Effect of livestock grazing intensity on native bee communities of a Pacific Northwest Bunchgrass Prairie

Background/Question/Methods

Livestock grazing is a widespread land use in western North America, which may impact native bees by affecting floral and nesting resources. Native bees are considered to be one of the most important pollinators. However, few studies have investigated how livestock grazing impacts native bees in western North America. Our study is the first, large-scale, manipulative experiment to examine the effect of a gradient of livestock grazing intensity on native bees. We conducted the study in 16 40-ha pastures on the Zumwalt Prairie, one of the largest remnants of the once expansive Pacific Northwest Bunchgrass Prairie. Each pasture was assigned one of four cattle stocking rates (high, medium, low, and no cattle), and grazing intensity was quantified by measuring utilization. Treatments were applied for two years. We measured soil and vegetation characteristics that related to floral and nesting resources as well as several metrics of the bee community, including diversity, richness, abundance, and community composition.

Results/Conclusions

Vegetation structure, the abundance of floral resources, soil stability, and the percent of ground covered by herbaceous litter decreased with increased grazing intensity, while soil compaction and the amount of bare ground increased. Native bee communities showed corresponding responses to grazing, with changes in abundance, richness, diversity, and community composition. Different groups of bees responded to grazing intensity differently and there was a seasonal difference in the response of bee communities to grazing intensity. Bumble bees were sensitive to grazing intensity early in the season, showing reduced abundance, diversity, and/or richness with increased grazing intensity. However, late in the season, no responses were detected. In contrast, halictid bees appeared unaffected by grazing throughout the season. These results suggest that maintaining some ungrazed or lightly grazed areas will benefit the conservation of this important and diverse pollinator group in the Pacific Northwest Bunchgrass Prairie.