The western bumble bee (Bombus occidentalis) was once a common bee in the western United States; however, over the last 20 years, this species has drastically declined across its range. The western bumble bee was petitioned for listing under the Endangered Species Act and received a positive 90-day finding. Several studies documented declining B. occidentalis populations in California, Oregon, and Washington, however, few studies sampled in Wyoming. We expect to find western bumble bees throughout Wyoming because much of the state contains habitat likely suitable for this species. Our objective is to investigate the current range of B. occidentalis in Wyoming by revisiting historic sites and sampling sites that vary in elevation and land cover to create a species distribution model. Specifically, we will address three questions: 1) Is B. occidentalis present in predicted suitable habitats in Wyoming? 2) Do landscape characteristics (e.g. elevation, land cover, temperature, precipitation) predict B. occidentalis distributions in Wyoming? and 3) Is B. occidentalis still present at historical collection locations? In the summer of 2017, we sampled 45 sites across western Wyoming in locations of variable predicted suitability and revisited 14 historic locations.
Results/Conclusions
We produced a predictive species distribution model using historic locations (90% from before 1990) that predicted the bee would occur in areas with low forest canopy density, high snow pack, more temperature evenness (a comparison of daily and yearly temperature fluctuations), and high herbaceous cover. During the summer of 2017, we found B. occidentalis at 22% of all locations sampled around western Wyoming, and at 36% of historic locations. A new model using historic and current data predicted that the total area of suitable habitat has declined around the state and that seasonal heat accumulation and spring precipitation are more important for predicting suitable habitat. Climate change may be a contributing factor to the bees’ population decline as it heavily influences these two variables, which may help clarify what conservation efforts should be implemented.