Eva Drukker

Insects are among the most diverse groups of organisms, and with their widespread population declines, their crucial roles in ecosystem functioning – such as pollination, decomposition, and trophic regulation – have become increasingly evident to society. For green roof designers, it has been a growing interest: to what extent can insects colonize, persist, and interact within such novel environments, and how can we support it? As highly artificial and spatially isolated habitats, green roofs are generally not expected to sustain diverse or functionally complex assemblages of insects, even among ecologists. This thesis examined how arthropod communities develop across multiple trophic levels, linking belowground and aboveground processes from soil organisms to plants and their associated insect assemblages. To my knowledge, this work is among the first to document these successional trajectories across the full interaction network in green roof ecosystems. In Chapter 2, I synthesized the existing literature on arthropods in green roofs. I demonstrated that not all roofs are equal in supporting biodiversity and that distinguishing roof types is essential. From a biodiversity perspective, roofs can be categorized as Sedum roofs, nature roofs, and roof gardens. Sedum roofs offer basic habitats with limited biodiversity potential, whereas nature roofs and roof gardens can support more complex food webs and specialist insects. The latter types remain poorly studied due to their relative scarcity. I also examined how arthropod communities develop over time and the environmental factors influencing them, highlighting the interplay of stochastic and deterministic processes in colonization and succession. Chapter 3 translated these insights into experimental validation. Building on literature predictions, I confirmed that plant diversity – particularly flower seasonality and abundance – is a key driver of pollinator diversity. I also revealed that landscape-scale factors, such as roof height and size, affect species groups differently: bees were negatively influenced by height, while butterflies and hoverflies were less sensitive. These findings aligned with previous studies showing butterflies tend to decline above 50 m, whereas bees respond negatively already at lower elevations. Chapters 4 and 5 leveraged these insights in a unique experimental design, following newly built roofs of similar height over three consecutive years. This standardized approach allowed me to isolate the effect of time on community assembly. Chapter 4 showed that aboveground plant communities exhibited clear successional trajectories: irrigation shifted plant communities from stress-tolerant Sedum analogues toward herbaceous “nature roof” systems, with corresponding arthropod responses. Irrigated roofs supported more spontaneous plant species coming from the surrounding urban landscape and followed trajectories resembling grassland communities. Belowground communities, assessed through nematodes, assembled rapidly but followed distinct patterns: high predator dominance, elevated Maturity Index values on non-irrigated roofs, and no irrigation or vegetation responses. Nematode assemblages diverged remarkably from global feeding guild patterns, suggesting green roofs select for predator-dominated soil communities. Chapter 5 examined arthropod species-level dynamics, revealing that early colonization was dominated by highly mobile taxa, including aphids, leafhoppers, small Diptera, and ballooning spiders, highlighting the importance of stochastic aerial dispersal. Host-plant associations structured herbivore communities, and environmental filtering was evident at finer scales, with hygrophilous taxa favouring irrigated roofs. Rapid colonization by predators and parasitoids demonstrated that trophic interactions establish early, confirming that green roof communities are shaped by a dynamic interplay of dispersal, environmental filtering, plant–herbivore associations, and trophic processes. Although this thesis has advanced understanding of green roof ecosystems, it represents only the beginning. Green roofs are complex and dynamic, and long-term monitoring remains essential to unravel successional trajectories, species persistence, and the ongoing influence of environmental filtering.