First encounters in a warming world: Floral constancy, generalist pollinators, and shifting plant phenology

Background/Question/Methods

Individual bees often show flower constancy, specializing on a floral resource while avoiding switches to newly available resources. The familiar plant species benefits by receiving high rates of visitation even when competing floral resources become available. However, the phenology of some plants is shifting more rapidly than others in response to climate change, resulting in a changed sequence of floral resources bees encounter. To determine whether this has implications for plant fitness, we asked how pollinator visitation to competing plant species depends on the order in which those species bloom. 

We used two sympatric plant species with overlapping phenology and a common bumblebee pollinator, Hypochaeris radicata and Campanula rotundifolia. We allowed Bombus impatiens to forage in greenhouse floral arrays representing three phenological progressions: (1) CR Early, with C. rotundifolia at 100% initially and gradually decreasing to 0% while H. radicata gradually increased from 0% to 100%; (2) HR Early, with H. radicata at 100% initially and gradually decreasing to 0% while C. rotundifolia gradually increased from 0% to 100%; and (3) a Control with both species at 50% throughout. We quantified visitation rate to both species, the number of foraging switches between plant species, and seed production by H. radicata.

Results/Conclusions

Overall, the number of bee visits received by both species was significantly higher when that species was introduced first than when it was introduced after its competitor.  In treatments where the proportion of the early species decreased over time, the visitation rate per flower to that species increased, for both species and across all stages of experimental treatments. Seed set for H. radicata showed a similar pattern to visitation. The proportion of switches between floral resources increased as bees trained on H. radicata were introduced to C. rotundifolia, but not when bees trained on C. rotundifolia were introduced to H. radicata.  Our results corroborate other work suggesting that individual bees learn and become specialized on a single floral resource and are slow to switch to a newly emerging resource. As a result, the flower species that blooms first can benefit from higher bee visitation and thus, potentially higher reproductive success. Our study suggests that a plant species able to shift its flowering phenology earlier in response to climate change, while neighbor species do not shift, might benefit from receiving a higher visitation rate from generalist pollinators.