Body size can vary dramatically between animal species. In bees, interspecific size variation is substantial and functionally significant. However, considerable size variation also exists within species, and, for eusocial bumble bees, size variation can be seen across different castes (queens, workers, males). Large-bodied bee species, such as bumble bees, appear to be more sensitive to environmental change than small-bodied species. Yet, we have few records of intraspecific size variation for wild bees at the population or community level. This lack of knowledge on a fundamental trait hampers our ability to understand how within-species size variation may relate to other aspects of bees’ natural history and their ability to deal with abiotic variation and potential disturbances. Here, I quantified the breadth of size variation within a bumble bee (Bombus) community near the Rocky Mountain Biological Laboratory (RMBL) in Gothic, CO and explored how intraspecific size variation relates to emergence timing and diet breadth. To do this, I measured specimens across castes (queen, workers, males) from the three most commonly encountered bumble bee species at RMBL (Bombus bifarius, B. flavifrons, and B. appositus) and combined this size information with existing data on their emergence timing and diet breadth.
Results/Conclusions
Our results from the RMBL Entomology Collection illustrate considerable size variation across our three focal species, within species across each bumble bee caste, and within each caste. For queens, Bombus bifarius was similar in size to B. flavifrons (mean intertegular distance = 4.86 mm, 4.49 mm respectively), followed by B. appositus (mean intertegular distance = 5.67 mm). The earliest emerging species (B. bifarius) had the largest coefficient of variation in body size (CV = 17%) and the widest diet breadth (~25 resources). In contrast, the latest emerging species (B. appositus) had the largest mean size, the smallest coefficient of variation (CV = 4%), and the narrowest diet breadth (~10 resources). Bombus flavifrons emerges intermediately between the two other species and had an intermediate coefficient of variation (CV = 8%), and an intermediately sized diet (~ 17 resources). In the RMBL area, the growing season is characterized by harsh and variable abiotic conditions. These results suggest that greater intraspecific size variation may allow bees to tolerate a greater range of biotic and abiotic conditions.