The Urban Densification Dilemma: How Much Is Too Much for Urban Nature?

Urban densification is widely promoted as a strategy to limit sprawl and preserve surrounding agricultural and semi-natural habitats, thereby supporting biodiversity at the landscape scale. Yet densification often replaces low-density residential structures with multi-story buildings of larger footprints, resulting in the progressive loss of intra-urban green spaces. This raises a critical question: Is there a threshold of urban densification beyond which biodiversity declines sharply, and to what extent can urban greening measures counterbalance these losses? Urban gardens, both private gardens and allotment sites, are increasingly recognized as key refuges for biodiversity and human–nature interactions, but their capacity to sustain diverse invertebrate communities under ongoing densification remains uncertain, particularly for many non-pollinating taxa.To address these knowledge gaps, we analyzed multi-taxa biodiversity patterns across 85 gardens (43 private and 42 allotments) in Zurich, Switzerland, spanning gradients of urban density within 50 m and 500 m radii and varying in local management intensity, vegetation structure, and habitat diversity. Using standardized pitfall and coloured pan traps, we collected more than 260’000 individuals representing 756 species from eight taxonomic groups, encompassing pollinators, herbivores, predators, and decomposers, and compiled functional traits related to trophic roles, abiotic tolerances, morphology, reproduction, life history, mobility, and phenology.We found urban density to be the main driver negatively impacting invertebrate diversity across groups in a non-linear way, followed by plant diversity and management practices. While increased plant diversity, reduced management intensity, and greater habitat heterogeneity can partially mitigate these impacts, such measures and even the installation of green roofs cannot fully compensate for habitat reduction associated with densification. Space-for-time projections further indicate that future densification will lead to additional species losses, despite a degree of resilience within some gardens.Overall, urban gardens are crucial for maintaining multifunctional urban biodiversity and people quality of life, yet they are increasingly threatened by densification. Defining ecological thresholds and strengthening biodiversity-friendly management, while preserving and enhancing networks of high-value garden habitats, are essential steps to reconcile urban development with biodiversity conservation.