Corey Thomas Callaghan, Diana Bowler, Henrique Pereira

Urban expansion poses a serious threat to biodiversity. Given that the expected area of urban land cover is predicted to increase by 2-3 million km2 by 2050, urban environments are one of the most widespread human-dominated land-uses affecting biodiversity. Responses to urbanization differ greatly among species. Some species are unable to tolerate urban environments (i.e., urban avoiders), others are able to adapt and use areas with moderate levels of urbanization (i.e., urban adapters), and yet others are able to colonize and even thrive in urban environments (i.e., urban exploiters). Quantifying species-specific responses to urbanization remains an important goal, but our current understanding of urban tolerance is heavily biased towards traditionally well-studied taxa (e.g., mammals and birds). We integrated a continuous measure of urbanization — VIIRS night-time lights — with over 900,000 species’ observations from GBIF to derive a comprehensive analysis of species-specific (N=158 species) responses of butterflies to urbanization across Europe. The majority of butterfly species included in our analysis avoided urban areas, regardless of whether species’ urban tolerances were quantified as a mean score of urban tolerance across all occurrences (79%) or as a species’ response curve to the whole urbanization gradient (55%). We then used the species-specific responses to urbanization to assess which life history strategies promote urban tolerance in butterflies. These trait-based analyses found strong evidence that the average number of flight months, likely associated with thermal niche breath, and number of adult food types were positively associated with urban tolerance, while hostplant specialism was negatively associated with urban tolerance. Overall, our results demonstrate that specialist butterflies, both in terms of thermal and diet preferences, are most at risk from increasing urbanization, and should thus be considered in urban planning and prioritized for conservation.