Nectar sugar concentration contributes to structuring bumblebee and plant interactions

Abstract Understanding the biology of floral traits underlying plant‐pollinator relationships in a community context is important to predict the occurrence and frequency of pairwise mutualistic interactions. Flower nectar is a sugar‐rich solution that is key in plant–pollinator interactions. Nectar sugar concentration directly determines viscosity, regulating nectar ingestion rates by pollinators and, together with nectar volume, shapes foraging decisions. Furthermore, the link between nectar ingestion efficiency and sugar concentration in bee pollinators is mediated by their proboscis length and the spatial positioning of nectar within flowers. We therefore hypothesize that nectar concentration can be related to the structure of bee–flower interactions in a community. We tested the hypothesis in a species‐rich alpine meadow in the Hengduan Mountains, China, a global biodiversity hotspot. The study meadow sustains an extremely diverse bumblebee community. We constructed the bee–flower network in the 2020 flowering season. Visitation frequency of the bumblebees to each flowering plant was recorded while nectar sugar concentration and initial volume of those plants were quantified in addition to other pollination‐related floral traits. We conducted phylogenetically comprehensive analyses to assess factors influencing the network structure, bees' visitation frequency and flower preference, and to detect the correlations of bees' proboscis length with nectar sugar concentration and corolla tube length and the relationships among the pollination‐related floral traits. Modularity analysis indicated that the community bee–flower network was structured by nectar sugar concentration and corolla tube length, which was associated with bees' proboscis length. However, the overall bees' visitation frequency to flowers was correlated to nectar sugar concentration but not to corolla tube length. Flowers visited by long‐proboscid bee species had comparatively dilute nectar. Moreover, the bumblebees' flower preferences were positively influenced by floral abundance. Additionally, variation in nectar sugar concentration was linked with pollination‐related floral traits and pollinator composition, suggesting a diversity of pollination strategies among the plants in the community. Synthesis : To our knowledge, this is the first field investigation uncovering how bee‐flower interactions and nectar sugar concentration are linked at the community level, which broadens the niche dimensions for resource partitioning among co‐occurring species and thus fosters biodiversity.