Pollen nutrition structures bee-wildflower community interactions in the Eastern Sierra

Pollination is often driven by the exchange of floral nutritional resources for pollen transfer. Bees for example rely on pollen as their main source of proteins and lipids, essential to adult reproduction and larval development. Yet pollen protein and lipid quality varies widely between plant species, therefore requiring bees to select their diet among the nutritional landscape. Our previous research presented the hypothesis that host-plant pollen protein:lipid ratios (P:L) may shape broad patterns of bee species’ foraging behavior, and that groups of particular bee and host-plant taxa may occupy nutritional niches. To test this hypothesis, we observed bee-flower interactions in Sierra Nevada (NV, CA) wildflower communities along an elevation gradient from 1550-2850m. We systematically sampled pollen-collecting bees to create bee-flower visitation networks, analyzed host-plant pollen nutrition from ~100 species, and analyzed pollen loads from ~80 bee species. From the networks, we asked if 1) plants offering nutritionally similar pollen share bee visitors, 2) bees collecting nutritionally similar pollen share host-plants, and 3) if modules of interactions form nutritional niches. Supporting our hypothesis, our metanetwork across space and time indicates that the major modules are delineated by plant species and bee species with similar pollen P:L within, and distinct differences in P:L between modules. This finding offers new insight into how reward composition at the community level mediates bee foraging strategies and interactions between co-flowering plant species. This study presents an novel approach to understanding how the macronutrient content of floral rewards drives the dynamics of plant-pollinator communities.