An examination of the ecological trade-offs among bumble bee queens

Background/Question/Methods . Bumble bees (Bombus spp.) are considered to be some of the most important pollinators in temperate, alpine, and arctic ecosystems. Although many bumble bee species are threatened by rapid population declines, we do not have a complete understanding of what governs bumble bee population dynamics in the wild. Each spring, bumble bee queens emerge from overwintering to initiate new nests that, if successful, will produce many workers and reproductive bees later in the season. Thus, the nesting success of queens is paramount to bumble bee population dynamics. Spring queens face numerous ecological challenges, including finding suitable nest sites and sufficient floral resources, while simultaneously experiencing harsh abiotic conditions. Therefore, queens likely face numerous trade-offs in order to maximize colony establishment and fitness. We ask whether there are trade-offs among emergence timing, diet breadth, and nesting preferences of queen bumble bees. We investigated these potential trade-offs by monitoring queen bumble bee emergence phenology, abundance, foraging patterns, tongue length, and nesting habitat preferences in six species across four years in a subalpine ecosystem in the Colorado Rocky Mountains.

Results/Conclusions . Soon after snowmelt, when abiotic conditions are harsh, floral resources are scarce while nest sites are most abundant. Owing to harsh abiotic conditions during early season, we hypothesized that species emerging soon after snowmelt would have the most generalized diets and nesting preferences (compared to later emerging species). Across all four years of study we saw considerable variation in queen emergence timing, diet breadth, and nesting preferences across species. In particular, we find evidence consistent with our hypothesis, in that earliest emerging species (B. bifarius) has the widest diet breadth, shortest tongue length, and broad nesting habitat preference, whereas the latest emerging species (B. appositus) has the narrowest diet breadth, longest tongue length, and narrower nesting preference. Evidence for trade-offs were mixed for species with intermediate emergence times (B. mixtus, B. rufocinctus, B. flavifrons, and B. fervidus). We suspect that the lack of evidence for trade-offs for species with intermediate emergence times is likely driven by differences in each species ability to cope with environmental conditions and biotic constraints. By improving our understanding of the trade-offs faced by spring bumble bee queens, we are closer to gaining a predictive knowledge of key constraints on their population dynamics.