City as a filter: urban density affects taxonomic and functional diversity of foliage dwelling spiders

Urbanization affects landscape structure, functions and local environmental conditions, with major impacts on biodiversity. An evaluation of its effects on biodiversity, including both taxonomic and functional diversity, is thus compelling, with a specific focus on taxonomic groups providing fundamental ecosystem services. Spiders are ideal biological models for urban ecology studies because they are renowned bioindicators and can be found abundantly along urbanization gradients. In this work, we aim at evaluating the filtering role exerted by urbanization at landscape scale on foliage-dwelling spider communities both at functional and taxonomic level. We assessed the response of foliage-dwelling spiders to urbanization in Torino (NW-Italy), by sampling their communities in urban parks along an urbanization gradient and in a control area located in a nearby natural park. We tested their response in terms of taxonomic and functional diversity to urban density and six landscape metrics. Results of statistical models (GLMMs) were used to predict values of the current biodiversity in the city and its values under different future scenarios of urbanization (i.e. 2040, 2050). Spider abundance and species richness decreased in the city compared to the control area and along the urbanization gradient. Variation in the community composition was mostly due to species replacement (67%) within the control area, and to species loss in the urban area (62%). This pattern was mostly due to the loss of specialized foraging guilds, such as pollinator-feeding spiders. Functional dissimilarity among samples within the urban area was mostly explained by functional loss (69%), suggesting an environmental filter favoring species preadapted to urban conditions. By projecting biodiversity measures in two “greener city” scenarios, we identified 8 priority areas in the city where management actions should be implemented. Our findings underscore the role of urbanization in shaping spider communities, favoring generalist species and specific functional traits. The prediction on future scenarios proved to be useful to identify areas where increasing the surface of urban parks may contribute most effectively to spider biodiversity.