Native bee communities associated with isolated aspen stands in the Pacific Northwest Bunchgrass Prairie

Background/Question/Methods

The decline of quaking aspen (Populus tremuloides) has been a long-standing conservation concern. In the western United States, a loss of 60-90% of aspen is estimated to have occurred since European settlement. Aspen is threatened by a variety of factors, including agricultural conversion, overgrazing, changes in fire regime, and urbanization. This loss poses a significant threat to many forms of biodiversity found to be associated with aspen. Previous studies have shown aspen stands support high levels of floral, avian, and mammalian diversity. However, no work has investigated native bee diversity associated with this habitat. The objectives of our study were to 1) describe the diversity, taxonomic composition, and sex ratios of native bees in aspen stands in the Zumwalt Prairie of northeastern Oregon, 2) compare bee communities found in aspen with native bees in adjacent native bunchgrass prairie, and 3) document differences in floral resources associated with each habitat type. We sampled native bees in four aspen stands and at four nearby bunchgrass prairie locations three times during the summer of 2010 using blue vane traps. We also quantified blooming forb richness at each site.

Results/Conclusions

Bee abundance increased in both grasslands and aspen stands as the summer progressed. Although taxa richness and overall abundance of bees did not differ significantly between aspen and prairie sites, genus and species composition did. Bumble bees (Bombus) were significantly more abundant in aspen stands, while sweat bees (Lasioglossum and Halictus) were significantly more abundant in grassland locations. Two species of bumble bees (B. appositus and B. insularis) were highly associated with aspen stands. In addition, sex ratios of one common bumble bee species (B. californicus) was significantly more male-biased in aspen stands compared to grassland locations. One potential driver of observed differences between aspen stands and prairie sites may be differences in floral resources. In late summer, when bees were most abundant, aspen stands had a significantly higher blooming plant richness, as well as a distinctive floral community. This study suggests that the conservation of aspen stands in the western United States will not only benefit many vertebrate species of concern, but also one group of invertebrates, native bees, that form an important component of overall biodiversity and provide the valuable ecosystem service of pollination.