Linked networks reveal dual roles of insect dispersal and species sorting for bacterial communities in flowers

Due to the difficulty of tracking microbial dispersal, it is rarely
possible to disentangle the relative importance of dispersal and species
sorting for microbial community assembly. Here, we leverage a detailed
multilevel network to examine drivers of bacterial community assembly
within flowers. We observed flower visitors to 20 focal plant species in a
coflowering community in the Sierra Nevada, revealing 289 species of
arthropods. We also analyzed bacterial communities on flowers of each
species. We found that plant species with similar visitor communities tend
to have similar bacterial communities, and visitor identity to be more
important than plant relatedness in structuring floral bacterial
communities. However, plant species that were hubs of arthropod visitation
were not necessarily hubs of floral bacteria, suggesting an important role
for species sorting. Across plant species, the composition of
flower-visiting Diptera (flies), bees and non-bee Hymenoptera best
predicted bacterial species composition on flowers. Taken together, our
analyses suggest dispersal is important in determining similarity in
microbial communities across plant species, but not as important in
determining the overall macrostructure (nestedness, modularity) and
microstructure (connectedness based on shared interactors) of the floral
bacterial network. A multilevel network approach thus allows us to address
features of community assembly that cannot be considered when viewing
networks as separate entities.