Beyond species loss: Community reshuffling shapes biodiversity along the urban–rural gradient

Urbanization is rapidly transforming landscapes and reshaping wildlife communities, yet it remains unclear whether assemblages along urban gradients are structured mainly by processes of selective species loss/colonization that generate nestedness or by species replacement that generates turnover; we tested these alternative mechanisms with a multi-taxa approach in a major European Metropolitan area. 2. Data from three vertebrate taxa characterized by contrasting ecological traits, dispersal ability and spatial scales of response—bats, small mammals and birds— were collected along an urbanization gradient in the Metropolitan City of Milan (Italy) and analysed to quantify beta diversity and its turnover and nestedness-resultant components. We disentangled environmental and spatial drivers of all beta components using distance-based redundancy analyses and variation partitioning. Finally, we evaluated whether community patterns were shaped by species-specific responses to environmental predictors and/or interspecific associations using Joint Species Distribution Models. 3. Beta diversity was high and consistently dominated by turnover across all taxa. Bat and small-mammal communities were primarily driven by environmental predictors, with bats additionally reflecting broad-scale spatial structuring of environmental conditions along the urban gradient, whereas small mammals responded mainly to local habitat characteristics. In birds, strong broad-scale spatial components also emerged. Consistently, community-and species-level analyses showed that bats mainly responded to a gradient of distance from the city centre associated with changes in vegetation structure; small mammals to urban-fabric characteristics and local habitat features such as grass cover; and birds to a broad-scale urbanization gradient and to a combination of green area extent, patch age, local landscape composition and vegetation features such as dead wood and fallen branches. Credible residual species associations were consistently absent across all taxa. 4. Synthesis and applications. Our results indicate that species turnover is a general ecological mechanism shaping vertebrate communities along the urban gradient, driven by taxon-and species-specific responses to environmental and spatial features. Community differences primarily reflect habitat suitability rather than dispersal limitation, highlighting the potential of targeted habitat restoration to support disturbance-sensitive species within urban landscapes. Such actions would help maintain a broader range of ecological niches and overall urban ecosystem functioning.