Bumble bee responses to climate and landscapes: investigating habitat associations and species assemblages across geographic regions in the united states of america

Bumble bees are integral pollinators of native and cultivated plant communities, but species are undergoing significant changes in range and abundance on a global scale. Climate change and land cover alteration are key drivers in pollinator declines; however, limited research has evaluated the cumulative effects of these factors on bumble bee assemblages. This study tests bumble bee responses to climate and land use by modeling 1) occupancy (presence/absence); 2) species richness; 3) species-specific habitat requirements; and 4) assemblage-level responses across geographic regions. We integrated richness, abundance, and distribution data for 18 bumble bee species with site specific bioclimatic, landscape composition, and landscape configuration data to evaluate the effects of multiple environmental stressors on bumble bee assemblages throughout 433 agricultural fields in Florida, Indiana, Kansas, Kentucky, Maryland, South Carolina, Utah, Virginia, and West Virginia from 2018 to 2020. Increased prevalence of attractive crops was associated with increased bumble bee presence, while higher maximum temperature of warmest month and unattractive crops were linked to bumble bee absences. Bumble bee species richness was positively correlated with attractive crops and elevation but declined with precipitation of the wettest month. Moreover, species richness increased as maximum temperature of warmest month approached 29°C but declined at they rose to 36°C, suggesting a potential temperature threshold around 33°C. Distinct east vs. west groupings emerged when evaluating species-specific habitat associations, prompting a detailed evaluation of bumble bee assemblages by geographic region. Overall, climate and land use combine to drive bumble bee occupancy and assemblages, but how those processes operate is idiosyncratic and spatially contingent across regions. From these findings, we suggested regionally specific management practices to best support rich bumble bee assemblages in agroecosystems. Results from this study contribute to a better understanding of climate and landscape factors affecting bumble bees and their habitats throughout the USA.