Climate change has led to shifts in both the timing of life history events and the spatial distribution of many organisms, including flowering plants and insect pollinators. These shifts can alter plant-pollinator interactions, with potential fitness consequences. In particular, while bumble bees are shifting up in elevation in some subalpine ecosystems, perennial plants are likely to lag behind. In this case, seed set might be lower at lower elevations if bumble bee visits are less frequent, or upward shifts in bumble bees could facilitate establishment of plant populations at higher elevations. As a first step in evaluating the relationships between elevation, visitation, and seed set, we focused on a subalpine community in the Rocky Mountains of Colorado, where elevational shifts in bumble bees have been documented. We examined patterns of bumble bee visitation and seed set for five perennial wildflowers that have shifted phenologically to varying extents and are visited by bumble bees. We investigated how bumble bee visitation rates and seed set vary with elevation, and whether visitation rates and/or elevation can explain variation in seed set. We also compare our findings to surveys of bumble bee abundances along the same elevation gradient completed 41 years prior.
Results/Conclusions
The patterns we found are species-specific: for three species seed set was positively related to elevation, for two species seed set was positively related to visitation, and for one species seed set was positively related to both elevation and visitation. We find evidence in support of upward elevational shifts for two bumble bee species, declines in relative abundance for two other species, and increased abundance for a third. Overall, our results point to idiosyncratic responses among plant and bumble bee species that suggest climate change will alter communities and interactions in non-uniform ways.