Pollinators on Green Roofs: Diversity and Trait Analysis of Wild Bees (Hymenoptera: Anthophila) and Hoverflies (Diptera: Syrphidae) in an Urban Area (Geneva, Switzerland)

Abstract

Green roofs can provide food resources to several insect groups. For pollinators found in cities, as are wild bees and hoverflies, the existence of a wide variety of green infrastructures is crucial to ensure their development and survival. In order to investigate if wild bees and hoverflies use green roofs and how local and landscape factors influence their abundance and diversity, sampling of these insects was done in 2017 using cornet traps on extensive green roofs of two types: 1) urban green roofs (30% of green spaces in a 200m radius). There were 62 wild bee species and 10 hoverfly species identified during the 22-week sampling period. For the latter, no differences in richness and abundance were found between roofs and between roof types. Most hoverfly species were associated with xero-thermophilic habitats. Regarding wild bees, no difference in abundance and richness was observed between roofs. However, urban roofs showed significantly lower abundances compared to mixed landscape roofs. Local and landscape factors influenced the pollinator communities: the percentage of attractive plant species on roofs was positively correlated with the abundance of wild bees and the percentage of green areas in a 600 m radius was positively correlated with their richness. The traits analysis showed no difference between roofs and between roof types. Our results highlight the important role of green roofs in supplying food resources for urban pollinators instead of providing suitable nesting habitats. The abundance of attractive plant species for pollinators and diversified landscape surrounding green roofs seem to be key factors in order to promote these wild pollinators in cities.