Floral preferences of mountain bumble bees are constrained by functional traits but flexible through elevation and season

Patterns of resource use by animals can clarify how ecological communities
have assembled in the past, how they currently function, and how they are
likely to respond to future perturbations. Bumble bees (Hymentoptera:
Bombus spp.) and their floral hosts provide a diverse yet tractable system
in which to explore resource selection in the context of plant-pollinator
networks. Under conditions of resource limitation, the ability of bumble
bees species to coexist should depend on dietary niche overlap. In this
study, we report patterns and dynamics of floral morphotype preferences in
a mountain bumble bee community based on ~13,000 observations of bumble
bee floral visits recorded along a 1400 m elevation gradient. We found
that bumble bees are highly selective generalists, rarely visiting floral
morphotypes at the rates predicted by their relative abundances.
Preferences also differed markedly across bumble bee species, and these
differences were well-explained by variation in bumble bee tongue length,
generating patterns of preference similarity that should be expected to
predict competition under conditions of resource limitation. Within
species, though, morphotype preferences varied by elevation and season,
possibly representing adaptive flexibility in response to the high
elevational and seasonal turnover of mountain floral communities. Patterns
of resource partitioning among bumble bee communities may determine which
species can coexist under the altered distributions of bumble bees and
their floral hosts caused by climate and land use change.