provided by beneficial insects, and supporting their populations in these areas is important. We tested for differences in bee communities between pastures and unmanaged (natural) grasslands in Manitoba, Canada. Pastures were either grazed by cattle or cut for hay. Hayed pastures were planted with an enhanced seed mix to benefit both bees and livestock. Nevertheless, enhanced hay fields were dominated by alfalfa. We found that natural and grazed sites supported more bees than hay sites did. Grazed sites tended to support oligolectic (plant specialist) bees, whereas natural sites supported more polylectic (plant generalist) bees. Bee capture rates were positively associated with overall field size. Associations between bee functional traits (diet, nesting, and sociality) and environmental variables are discussed. Our results indicate that grazing may be an important management tool for conservation of oligolectic and ground-nesting bees. In addition, polylectic and stem-nesting bees may use natural landcover types more than agricultural landcover types, and increasing the amount of natural landcover for insects should be a priority.