Influence of native ungulate herbivory on riparian floral resources and native bee communities

Bees are essential pollinators in many natural and agricultural systems and declines of some species are well-documented. Threats to bees include habitat loss and degradation and uncertainty remains about how management practices influence bee communities. Globally, ungulates are common in many systems and though numerous studies have examined livestock effects on bees, native ungulate effects on bees are poorly studied. In the western US, two native ungulates, deer ( Odocoileus spp.) and elk ( Cervus canadensis ), are widespread and may occupy large annual home ranges. Public land managers are tasked with managing deer and elk habitat in ways compatible with other land uses, including as pollinator habitat. To address the question of how these large herbivores influence bee communities, we conducted a three-year manipulative study in a Pacific Northwest riparian system. Half of 12 sites were protected from native ungulate herbivory and half were unprotected. We measured bloom availability and diversity each year, and quantified herbivory pressure and sampled bees in the third year. We found fewer blooms of plant species preferred by elk, the dominant large herbivore in the system, in unprotected sites than protected sites in the second and third years, and that blooming plant composition differed between protected and unprotected sites. Bee species richness was lower in unprotected areas in July of the third year and evidence suggests that bees specializing on plants consumed by ungulates may be at particular risk. Our work illustrates the importance of considering dietary overlap when investigating effects of ungulates on bee communities.