No 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.

Extensive green roofs are said to increase local arthropod diversity in urban environments. Most roofs are dominated by either generalists or xerothermic species, as a result of their isolated locations and shallow substrate layers. We investigated whether biodiverse extensive green roofs (Sedum spp., forbs and grasses) attract more arthropod species than conventional extensive green roofs (only Sedum spp.) and differ in their respective community compositions. Six conventional and six biodiverse green roofs were sampled continuously for one year via pitfall traps. Ants, carabids and spiders were identified to the species level. We found 63 spider species, 47 ground beetle species and 22 ant species during our study. Spider communities were dominated by easily dispersible species of Linyphiidae, ground beetle communities by either eurytopic or xerothermic species, and ant communities by the synanthrope Lasius niger. While ant communities differed significantly between conventional and biodiverse roofs, no effect of roof type was observed for spider or ground beetle communities. We conclude that both types of green roof give similar results in the context of conserving urban arthropod diversity, at least for the taxa we investigated. In order to attract a wider variety of arthropods in urban areas we recommend the installation of green roofs that are specifically designed to increase biodiversity.

Here, the abundance of macro-invertebrates (Arthropoda and Gastropoda) of eight green roofs and their adjacent ground level habitats in the city of Antwerp, Belgium, is compared. All higher-level taxa found were present in both types of habitats without significant differences in their overall abundance between green roofs and ground level habitats. However, we found significant differences in abundances between the two types of habitats, when specific taxa were compared. Beetles (Coleoptera), isopods (Isopoda) and bees (Anthophila) were more abundant at ground level sites compared to green roofs, while for true bugs (Heteroptera) and cicadas (Auchenorrhyncha) the opposite was found. Our results support the idea that extensive green roofs in Belgium can provide a suitable habitat for different invertebrate taxa, but further research is needed to identify the true drivers behind differences in abundance between ground level and adjacent green roofs.