Livestock grazing is associated with seasonal reduction in pollinator biodiversity and functional dispersion but cheatgrass invasion is not: variation in bee assemblages in a multi-use shortgrass prairie

Livestock grazing and non-native plant species affect rangeland habitats
globally. These factors may have important effects on ecosystem services
including pollination, yet, interactions between pollinators, grazing, and
invasive plants are poorly understood. To address this, we tested the
hypothesis that cattle grazing and site colonization by cheatgrass (Bromus
tectorum) impact bee foraging and nesting habitats, and the biodiversity
of wild bee communities, in a shortgrass prairie system. Bee nesting
habitats (litter and wood cover) were marginally improved in non-grazed
sites with low cheatgrass cover, though foraging habitat (floral cover and
richness, bare soil) did not differ among cattle-grazed sites or
non-grazed sites with low or high cheatgrass cover. However, floral cover
was a good predictor of bee abundance and functional dispersion. Mean bee
abundance, richness, diversity and functional diversity were significantly
lower in cattle-grazed habitats than in non-grazed habitats. Differences
in bee diversity among habitats were pronounced early in the growing
season (May) but by late-season (August) these differences eroded as
Melissodes spp. and Bombus spp. became more abundant at study sites.
Fourth-corner analysis revealed that sites with high floral cover tended
to support large, social, polylectic bees; sites with high grass cover
tended to support oligolectic solitary bees. Both cattle-grazed sites and
sites with high cheatgrass cover were associated with lower abundances of
above-ground nesting bees but higher abundance of below-ground nesters
than non-grazed sites with low cheatgrass cover.  We conclude
that high cheatgrass cover is not associated with reduced bee biodiversity
or abundance, but cattle grazing was negatively associated with bee
abundances and altered species composition.  Although floral
cover is an important predictor of bee assemblages, this was not impacted
by cattle grazing and our study suggests that cattle likely impact bee
communities through effects other than those mediated by forbs, including
soil disturbance or nest destruction. Efforts aimed at pollinator
conservation in prairie habitats should focus on managing cattle impacts
early in the growing season to benefit sensitive bee species.