Scaling up nature-based solutions: dose-response effects of urban greening on avian biodiversity

Nature-based solutions (NbS) are emerging as promising strategies for addressing societal and environmental challenges, striving to transform cities into more resilient and sustainable environments. NbS harness nature and ecological processes to provide multiple benefits and while supporting biodiversity. Although urban greening is widely promoted through NbS, evidence on their effectiveness in supporting biodiversity remains limited. A key challenge is determining optimal implementation levels to maximize biodiversity gains, as most studies simply compare presence versus absence of NbS. To address this gap, we conducted a metropolitan-scale survey of over 1, 000 sites across four NbS types-street trees, urban green spaces, green roofs, and water sites-analyzing dose-response relationships between natural elements and urban bird diversity. Our findings reveal predominantly non-linear biodiversity responses, suggesting that 'more is not always better' and that effectiveness depends on ecological context. Tree cover, vegetation complexity, and bank vegetation emerged as key positive drivers of bird diversity, yet their benefits often peaked at intermediate levels. Street trees supported bird diversity up to ∼30 % cover, after which increases were marginal. In green spaces, species richness was highest at moderate levels of tree and shrub cover, emphasizing the importance of structural heterogeneity. Water sites with naturalized banks had significantly higher bird diversity than artificial water bodies, while green roofs contributed minimally to avian biodiversity. Altogether, these results demonstrate that NbS must explicitly integrate biodiversity objectives. Maximizing biodiversity benefits requires developing and implementing NbS designs that account for non-linear biodiversity responses and biodiversity thresholds, and consider the regional ecological context.