Livestock grazing but not exotic grass invasion are associated with a
seasonal reduction in wild bee biodiversity and functional variation in
shortgrass prairie
Abstract
1. 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. 2. Bee nesting habitats (litter and wood cover) were marginally
improved in non-grazed sites, though foraging habitat (floral cover and
richness) did not differ among grazed, non-grazed, or cheatgrass
colonized sites. However, floral cover was a good predictor of bee
abundance and functional dispersion. 3. Mean bee abundance, richness,
α-diversity and functional diversity were significantly lower in
cattle-grazed habitats than in cheatgrass-colonized or 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. 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 and cheatgrass-colonized sites were associated with lower
abundances of above-ground nesting bees but higher abundance of
below-ground nesters. 4. Synthesis and applications. Cheatgrass-invaded
sites do not appear to be 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 grazing
and our 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.