Gardeners’ Contributions to Urban Biodiversity: A Multi-Taxa Assessment of α- and β-Functional and Taxonomic Diversity in a Densifying City

Urban ecosystems are playing a growing role in conserving and enhancing biodiversity and ecosystem functioning, yet ongoing urban densification can challenge them by reducing urban green spaces, including urban gardens, limiting their contribution to achieving conservation targets. However, it is first critical to understand how different biodiversity metrics and facets, which might show contrasting responses, are affected by local and landscape factors in gardens. Here, we use a large, multi-taxa invertebrate diversity dataset from 85 urban gardens in the city of Zurich, Switzerland, to investigate their role in supporting multifaceted urban invertebrate biodiversity, both currently and considering future urban densification. To do so, we use a combination of social-ecological factors, including management practices, urban density, vegetation and habitat diversity. We sampled 264’972 individuals belonging to 756 species from eight taxonomic groups of invertebrates, including pollinators, herbivores, predators and decomposers, as well as functional traits related to abiotic tolerances, trophic preference, life history, mobility, morphology, reproduction and phenology for all species. We found that gardens’ γ-diversity was mostly driven by α -diversity. We found urban density to be the main driver negatively impacting invertebrate α and 𝛽 taxonomic and functional diversity across groups in a non-linear way, followed by plant diversity, with a positive effect, and management intensity, with a negative one. Thus, local gardens may partly compensate for the effects of landscape urban density on invertebrate diversity, both by increasing local plant diversity and by indirectly increasing habitat diversity and reducing management intensity through human actions. Finally, future urban densification led to species losses across taxa. Overall, our study showed that urban gardens represent an important asset for multifaceted urban biodiversity, yet urban densification represents a major threat. Measures to reconcile urban development with biodiversity preservation are critically needed, especially by improving local garden conditions via biodiversity-friendly management.