The Interplay of Climate Change, Urbanisation, and Species Traits Shapes European Butterfly Population Trends

Aim: To examine how butterfly population trends respond to climate change and urbanisation at a continental scale, andwhether responses differ between urban and rural environments. Location: 869 sites across 12 European countries, spanning six bioclimatic zones. Time Period: 1976–2021. Major Taxa Studied: Butterflies (Lepidoptera). Methods: We analysed long-term monitoring data from > 8400 populations of 145 species representing a wide range of eco-logical and life-history traits. Population trends were modelled in relation to climate variables (temperature, precipitation andaridity), urbanisation (built-up surface), and their interactions with urban context (urban vs. rural) and species traits (trophicspecialisation, body size, reproductive rate and thermal adaptation). Results: Climate warming and aridification were consistently linked to population declines in both rural and urban contexts,while precipitation effects varied by location and species. Urbanisation alone did not predict trends, but the urban–rural contextstrongly modulated species' responses to warming, indicating potential synergies between climate change and urbanisation. Thestronger impact of warming in urban populations likely reflects elevated baseline temperatures and reduced habitat suitabilityand connectivity in urban landscapes, limiting thermal buffering. Species with colder thermal niches and lower reproductive rates were most vulnerable to warming, as warming exceeds the thermal optima of cold-adapted species and lower reproductiverates limit their capacity to buffer climate-driven population declines. Under aridification, which can reduce host-plant availa-bility, trophic specialists declined more in urban areas, whereas generalists unexpectedly declined more in rural sites, suggestingcontext-dependent constraints under increasing water limitation. Main Conclusions: Our findings highlight the complex interplay between climate change, urban context, and species traitsin driving population dynamics. Importantly, our results suggest that urbanisation generally amplifies the negative impact ofclimate change on insect population trends.