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

AimAs cities densify and expand, the careful planning and design of green spaces are essential for supporting urban biodiversity. Here, we evaluate the relative contribution of habitat patches of varying size, quality, and connectivity to urban biodiversity and assess environmental factors driving differences in species richness and community composition. LocationZurich, Switzerland. Time period: 2008-2018.Major taxa studied: Invertebrates, vertebrates, and trees. MethodsWe analyzed species occupancy data from 452 habitat patches. We quantified alpha, beta, and gamma diversity, assessed species-area relationships, and applied generalized dissimilarity modelling to test the role of patch area, connectivity, and habitat quality--proxied through environmental variables, including vegetation complexity, water presence, and forest isolation--in shaping community composition. ResultsAlpha diversity increased significantly with area, although small patches (usually < 5 ha) disproportionately contributed to beta diversity. Per unit area, groups of small patches yielded higher gamma diversity than equivalent areas of large patches, particularly for trees and invertebrates. Community composition was strongly influenced by patch area, with effects mediated by vegetation complexity, water, and isolation, with responses differing among taxa. Main conclusionsSmall habitat patches play a critical role in enhancing overall urban biodiversity. They increase species richness through cumulative area effects and promote community turnover (mediated by environmental heterogeneity). Maintaining networks of small patches alongside large green spaces is therefore key to conserving biodiversity in urban landscapes.