In the floral marketplace, plants might facilitate each other or compete for pollination services. These pollinator-mediated interactions likely depend on both the spatial distribution of plants and the resources they provide, but how these two factors together influence interactions among plants has been difficult to predict. This is primarily because it has not previously been feasible to measure floral distributions at pollinator-relevant scales; further, the nutritional quantification of floral rewards has been incomplete. Here we connect perspectives from nutritional and spatial ecology to understand how the identity and rewards of flowers surrounding a focal plant impact its pollen receipt. Using unmanned aerial vehicles, we collected high-resolution aerial imagery (~2px/cm) of large co-flowering plant communities (2500-4000 m2) in the Sierra Nevada (NV, CA). Across ten patches we measured con- and heterospecific pollen receipt on stigmas from five individuals of every flowering species that represented more than 10% of the community (155 plants, seven focal species). We identified all co-flowering inflorescences within 10 meters of focal plants from aerial images. To quantify focal species’ floral rewards, we measured the amount and sugar concentration of nectar and analyzed pollen nutrition (crude protein and lipids, which impact bee foraging).
Results/Conclusions
Overall, focal plants received significantly more conspecific than heterospecific pollen from other focal species (Poisson GLMM, LRT, p < 0.001), suggesting that perhaps competition for pollination services among these focal species is low. Nevertheless, there were significant differences among plant species in con- and heterospecific pollen receipt (species*source interaction, LRT, p < 0.001). In a preliminary analysis, we calculated the Euclidian distance in three-dimensional nutrient space among the rewards offered by four focal plant species. Using these distances, we found that for focal plants that had heterospecific pollen on their stigmas, deposition from competitors that had more nutritionally similar rewards was highest (Poisson GLMM, LRT, p<0.01). Further, in a preliminary analysis of one plot, we found that neighborhood composition had diverse impacts on pollen transfer. In some cases, there was apparent competition among co-flowering plants where conspecific pollen receipt decreased when competitors grew nearby (Log LMM, p<0.05). In other cases, plants showed little variation in pollen receipt regardless of neighborhood composition. These findings suggest that floral reward nutrition can be important in shaping plant-plant interactions and raise novel questions about how floral reward nutrition may interact with plants’ spatial distribution to drive pollinator-mediated associational effects.