Pollinator nutrition from a plant's-eye view: Floral reward macronutrients predict patterns of pollen movement in a co-flowering plant community

Bees acquire nearly all of their protein, lipids, and carbohydrates from pollen and nectar. However, how variation in the nutritional composition of the floral rewards produced by different plant species impacts interactions with pollinators and consequently indirect interactions among co-flowering plants is unknown. Here we asked how pollinator-mediated plant-plant interactions are impacted by 1) absolute floral reward macronutrition, and 2) similarities among species’ macronutrition in a community context. We measured the macronutrient composition of six focal species’ rewards (nectar sugars, pollen proteins and lipids), and documented a large pollen movement network by characterizing conspecific and heterospecific pollen transfer across 712 stigmas from 149 focal bumblebee-visited (Bombus spp.) plants in Sierra Nevada dry meadows. We found that heterospecific pollen transfer correlated with both pollen and nectar nutrition but conspecific pollen transfer did not. Plants with high protein:lipid ratios in their pollen received fewer heterospecific pollen grains, and plants with higher quality nectar trended towards receiving more. Further, species pairs that had similar reward macronutrition were more likely to have heterospecific pollen transfer between them. These findings highlight that pollinator-mediated plant-plant interactions are likely driven by complex and combined effects of pollen and nectar quality. Further these results suggest that the relative success of any given floral reward strategy may depend on how unique that strategy is in a community context, and raise the possibility that plants could escape competition for pollinators if they have a reward strategy that differs from co-flowering species.