Native pollinators: the effects of livestock grazing on Montana rangelands

Abstract

Although native pollinators on rangelands serve, in part, as food at higher trophic levels, their primary ecosystem function is pollination. With 70% of western U.S. rangelands grazed by livestock, understanding how grazing affects native pollinators is a key component to managing rangelands, yet it is not well understood. In this study, I investigated how cattle grazing influences both diversity and abundance of native pollinators, including bees and wasps, syrphid flies, butterflies, and moths at two research locations for six to 10 weeks during the spring of 2016, 2017, and 2018. The first site, near Sidney, MT, served to understand if pollinators were more closely associated with active cattle grazing or rested pastures. The second site, near Roundup, MT, served to understand if pollinators were more associated with pastures either enrolled or not enrolled in the Sage Grouse Initiative, or pastures that had not experienced livestock grazing in previous seven years. Colored pan traps were deployed weekly in each treatment at each site. In addition to pollinator collections, weekly measurements of vegetation via Daubenmire frame were also collected. I collected 17,078 specimens at Sidney and 13,683 specimens at Roundup. My results suggest that in drier sagebrush landscapes, native pollinators are positively to neutrally associated with pastures moderately grazed by livestock. However, in mixed grass prairie landscapes that receive high precipitation, rest-rotational grazing does not appear to have a significant effect on primary native pollinators.