Green roofs are a promising tool to return nature to cities and mitigate biodiversity loss brought about by urbanization. Yet, we lack basic information on how green roofs contribute to biodiversity and how their placement in the urban landscape affects different taxa and community composition. We studied the effects of local and landscape variables on beetle communities on green roofs. We expected that both local roof characteristics and urban landscape composition shape beetle communities, but that their relative importance depends on species characteristics. Using pitfall traps, we collected beetles during two consecutive years from 17 green roofs in Basel, Switzerland. We evaluated the contribution of six local and six landscape variables to beetle community structure and to the responses of individual species. Communities on the roofs consisted of mobile and open dry-habitat beetle species, with both local and landscape variables playing a role in structuring these communities. At the individual species level, local roof variables were more important than characteristics of the surrounding urban landscape. The most influential factors affecting the abundances of beetle species were 2 vegetation, described as forb and grass cover (mainly positive), and roof age (mainly negative). Therefore, we suggest that the careful planning of green roofs with diverse vegetation is essential to increase their value as habitat for beetles. In addition, while beetle communities on green roofs can be diverse regardless of their placement in the urban landscape, the lack of wingless species indicates the need to increase the connectivity of green roofs to ground level habitats.

Green roofs have recently gained recognition for their potential contribution to urban ecosystems by providing, among other services, habitat for plants and animals, and stepping stones for mobile organisms, thereby enhancing permeability among habitat patches across densely built cities. In Switzerland, investigations over the past 20 years on more than one hundred distinct green roofs across six cities have provided an unprecedented dataset on ground beetles, albeit with information that is scattered across unpublished reports and local databases. We present here for the first time a synthesis of the state of knowledge of ground beetle communities from green roofs in Switzerland. We describe 91 ground beetles species (19,428 individuals) and highlight patterns of species occurrence and composition across green roofs and cities. Most of the roofs host ground beetle communities dominated by five common mobile species with quite diversified ecological requirements. In addition, we observed nine species (10% of all species collected) that are conservation concerns in Switzerland and Central Europe as well as numerous stenotopic species (from grasslands and pioneer vegetation). This indicates that, besides sustaining local populations of common species, green roofs can also offer suitable ecological conditions of high conservation interest. We suggest both improving the design of green roofs (composition and configuration of vegetation, as well as soil depth and substrate composition) to increase their ecological value for species with the most different ecological needs, and to integrate green roofs into urban planning to make them more efficient as biodiversity supports.Keywords Biodiversity . Urban ecology . Urban green . Vertical green . Endangered species . Carabid beetles . Green infrastructures . Red list species * Gaël Pétremand

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