Pollinator-Promoting Interventions in European Urban Habitats-A Synthesis.

Pollinators receive considerable interest due to their fundamental role in ecosystem functioning and human well-being. Unlike farmlands, studies of urban pollinator-promoting interventions are scarce and have not been synthesised, hampering policy implementation. To fill this gap, we compared pollinator-promoting interventions (treatment) with conventionally managed (control) sites regarding vegetation, floral resources, and pollinators. Our synthesis investigated 1051 sampling sites with different interventions (abandonment, extensive mowing, flower sowing, and combined practices) and habitats (parks, grasslands, road verges, private and public gardens) from 28 European datasets at pooled- and study-levels. Urban pollinator-promoting interventions generally benefited plants and pollinators with taxon, intervention, habitat, and spatio-temporal specific differences. Pooled analyses showed mostly positive and never negative treatment effects, while study-level details described primarily positive and neutral but rarely negative effects. Bumblebees and butterflies benefited most from the interventions. Some effects were stronger for interventions involving flower sowing, interventions occurring in road verges, and interventions located in Northwestern Europe. Although regulations, guidelines, and monitoring are improving, knowledge gaps remain for some pollinator taxa (e.g., beetles), regions (e.g., Mediterranean), and novel interventions (e.g., for ground-nesting insects). Further collaborative studies from around the world could help cities bring people, plants, and pollinators together by creating resilient, multi-functional urban spaces.