Urbanisation is associated with changes in stable isotopes across multiple trophic levels.

The increase in urbanisation imposes important threats to biodiversity through habitat destruction, reduced availability of preferred food resources and higher pollution. To support future urban planning, it is necessary to gather more knowledge on how the ecology of organisms from different trophic levels varies across the urbanisation gradient. In our study, we employed carbon and nitrogen stable isotope analysis to investigate the relationship between increasing urbanisation and δ¹³C and δ¹⁵N values across a tri-trophic system of trees (birch and oak), invertebrates (aphids, other Hemiptera, and caterpillars) and a model avian species for urban ecology (the blue tit Cyanistes caeruleus). For the blue tits, we measured the isotopic niche to assess how urbanisation affect niche width at different life stages (adults and nestlings). We observed higher δ¹⁵N values in all taxa in urban areas and δ¹³C values were also higher in urban trees and blue tit nestlings. Exposure to increased air pollution in urban areas, mainly derived from anthropogenic NO_x gas emissions, is one of the main causes of the increase in δ¹⁵N in urban organisms. Furthermore, in urban areas covered by impervious surfaces there is greater water scarcity in the soils, leading to physiological responses in plants that increase the δ¹³C in leaves. We observed that the isotopic niche of urban blue tits is 4.5-18 times smaller in adults and nestlings, respectively, than that observed for forest individuals. Forest blue tits exhibit broader niches, likely reflecting a greater availability and diversity of optimal resources in less disturbed habitats. Conversely, urban blue tits exhibited narrower isotopic niches, suggesting an impact associated with lower diversity and abundance of profitable prey in urbanised habitats. Our study highlights that urbanisation can affect organismal physiology across different trophic levels in similar fashion.