Multifunctional bioretention basins as urban stepping stone habitats for wildflowers and pollinators

Sustainable Urban Drainage Systems (SUDS) are increasingly used to manage precipitation events in cities and create decentralized stormwater management. To enhance ecological benefits of bioretention basins, we investigated development of native forb communities composed of 26 plant species common in northwest Europe, their provision of foraging resources, and functional connectivity between basins for pollinating insects. We used two types of engineered infiltration soil to test effects of growing conditions on plants in four basins under field conditions over three years, and in a controlled environment. Hemi-rosette grassland forbs associated with neutral to calcareous sandy soils tolerated the environmental conditions and persisted during early succession, while there was no effect of infiltration soil on community composition. Furthermore, colonisation by non-sown spontaneous plants was limited, but differed between soil types. Sown forbs and spontaneous wildflowers contributed foraging resources to local pollinators. Fluorescent dye was moved between forbs in all basins, while bi-directional dye dispersal up to 86 m demonstrated basins facilitate functional connectivity for pollinating insects whereby structurally disconnected basins contribute as stepping-stone habitat patches. Finally, in the controlled experiment testing the effect of soil types on plant growth (aboveground biomass) and reproduction (floral units) of eight native forbs, results indicated that infiltration soils should maximise organic content to enhance species richness in bioretention basins, albeit without compromising system performance. Despite low replication in the field study, we demonstrate bioretention basins can support urban biodiversity conservation by providing valuable foraging habitat and functional connectivity for pollinators while requiring minimal management.