Food webs on green roofs are unique but less robust than their ground‐level counterparts

Abstract Green roofs are increasingly acknowledged as novel habitats capable of sustaining diverse species assemblages. Due to their distinct abiotic conditions and stochastic colonization pathways, they are hypothesized to support ecological communities that differ from those of ground‐level habitats, yet the capacity of these habitats to sustain coherent and robust food webs is hardly known. Combining environmental DNA metabarcoding from soil samples with a metaweb approach, we compared the composition, structure and robustness of ground‐level and green roof food webs across 52 paired green roof and ground‐level sites along an urban densification gradient in Zurich, Switzerland. Food webs on green roofs were compositionally and structurally distinct from those at ground level. These networks were denser (higher connectance), more diffuse (greater omnivory) and less structured (increased trophic incoherence) than ground‐level webs. Together, this made the food webs on green roofs more prone to secondary extinctions. These patterns were largely driven by the predominance of predators and a lack of basal consumers, such as detritivores. Variation in food web properties was mediated by green roof design. Specifically, older roofs with deeper substrates supported sparsely connected, vertically diverse food webs that exhibited greater robustness to secondary extinctions than more recently built green roofs. Synthesis and applications . Green roofs operate as distinct habitats, offering novel environmental conditions, thus increasing urban habitat diversity and supporting denser but less robust food webs compared with ground‐level habitats. Increasing substrate depth and allowing for long‐term development can promote trophic diversity and robustness, informing strategies for urban biodiversity conservation and ecosystem management.