Native Bees Associated with Isolated Aspen Stands in Pacific Northwest Bunchgrass Prairie

In the western United States, up to 90% of quaking aspen (Populus tremuloides Michx.) is estimated to have disappeared since European settlement due to human activities. This loss poses a significant threat to many forms of biodiversity associated with aspen, including plants, birds, and mammals. However, no work has investigated native bee diversity associated with aspen. Our objectives were to: (1) describe the diversity, taxonomic composition, and sex ratios of bees in aspen stands in native bunchgrass prairie of northeastern Oregon, (2) compare bee communities in aspen with those in an adjacent grassland, and (3) document differences in floral resources associated with each habitat. We sampled native bees and quantified blooming non-anemophilous plant richness in four aspen stands and four locations in bunchgrass prairie three times during the summer. Bee abundance increased in both habitats as the summer progressed. Although taxa richness and overall abundance of bees did not differ significantly between habitats, community composition did. Bumble bees (Bombus) were more abundant in aspen stands, while sweat bees (Lasioglossum and Halictus) were more abundant in grassland locations. Four species of bumble bees were significantly associated with aspen stands and sex ratios of bumble bees were significantly more male-biased in aspen stands compared to grassland locations. Floral resources are one potential driver of observed differences between habitats. In late summer, aspen stands had higher blooming plant richness, as well as a distinctive floral community. This study suggests that conserving aspen benefits not only many plants and vertebrates, but also ecologically significant invertebrates, such as native bees