Pollen loads vary across species, populations, and individuals of three bumble bee species

Bumble bees (Apidae:Bombus spp.) are known to be generalist foragers with a wide range of possible floral partners from which they collect pollen. Yet, we have little information about how the niche dynamics of this diverse taxon vary between species and among populations or individuals within the same species. Here, we test ecological theory suggesting that not all individuals in a species or population will exhibit identical patterns of resource use by metabarcoding the pollen loads collected by individual bumble bees. This molecular analysis provides fine-scale understanding of the plant species present in bumble bees' pollen loads, allowing us to assess variation in the richness and composition of pollen loads across individuals, populations, and species in a high-diversity subalpine region. Our results show that bumble bee pollen loads indeed vary across multiple levels of biological organization. While B. bifarius had the highest pollen load richness at the species-level, individuals of this species had far more variable pollen loads that rarely reflected the species-level niche breadth. B. bifarius also exhibited a distinct composition of plant species present in their pollen loads, though the strongest driver of niche differentiation among the analyzed pollen loads appeared to be a strong association between B. appositus and Delphinium. Ultimately, this work illustrates the importance of assessing niche dynamics across multiple levels of biological organization in order to understand important patterns in resource use, ecosystem function, and resilience to perturbations.