Individual-Based Networks Reveal the Importance of Bee Fly (Bombyliidae) Pollination in a Diverse Co-Flowering Community

Flowering plants can be visited by a wide diversity of pollinating insects; however, the structure of plant–insect interactions for non-bee pollinators is not well-known, even though non-bee insects can play a central role in the pollination of many plant species. Pollination by non-syrphid flies, such as bee flies (Bombylius major L., Bombyliidae, Diptera), has often been underappreciated. Bee flies represent a diverse group of long-tongue nectar-feeding insects that are often reported as generalists who 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. Using this approach, we uncover the structure (e.g., modular vs. nested) of bee fly individual foraging and the degree of individual specialisation. We further evaluate the role of resource availability (floral abundance) and intraspecific trait variation (proboscis length and body size) in shaping individual specialisation. Overall, bee flies visited 20 different plant species. However, network analysis shows that individuals are more specialised and tend to partition the floral resource as reflected by the high degree of network modularity. Most bee fly individuals concentrate their foraging on only a few floral resources (two to four plant species) suggesting strong niche partitioning in this group of pollinators. This modular foraging pattern was not explained by differences in resource availability over the season. Proboscis length, however, was negatively related to the level of individual specialisation. Individuals with larger proboscis had larger foraging niches (less specialisation) perhaps due to easier access to a wide range of plant species with different floral tube sizes. Overall, our study reveals high individual specialisation and niche partitioning in bee-fly interactions, mediated by differences in proboscis length, and with important implications for pollen transfer dynamics, plant–plant competition and plant reproductive success in diverse co-flowering communities.