No need for fat: Nutritional preferences affect pollen foraging networks in alpine pollinator communities

Most flowering plant species rely on insect pollinators for successful cross-pollination. While floral color, scent, and morphology help attract visitors, the primary motivation for visiting a flower is the nutritional reward - nectar and, especially, pollen. Pollen provides a complete set of macro- and micronutrients essential for adult and larval provisioning in many insect flower visitors. However, pollen nutrient composition (i.e., quality) varies widely among plant species, and different visitors may have distinct nutritional requirements. Whether these differences are linked to the selection of plants visited remains poorly understood. In this study, we investigated whether the nutrient composition of pollen is linked to the visitation patterns of two main alpine pollinator groups: bumblebees and hoverflies. We observed flower visits in the field and identified the origin of pollen collected by bumblebees and hoverflies via metabarcoding. In addition, we analyzed the nutrient profiles of alpine flowering plants from the same habitat, including amino acids, fatty acids, and sterols. We hypothesized that both bumblebees and hoverflies (i) differ in the plant spectrum they were observed on/collected from and visited for pollen collection, (ii) and that preferred nutrient profiles differ between the two pollinator groups. We additionally expected (iii) that, in the studied alpine plant communities, the nutrient composition of pollen of plant species collected by flower visitors is more strongly associated with pollinator identity than with plant phylogeny due to competition for pollinators. Our results revealed that alpine bumblebees and hoverflies were more similar in their pollen hosts and nutritional preferences than expected, but, for both groups, the network based on observed flower visits differed significantly from that based on pollen collection. Pollen fatty acids and amino acids content was positively correlated, and both bumblebees and hoverflies preferred pollen with low fatty and amino acid content. Pollen sterols did not differ between plants collected by pollinators and non-collected plants. Neither fatty acid, amino acid, nor sterol composition was linked to plant phylogeny. Our findings suggest that not only amino acid content, but also fatty acid content, plays a key role in shaping pollen-collection patterns of flower visitors in alpine ecosystems.