Wild bees and wildlife: Investigating vertebrate and invertebrate interactions in riparian areas of the Blue Mountains, Oregon

Background/Question/Methods: Grazing mammals can alter ecosystem structure and function through multiple pathways. Many aspects of mammalian herbivory have been well studied, including plant responses to herbivory and consequences of top predator losses on native ungulates in trophic cascades. However, less attention has focused on how grazing mammals, especially native ungulates, interact with invertebrates, even though invertebrates provide key ecological functions, such as pollination. As part of a larger, multidisciplinary project evaluating ungulate grazing management and riparian restoration at the USFS Starkey Experimental Forest and Range (Starkey) in the Blue Mountains of Oregon, the goal of this study was to examine how vertebrate wildlife may influence wild bees through dietary overlap. The objectives of this three-year study were to 1) document which flowering species are most commonly visited by wild bees in riparian areas at Starkey, and 2) quantify how the exclusion of deer and elk in these areas influences flowering plant communities. To address these objectives, we sampled wild bees and their floral resources from spring to fall in 2014-2016 at 12 riparian sites along a 14 km stretch of Meadow Creek at Starkey. Ungulates were excluded at half the sites beginning in 2014.

Results/Conclusions: We recorded over 150 species of flowering forbs and shrubs at the 12 Starkey riparian sites, and over 900 native bee visitors (> 80 species and 22 genera). Most flowering plant species (75%) had no observed bee visitors. Of the 38 plant species that did, the four with the greatest number of observed bee visitors were three native species (Solidago missouriensis, Potentilla gracilis, and Symphyotrichum spathulatum) and one exotic species (Cirsium vulgare). Flowering stems were generally more abundant in sites where ungulates were excluded vs. grazed sites; however, patterns were highly variable in time and space and across species. Blooming plant species composition changed over the three-year period, with some plants frequently visited by bees and also known to be important in elk diets increasing in deer and elk excluded sites relative to grazed sites. The results of this study illustrate the importance of understanding dietary overlap when investigating the consequences of wildlife – pollinator interactions. Native ungulates are common ecosystem engineers, whose spatial and temporal patterns of herbivory appear to have important effects on ecologically significant invertebrates, such as wild bee pollinators. The results of our work can inform pollinator conservation and ungulate management when multiple objectives must be considered.