Given widespread bumble bee declines, there is a need to better understand the key factors driving bumble bee population dynamics in the wild. Bumble bees have an annual life cycle. In early spring queens emerge from hibernation to search for nests and found colonies. Colonies grow in the number of foraging workers over the growing season, and after a peak in summer, they produce reproductive bees (males and autumn queens). The success of each of these life stages depends on a continuous supply of floral resources, but may also be influenced by the abundance of previous life stages. Furthermore, variation in abiotic factors may directly or indirectly influence the success of each life stage. These are fundamental yet poorly understood relationships for wild bumble bees. We investigated these relationships by monitoring floral resource and bumble bee abundance across the entire colony lifecycle for multiple species over five years (2015-2019) in a subalpine ecosystem in the Colorado Rocky Mountains. We asked: (1) how do floral resources and climate variables influence inter-annual variation in the abundance of each bumble bee life stage, and (2) how does the abundance of the previous life stage influence the abundance of later life stages?
Results/Conclusions
Across all five years of the study there was considerable variation in floral resources, abiotic factors, and the abundance of all bumble bee life stages. We observed a strong positive relationship between floral resource abundance, worker abundance, and colony output, but this was not the case for spring queens. Instead, we saw a steady decline in the abundance of spring queens for multiple species over our study period, which appeared to be related to abiotic conditions over winter. The abundance of workers and males was also influenced by abiotic factors (during summer), but only indirectly through floral resources. Our research provides evidence of contrasting responses of bumble bee life stages to biotic and abiotic variation. Our results also suggest that bumble bee population sizes are dependent on floral resource abundance—even in a wild ecosystem with rich floral resources—and that worker abundance and colony output may be resilient to fluctuations in spring queen abundance, at least given the queen numbers encountered during our study.