A directed network analysis of heterospecific pollen transfer in a biodiverse community

Community studies have shown that plant species are often pollinated by multiple pollinators; however, networks of heterospecific pollen transfer (HPT) in natural communities remain largely unexplored. We analyzed pollen deposition on stigmas of 57 flowering species to build a picture of plant-plant interactions via HPT in a biodiverse alpine meadow in southwest China. Plant species were categorized as pollen donors or recipients by their link numbers and link qualities. We identified 3609 heterospecific pollen grains, representing 410 links among 69 pollen species. Each plant species received on average 7.2 pollen species and donated its pollen to 5.5 species; only a few species donated or received large amounts of pollen or pollen from a large number of species. Compared to specialized plants, generalized plants tended to receive more heterospecific pollen but exported no more pollen to other species. Plant position in the network was related to both floral traits (stigma position) and pollinator generalization level. When different species share the same pollinator, bidirectional HPT may occur, but this was rarely observed in the species-rich community, indicating that interspecific pollen interference was largely unidirectional. Our study highlights the importance of understanding how sympatric flowering plants reduce deleterious effects of HPT, for example via stigma position. This study is the first to present a pollen transfer network for an entire community and to unravel its properties using directed network analysis.