Individual-based networks reveal the importance of bee fly (Bombyliidae) pollination in a diverse co-flowering community

Co-flowering species are visited by a wide diversity of pollinating insects; however, the structure of individual foraging patterns for non-bee insect pollinators is not well-known even though this is central for the effectiveness of the pollination processes. Pollination by non-syrphid flies, such as bee flies (Bombyliidae, Diptera), has often been underappreciated. Bee flies represent a diverse group of long-tongue nectar-feeding insects that are often reported as generalists that visit flowers indiscriminately. Here, we used individual-based pollen transport networks to assess patterns of individual foraging in bee flies over two flowering seasons in a diverse co-flowering community. Thus, we determined the structure of individual foraging and specialization. We also quantified the abundance of floral resources over time and individual proboscis length and body size to evaluate the role of resource availability and intraspecific trait variation in shaping individual specialization. Although the bee fly population is generalist (20 plant species as resources), individuals partition their niche at the foraging-bout level reflected by a modular pattern of interactions, concentrating their foraging on a few floral resources (2-4 plant species). This indicates that individual foraging in bee flies is nonrandom and can ensure high specificity in pollen transfer among conspecifics in the community. This modular foraging pattern was persistent over the two sampled flowering seasons (2021-2022) and not explained by resource availability. Intraspecific variation in proboscis length, however, was related to the level of individual specialization. An increase in proboscis length was related to an expansion of individual niche, which can be related to broader access to plant species with different floral tube sizes. Furthermore, specialization varied between years as well as the plant composition within interaction modules. Our study suggests that individual foraging patterns can persist over time, although specialization levels and niche partitioning of available resources may be dynamic in response to individual phenotypic variation and other ecological factors.

Methods
This dataset is part of the paper titled "Individual-based networks reveal the importance of bee fly (Bombyliidae) pollination in a diverse co-flowering community", considered for publication in the Journal of Applied Entomology. In this study, we used individual bee flies surveyed on a serpentine seep plant community, North California, during two flowering seasons (2021 and 2022) to assess individual specialization and the role of these neglected pollinators in the pollination process in this highly diverse co-flowering community. Individual bee flies were collected in the seeps by 2-3 people simultaneously by walking at a steady pace while observing all flowering species. Specimens were stored in tubes under freezing temperatures from collection to processing in the lab. Their pollen loads were collected from head, dorsal and ventral thorax using fuchsin jelly cubes that were later mounted on a single microscope slide. All pollen grains obtained from bee flies were counted under a microscope and identified based on pollen libraries previously established from anthers collected for each plant species at the study site. We also measured the proboscis and body length of each individual bee fly using a digital caliper (at the nearest 0.01 mm) after collecting their pollen loads. Finally, we provide data describing resource availability over time by quantifying the number of flowers per species across two flowering seasons (2021 and 2022). We set 1×2-meter plots along the seeps in both years totalizing 81 plots (40 plots in 2021 and 41 plots in 2022). Plant identity and number of open flowers per species were recorded within plots during sampling days (every two days within 21-day sampling in 2021 and 27-day sampling in 2022).