- Animal pollination is critical to plant reproduction in agricultural and wildland ecosystems. Much of the production of seeds and fruits in natural areas, which underlie many food webs, depends on pollination services by insects. The taxon responsible for delivering the bulk of these services in most temperate systems is bees. While colony collapse disorder in the nonnative European honey bee (Apis mellifera) is a significant concern and one that has generated much media and scientific interest, recent studies have indicated that native, unmanaged bees may be declining as well. To increase the likelihood of continued delivery of pollination services, land managers need to have a comprehensive understanding of how management actions may affect native bees. One dominant land type, especially in the Pacific Northwest of the United States (US), is rangelands. Rangelands are known to support diverse pollinator communities and face land management challenges such as multiple-use and historic degradation. My thesis examines how one potentially important stressor, livestock grazing, affects native bee and plant communities and how shrubs used in the restoration of riparian areas common in rangelands can provide resources to a diverse community of native bees.
My first chapter provides a general introduction to the topic of native pollinators and a broad overview of the importance of pollinators, their basic habitat and biological needs, and some of the stressors that may be affecting populations. The purpose of this chapter is to provide context and background knowledge for the chapters that follow.
Chapter two focuses on one potentially important stressor of native bee communities in the American West – livestock grazing. In this chapter, I examine the effects of, late-season, moderate intensity, rotational cattle grazing on native bees and the blooming plants they depend on. To understand the effects of grazing on bee and plant communities, we conducted bee and blooming plant surveys at 28 sites located at two eastern Oregon study locations in the summer of 2018. One of the locations was a riparian wet-meadow system and the other was a bunchgrass prairie system. At each location, half of the sites were grazed by cattle at some point in the summer and half were not. Bee and plant communities at both locations varied through the growing season, with peaks in species richness and diversity occurring early to mid-season. We found location specific effects of cattle grazing on bee and blooming plant communities. At the riparian meadow location, we found that cattle grazing had short-term effects in reducing bloom abundance, species richness, and Shannon diversity in July and August, but these effects did not translate to any significant effects on bee communities. At the bunchgrass prairie location, we found no significant short-term effects of grazing on blooming plant communities and no negative effects on native bees. In fact, we detected higher bee abundance and richness in grazed sites at this location. An analysis of longer term-grazing at the second location revealed no detectable effect of grazing on blooming plant or native bee communities.
Chapter three focuses on the interactions of native bees and flowering plants in a restored riparian area. To understand how bees interact with flowering plants, we conducted extensive hand-net surveys of bees in 2018 and 2019 from April to September in the restored area. To understand the changes in blooming plant community richness throughout the season, we conducted plant richness surveys throughout the 2018 growing season. We found a diverse community of bees that interacted with a diverse community of blooming plants. Early in the season (April), we found that blooming forbs were significantly more abundant and species rich than blooming shrubs. While forbs were more abundant in the early-season, we found no evidence that bees foraged on forbs at a higher rate than shrubs. Willow seemed to be important in supporting several apparent specialist bee foragers in the early-season. Later in the season, both shrubs and forbs remained important resources for bees, and we found bees that were apparent specialists on both shrub and forb blooms.
Chapter four summarizes the key findings of the previous two chapters and provides some basic recommendations for land management with objectives related to pollinator health. I also make some suggestions for future research directions that could build on the findings described in this thesis. Collectively, the results of this thesis provide hope for pollinator conservation in inland Pacific Northwest grasslands. The second chapter indicates that late-season, rotational, cattle grazing may help mitigate some of the negative effects of cattle grazing on native bees that have been observed in some systems. The third chapter indicates that riparian restoration activities, especially planting blooming shrubs, may provide important forage resources for native bees in Pacific Northwest rangelands.