Urbanization reshapes pollinator communities: contrasting patterns of taxonomic and functional beta diversity in bees and hoverflies

Urbanization is a pervasive driver of habitat transformation, yet its effects on the taxonomic and functional structure of pollinator communities remain insufficiently resolved. We sampled bees and hoverflies along a semi-natural–suburban–urban gradient in the Poznań city to examine how urbanization influences taxonomic and functional community structure and patterns of beta diversity differ between pollinator taxa. Semi-natural habitats supported the highest bee and hoverfly abundances and species richness and, for bees, the greatest alpha diversity, indicating strong negative impacts on diversity of local assemblages. Nonmetric multidimensional scaling revealed significant shifts in community composition for both groups, with hoverfly communities showing stronger separation among urbanization levels than bee communities, suggesting greater sensitivity of hoverflies to urban environmental change. Functional responses differed between taxa: in bees, functional dispersion declined in urban sites, indicating trait clustering and environmental filtering, whereas in hoverflies, functional richness was markedly reduced in suburban and urban habitats, pointing to the loss of larval and adult trait diversity. Partitioning of beta diversity revealed taxon-specific patterns in both taxonomic and functional dimensions. Overall, our findings show that urbanization restructures pollinator communities through taxonomic and functional reorganization, with responses differing between bees and hoverflies. The observed beta diversity patterns suggest that urbanization is associated with taxon-specific patterns of differentiation. Because our study is based on one city in Poland, caution is warranted when extrapolating these results to other urban systems. Maintaining a mosaic of semi-natural, suburban, and urban green spaces that provides diverse floral resources, nesting substrates, and larval microhabitats will be essential to conserve pollinator diversity in increasingly urban landscapes.