Here Comes the Heat: Urban Warming Increases Abundance but Compromises Fitness in the Common Woodlouse

Urbanisation and urban heat frequently co‐occur, and climate change is expected to amplify these effects, yet their consequences for population dynamics and life‐history traits remain unclear. Using the common woodlouse Armadillidium vulgare , a widespread decomposer and urban indicator species, we combined fine‐scale field surveys with climatic chamber experiments on offspring of field‐collected individuals to disentangle these effects. Field surveys revealed consistently higher abundances and occurrences in Urban Heat Islands (UHIs) compared to Urban Cool Islands (UCIs), but with lower reproductive success and elevated senescence, as measured by β‐galactosidase activity, pointing to a trade‐off between density and individual fitness. For offspring of field‐collected individuals reared in climatic chambers, growth remained unaffected, but survival and reproduction declined under future (2050) climate conditions relative to present (2024), suggesting only limited acclimatisation. Offspring of individuals from Urban Heat Islands (UHIs) showed earlier reproductive timing under future climate conditions, suggesting a potential adaptive response. Moreover, β‐galactosidase activity was lower in UHI offspring exposed to 2050 conditions, pointing to acclimatisation in stress physiology. Together, these results highlight that even highly opportunistic urban generalists may experience fitness costs when urban heat and urbanisation interact with climate change, with potential consequences for decomposer‐driven ecosystem functioning in cities.