Bumble bees are crucial native pollinators in natural and agricultural ecosystems. In recent years, dramatic population declines have been observed for a number of species in North America, although a comprehensive scientific evaluation of patterns and potential contributing factors for this decline has been lacking. I present data from the first large scale collaborative effort to study the status of bumble bees across the United States, including population surveys, comprehensive databasing of historical museum specimens, analysis of genetic structure and diversity, and levels of infection with the pathogen Nosema bombi.
Results/Conclusions
Results from nationwide field surveys in 46 states suggest that the geographic distributions of several bumble bee species have dramatically declined compared to historical ranges inferred from natural history collections. Genetic diversity and number of sampled colonies per site were lower for Bombus pensylvanicus, a declining species in the eastern U.S., than for healthy co-distributed species, although patterns for western taxa were less clear. Declining species also had higher levels of infection by N. bombi than stable species.
These results provide a number of possible hypotheses for bumble bee decline in North America, although assigning a definitive cause will require further research. Importantly, this study provides a baseline framework for future research and monitoring of the distribution and abundance of bumble bees in the United States.