A meta-analysis of the effects of habitat aridity, evolutionary history of grazing, and grazing intensity on bee and butterfly communities worldwide

A variety of habitat-associated factors moderate effects of grazing on
insect biodiversity. Here, we examine how aridity, evolutionary history of
grazing, and grazing intensity individually and interactively mediate the
effect of livestock grazing on pollinator diversity (native bees and
butterflies). Using a meta-analysis of 59 studies published in the primary
literature we characterized the response of pollinator communities to
grazing across several continents. In very humid habitats high grazing
intensities generally had negative impacts on pollinator abundance and
richness, but these effects were not found in semi-arid habitats, where
livestock grazing intensity did not interact with aridity to impact
pollinator abundance or richness. However, within semi-arid habitats
livestock grazing was associated with reduced pollinator richness in areas
with short evolutionary histories grazing. Pollinator life history
mediated effects of livestock grazing on pollinator communities: livestock
grazing had negative impacts on richness of social bees and butterflies
but not solitary bees, though abundances of all three pollinator
categories were consistently reduced under livestock grazing. Our
synthesis suggests that effects of cattle on pollinators may be driven by
impacts on nesting habitats (e.g., soil compaction), rather than
consumption or alteration of forb cover. Our collective findings have
importance for coordinating grazing management and pollinator conservation
efforts and help to distinguish how grazing practices could impact
pollinator biodiversity across ecoclimatic regions.