Urbanization is a key reason for connecting landscapes, but increasing connectivity within cities is an important challenge. Green roofs, rooftops designed with shallow soil and stress tolerant vegetation, have been used mostly for properties like draining, but what about their ecological value? Can green roofs increase habitat connectivity for arthropods? In a recent study, Braaker and colleagues tested if green roofs in Zurich, Switzerland can increase arthropod biodiversity. They sampled carabid beetles, spiders, weevils, and bees on green roofs and compared them to similar green areas at ground-level. Using redundancy analysis and Moran’s eigenvector maps, they compared community composition between sites and analyzed the importance of connectivity for creating neighboring communities with similar species composition. In addition to finding the ecological value of green roofs, Braaker wanted to find the relative importance of the local environment, microclimate, and connectivity for explaining species composition.
One key finding was that rooftops and ground areas had substantially different arthropod communities for most of the groups sampled. That means that the green roofs increase total biodiversity by providing a distinct habitat that draws in arthropods from outside the urban areas. In addition, Braaker et al. found that microclimate for mobile species was not as important as connectivity. For individuals that made informed dispersals (like attraction to chemical signals) and species that were highly mobile, connectivity shaped their movements. This research is the first of its kind to show that green roofs serve as stepping stone corridors for arthropods. One final thought on the paper is that there was no effect on green roof size – small green roofs had the same connectivity effect as large ones. This paper is the first of its kind to show that green roofs can increase urban arthropod biodiversity through connectivity.