Global climate change is expected to alter ecological communities through changes in species’ local abundances and geographic ranges. Species’ range shifts are thought to drive novel species interactions as species migrate at different rates in response to changes in climate. Plant-pollinator communities are expected to be particularly susceptible to interaction decoupling due to the different speeds at which plants and pollinators are able to migrate to “track” climate change. We asked whether plant species’ failure to migrate affected reproduction via changes in pollinator community composition and pollinator visitation rate. We transplanted replicate alpine plant communities downhill to simulate progressively warmer future climate scenarios, observed pollinator visitation to plants, and quantified seed set for three focal species. Additionally, we observed pollinators of plant species naturally occurring at each site, asking whether overall pollinator community composition changed, and whether pollinator visitation increased, in response to warming.
Results/Conclusions
Bumble bees (Bombus spp.) were the dominant pollinators across all elevations, followed by syrphid flies at high elevation sites and Lasioglossum bees at low elevation sites. Consistent with our expectations, visitation rate to naturally occurring plant species significantly increased in response to warming downslope; however, visitation rate to transplanted patches and focal species within patches was highest under intermediate warming. Furthermore, pollinator community composition visiting focal plant species changed dramatically in response to warming. In two focal plant species, female fitness suffered at elevations where bumble bees visited less abundantly, even if overall visitation rate (by all pollinator species) was the same. In a third focal species, overall visitation rate predicted female fitness, regardless of pollinator identity. Our results suggest that climate-induced range shifts may cause plant species to lose in competition for pollinators with novel plant competitors.