Small habitat patches are the largest contributors to urban biodiversity across taxonomic groups

As cities densify, green spaces play an increasingly critical role in sustaining urban biodiversity. Yet the relative contribution of habitat patches of varying size, connectivity, and quality has typically been assessed at the patch level, overlooking how these factors shape biodiversity at the landscape scale. Using a decade of monitoring data for invertebrates, vertebrates, and trees across 452 habitat patches in Zurich, Switzerland, we examined how patch area and environmental conditions shape alpha, beta, and gamma diversity. We found that larger patches harbored higher alpha diversity, consistent with the species-area relationship, but also that small patches disproportionately contributed to beta diversity by promoting community turnover. Further analyses also indicated that, per unit area, groups of small patches often supported higher gamma diversity than a few large ones, particularly for trees and invertebrates. Generalized dissimilarity models futher revealed that patch area was a consistent predictor of community composition, not solely via richness effects but through interactions with environmental conditions—including vegetation complexity, water availability, impervious cover, and connectivity to forested habitats. These drivers produced broadly parallel compositional gradients across taxa, with large peri-urban patches hosting more homogeneous and compositionally stable communities, and small inner-city patches supporting distinct, environmentally heterogeneous assemblages. Taxon-specific patterns also emerged: horticultural practices strongly structured tree communities, while invertebrates responded most strongly to landscape-scale factors and vertebrates to local habitat conditions. Collectively, our findings challenge prevailing assumptions in urban planning that prioritize only large patches, showing that even very small green spaces (<1 ha) make essential contributions to gamma diversity and maintain unique biological assemblages. Although small patches do not substitute for habitat loss, they retain high ecological value and can act as stepping stones that facilitate regional persistence. Maintaining a diversity of patch sizes and environmental conditions will therefore be critical for conserving multi-taxon biodiversity in urban landscapes.