Pollinator assemblages in grasslands along river valleys depend on the urban matrix and local habitat scale variables

Riverside urban grasslands (distinctive riparian ecosystems situated alongside rivers within cities) can provide several benefits for pollinators, which are crucial for the health and sustainability of ecosystems. Maintaining these habitats not only promotes the diversity of pollinators but also helps mitigate the negative impacts of human activities on the urban landscape. However, it is still unclear how local environmental and vegetation factors affect pollinators' functional and taxonomic distribution along riverside grasslands in cities and how to use these results to preserve and conserve urban wildlife. In this study, we examined how the taxonomic and functional diversity of wild bees, butterflies, and hoverflies and their traits in urban riverside grasslands are related to the environmental and vegetation factors. We studied bee, butterfly, and hoverfly assemblages in 30 (200 long and 1 m wide) transects along riverside grassland sites in Poznań, Poland. We found that higher cover of impervious surface area leads to lower butterfly abundance and species richness and taxonomical diversity of butterflies and hoverflies. Butterfly taxonomical and functional diversity increased with increasing distance to human settlements. Furthermore, the higher cover of plants and tall vegetation positively correlated with butterfly abundance, species richness, and diversity, as well as hoverfly species richness. Cover of water bodies was positively related to bee diversity and hoverfly taxonomical and functional diversity. As a result, specialized pollinators are more likely to decline than generalist pollinators adapted to urban environment. By creating and maintaining riverside urban grasslands that prioritize the needs of pollinators, cities policymaker and local communities can contribute to the conservation of these vital insects and promote a healthier, more sustainable urban environment.