Green roofs and pollinators, useful green spots for some wild bee species (Hymenoptera: Anthophila), but not so much for hoverflies (Diptera: Syrphidae)

Abstract

Urbanisation has become one of the major anthropogenic drivers behind insect decline in abundance, biomass and species richness over the past decades. As a result, bees and other pollinators' natural habitats are reduced and degraded. Green roofs are frequently recommended as ways to counter the negative impacts of urbanisation on nature and enhance the amount of green space in cities. In this study we evaluated the pollinator (more specifically wild bees and hoverflies) diversity, abundance and species richness on twenty green roofs in Antwerp, Belgium. We analysed the influence of roof characteristics (age, surface area, height, percent cover of green space surrounding each site) on species richness or abundance of pollinators. In total we found 40 different wild bee species on the green roofs. None of the physical roof characteristics appear to explain differences in wild bees species richness and abundance. Neither could we attribute the difference in roof vegetation cover, i.e. roofs build-up with only Sedum species and roofs with a combined cover of Sedum, herbs and grasses, to differences in diversity, abundance, or species richness. We found a positive trend, although not significant, in community weighted mean body size for wild bees with an increase in green roof surface area. Roof wild bee communities were identified as social polylectic individuals, with a preference for ground nesting. Only eleven individuals from eight different hoverfly species were found. Our results show that green roofs can be a suitable habitat for wild bee species living in urban areas regardless of the roofs' characteristics, but hoverflies have more difficulties conquering these urban green spaces. Urbanisation-the gradual shift in residence of the human population from rural to urban areas-combined with the overall growth of the human population causes an increase in habitat loss, more fragmentation and an overall change in habitat quality 1. One of the major effects of this urbanisation trend is a serious threat to biodiversity on a global scale 2,3. Increased city area results in species habitat loss, increased spatial distance between remaining pockets of green, and an overall change in habitat quality 1. These factors have caused an overall decline in insect abundance, biomass and species richness 4,5. Whether the rates of decline for insects are on par with or exceed those for other groups remains unknown 6. Pollinators are no exception, and together with other functional insect groups also suffer from stressors such as parasites, pesticides and a lack of flowers 7. Pollination is vitally important to ecosystems and crop production, with a staggering 87% (~ 310.000 species) of all flowering plants depending on animal pollination 8. An annual market value of $235-577 billion worldwide, is directly attributable to animal pollination 9. In the north-temperate zone (e.g. Europe) bees, hoverflies and lepidopterans dominate pollination, whereas, in other parts of the world other pollinators, such as wasps and beetles, may be just as important 8. Domesticated honeybees are often used in agricultural areas, although wild bees are the more efficient pollinators 10. In addition, the pollination service's long-term stability is dependent on bee species richness and abundance 11,12. Global honeybee stocks have increased in the past fifty years, while wild bees appear to have declined substantially over this period 7,13. The decline of pollinator species and their distribution is strongly influenced by habitat loss and fragmentation and is further magnified by global warming 9. Although it is clear that urbanisation has an effect on pollinator abundance and species richness, the effects can go both ways 14,15. Overall, lower pollinator species richness and OPEN