COS 76-3 - Drivers and consequences of bumble bee body size variation

Background/Question/Methods

Body size is arguably one the most important traits influencing the physiology and ecology of animals. For bumble bees, size variation between and within castes is essential to their ecology, both in their social organizations and the pollination services they provide to plants. Despite their ubiquity and importance, bumble bees are experiencing population declines globally. It is hypothesized that bumble bees are particularly at risk of declines in part because of their relatively large size, and there is also evidence that bumble bees are experiencing declines in body size. These size declines are consistent with broader trends of shrinking animal body size in response to climate change. However, the precise ecological drivers and consequences of size variation remains poorly understood for wild bumble bees. To characterize how climate and floral resources affect bumble bee size distributions, I quantified the breadth of size variation within the bumble bee (Bombus) community near the Rocky Mountain Biological Laboratory (RMBL) in Gothic, CO. I then analyzed these data to ask how size relates to the phenology, diet breadth, and relative abundance of species.

Results/Conclusions

Our preliminary analysis of the three most abundant species (Bombus appositus, B. bifarius, B. flavifrons) shows evidence of substantial bumble bee body size variation between years and within seasons. The largest of these species, B. appositus, declined in size, with queens in 2021 being 11% smaller on average than those in 2020, and workers 18% smaller. Similar trends were seen in B. bifarius (queens -7%, workers -17%) and B. flavifrons (queens -9%, -17%). In 2021, the earliest emerging bees within each species and caste tended to be larger than later emerging individuals. The 2020 season floral abundance and diversity was impacted by drought and low snowpack, resulting in fewer floral resources. In the following 2021 season, queen bumble bees (born the previous fall), were smaller, likely as a consequence of fewer resources. With fewer resources, queen larvae may have matured to a smaller size. While the 2021 season was less impacted by drought and low snowpack, these smaller queens appear to have produced smaller workers. This suggests that bumble bee size variation is strongly influenced by changes in climate conditions and floral resources.