No abstract

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.

The relative importance of top‐down and bottom‐up mechanisms in shaping community structure is still a highly controversial topic in ecology. Predatory top‐down control of herbivores is thought to relax herbivore impact on the vegetation through trophic cascades. However, trophic cascades may be weak in terrestrial systems as the complexity of food webs makes responses harder to predict. Alternatively, top‐down control prevails, but the top‐level (predator or herbivore) changes according to productivity levels. Here we show how spatial variation in the occurrence of herbivores (lemmings and voles) and their predators (mustelids and foxes) relates with grazing damage in landscapes with different net primary productivity, generating two and three trophic level communities, during the 2007 rodent peak in northern Norway. Lemmings were most abundant on the unproductive high‐altitude tundra, where few predators were present and the impact of herbivores on vegetation was strong. Voles were most common on a productive, south facing slope, where numerous predators were present, and the impacts of herbivores on vegetation were weak. The impact of herbivores on the vegetation was strong only when predators were not present, and this cannot be explained by between‐habitat differences in the abundance of plant functional groups. We thus conclude that predators influence the plant community via a trophic cascade in a spatial pattern that support the exploitation ecosystems hypothesis. The responses to grazing also differed between plant functional groups, with implications for short and long‐term consequences for plant communities.