Noise, light pollution, and human presence predict forest habitat degradation: A key agent in biodiversity decline

Expanding urbanization introduces various environmental stressors, such as artificial light at night, anthropogenic noise, and human presence. Although these stressors are commonly blamed for biodiversity decline, urban development also coincides with severe habitat transformations, leading to the loss of natural habitats and key ecological features essential for diverse biota. How these environmental changes interact to shape urban biodiversity remains unresolved, posing substantial challenges for conservation policies. Here, we address this issue using multilevel structural equation modeling (MSEM) across 90 wooded green spaces in Kraków, Poland, focusing on local communities of birds (28 species) and bats (5 genera). We found that environmental stressors are widespread correlates of bird and bat occurrences but also strongly correlate with habitat degradation, reflected in reduced green space size and diminished availability of structural features, such as deadwood, tree cavities, and epiphytes-critical resources for these taxa. In MSEM predictions, environmental stressors primarily affected communities indirectly by driving habitat changes. Secondarily, stressors acted as both direct and indirect predictors for some taxa (combined within a single model), though purely direct effects were rare and often co-occurred with habitat effects. Overall, habitat alterations were more significant drivers of taxon loss than stressors, with green space size, crown or lying deadwood, tree cavities, and epiphytic plants emerging as the most critical features for supporting biodiversity. Habitat degradation was primarily correlated with human presence, less strongly with light, and only weakly with noise levels. However, the direct effects of each were similarly rare and could be either positive or negative. Our findings suggest that the seemingly prominent effects of human-associated stressors on biodiversity may often be artifacts of coinciding habitat degradation, with habitat loss and the removal of nuanced habitat features playing a more direct and critical role. While reducing noise, light, and restricting human activity might be effective conservation strategies for some species, they are insufficient without preserving habitat remnants and fostering structural diversity to resemble that of natural ecosystems. These habitat-centric approaches are keystones that should be prioritized, offering a promising roadmap to reconcile human well-being with biodiversity preservation in future sustainable cities.