Green roofs harbor different and non-substituting invertebrate communities than surrounding ground-level habitats

Green roofs are increasingly promoted to support urban biodiversity, potentially mitigating habitat loss from urban growth. Given their standardized design, however, green roofs are expected to differ ecologically from the ground-level habitats they are intended to substitute. Yet the extent of this divergence remains unclear, especially for taxa with varying dispersal abilities, including low-mobility invertebrates that are less often associated with green roofs. To quantify this divergence, we used environmental DNA metabarcoding to assess invertebrate diversity and identify environmental drivers affecting species with different dispersal abilities across 52 pairs of extensive green roof and ground-level sites in Zurich, Switzerland. We found that green roofs shared around 10 % of species with ground-level sites, regardless of how spatially isolated they were. Instead, green roofs harbored unique communities, albeit marginally poorer in number of species. However, this pattern varied by species mobility, with soil-dwelling species forming more heterogeneous communities, likely due to dispersal limitations between roofs. Additionally, green roof design choices, including substrate type and depth, as well as vegetation cover, emerged as key drivers of species richness, regardless of species dispersal abilities. Although green roofs do not structurally or ecologically replicate the ground-level habitats they would substitute, they provide complementary habitats that enhance urban biodiversity when designed with ecological considerations. While further research is required to evaluate the capacity of green roofs to sustain viable populations despite their spatial isolation, this study highlights practical design interventions, such as deepening the substrate, to improve their role as complementary habitats in cities.