Using Species Distribution Models to Assess the Status of the Declining Western Bumble Bee (Hymenoptera: Apidae: Bombus occidentalis) in Wyoming, USA

Monitoring declining species is crucial to inform conservation but is challenging for rare species with limited information. The Western Bumble Bee (Bombus occidentalis) was previously common in the western United States but has drastically declined. Despite documented populations in the Intermountain West, many areas remain under-sampled. Species distribution models (SDM) can guide sampling efforts in large areas by predicting where the highest probability of suitable habitat may occur. We developed a sampling SDM using historical observations (1910–2010) in Wyoming to predict suitable habitat in the past. Using the model, we selected sampling sites that ranged from low to high predicted habitat suitability and we revisited historical locations where B. occidentalis were observed. Using all data (historical and current), we selected the predictors that explained the most variance, and created separate historical and current (2017–2018) SDM using the same variables to assess how predicted habitat suitability changed. We detected B. occidentalis at 30% of the revisited historical sites and 25% of all sites sampled. Areas predicted to be highly suitable for B. occidentalis in Wyoming declined by 5%; a small decrease compared to declines in the western portion of their range. Predicted habitat suitability increased the most in foothill areas. Creating SDM with landscape and climatic variables can bolster models and identify highly contributing variables. Regional SDM complement range-wide SDM by focusing on a portion of their range and assessing how predicted habitat changed.