Biological invasions are major drivers of biodiversity change. Alien mammals are particularly concerning in Europe, where their expansion remains unabated, though the driving factors are still unclear. Well-documented introductions and distributions in this continent provide a unique opportunity to understand how human activities influenced this expansion. We modelled the potential alien ranges of 46 established alien mammals in Europe using species’ introduction localities, residence time, dispersal ability, generation length, and climatic suitability. We compared potential and observed ranges through three range indices: range filling (portion of potential distribution occupied), overfilling (portion of observed distribution unexpectedly occupied), and unfilling (portion of potential distribution currently unoccupied), and we investigated the effects of native range size, introduction pathways (species’ sum of the known pathways of introduction across the European alien range, spanning 1492–2020), and socio-economic variables (spanning 1980–2017) on uncovered patterns. We show that the median range overfilling value was high (22%), suggesting that alien mammals are substantially spreading outside expected distribution areas. Conversely, median values of range filling (14%) and unfilling (17%) were lower, suggesting recorded introductions inadequately explain alien mammals’ distributions. Range patterns were strongly shaped by human population density, which positively influenced all three range indices, driving range patterns and influencing alien mammals’ introduction and establishment. Contrary, roads and railways were negatively related to range overfilling and unfilling, as was the number of introduction pathways to range filling and overfilling. Ultimately, the role of these socio-economic factors depends on human behaviour rather than environmental characteristics or species’ ecology. We confirm human agency as an important driver of alien mammals’ distribution and spread in Europe, highlighting that modifying human attitudes and regulations towards these taxa is key to limiting further spread.

Alien species can have massive impacts on native biodiversity, ecosystem functioning, and human livelihoods. Assessing which species from currently cultivated alien floras may escape into the wild and naturalize is essential for efficient and proactive ecosystem management and biodiversity conservation. Climate change has already promoted the naturalization of many alien plants in temperate regions, but whether it is similar in (sub)tropical areas is insufficiently known. In this study, we used species distribution models for 1,527 cultivated alien plants to evaluate current and future invasion risks across different biomes and 10 countries in southern Africa. Our results confirm that the area of suitable climate is a strong predictor of naturalization success among the cultivated alien flora. In contrast to previous findings from temperate regions, however, climatic suitability is generally predicted to decrease for potential aliens across our (sub)tropical study region. While increasingly hotter and drier conditions are likely to drive declines in suitability for potential aliens across most biomes of southern Africa, in some the number of potential invaders is predicted to increase under moderate climate change scenarios (e.g., in dry broadleaf forests and flooded grasslands). We found that climatic suitability is expected to decline less for aliens originating from continents with the tropical biome or from the Southern Hemisphere. In addition, we found that the climatically suitable area will decline less for aliens that have already naturalized in the region. While the number of potential invaders may decrease across southern Africa under future climate change, our results suggest that already naturalized aliens will continue to threaten native species and ecosystems.

Aim: The breadth of ecological niches and dispersal abilities have long been discussed as important determinants of species’ range sizes. However, studies directly comparing the relative effects of both factors are rare, taxonomically biased and revealed inconsistent results. Location: Europe. Time period: Cenozoic. Major taxa: Butterflies, Lepidoptera. Methods: We relate climate, diet, and habitat niche breadth and two indicators of dispersal ability, wingspan and a dispersal tendency index, to the global range size of 369 European-centred butterfly species. The relative effects of these five predictors and their variation across the butterfly phylogeny were assessed by means of phylogenetic generalized least squares models and phylogenetically weighted regressions, respectively. Results: Climate niche breadth was the most important single predictor, followed by habitat and diet niche breadth, while dispersal tendency and wingspan showed no relation to species’ range size. All predictors together explained 59% of the variation in butterfly range size. However, the effects of each predictor varied considerably across families and genera. Main conclusions: Range sizes of European-centred butterflies are strongly correlated with ecological niche breadth but apparently independent of dispersal ability. The magnitude of range size – niche breadth relationships is not stationary across the phylogeny and is often negatively correlated across the different dimensions of the ecological niche. This variation limits the generalizability of range size–trait relationships across broad taxonomic groups.