Urban areas are favouring the spread of an alien mud-dauber wasp into climatically non-optimal latitudes

Abstract

Insects are highly involved in accidental introductions in non-native areas. Potential distribution modelling is routinely used to predict the dynamics of such range expansions, giving insights on which areas are climatically suitable for establishment. However, even in areas where climatic conditions are unsuitable, colonization may be still possible in sub-areas with particular, human activity-driven microclimates, such as cities. We used as a model species the Asian mud-dauber wasp Sceliphron curvatum (Hymenoptera: Sphecidae), which arrived in Europe 40 years ago, to show that cities may be used by alien species to enlarge their distribution into climatically non-optimal areas. By using an average consensus from six different models, we predicted that, based on climate, S. curvatum would find the highest suitability in most part of Mediterranean basin, which are characterized by high summer temperatures and reduced climatic oscillations. The species is indeed often observed in such areas, but also in Central Europe, where suitability is overall lower. At such latitudes, however, the wasp was more often found in cities (which have the highest suitability) than in peri-urban and rural areas, possibly according to the urban ‘heat island’ effect. In Southern Europe, where climate is overall more favourable for the species, suitability tended to be more similar in both urban and rural environments, and urban detections were indeed rarer. The inclusion of population density in the model improved the suitability of Northern areas in an expected urbanization-driven jeopardized pattern. Hence, S. curvatum would be able to colonize in the future at least some climatically unsuitable Northern areas, reaching up to 70° latitude, by using cities as the main sites for establishment.