Patterns and drivers of range filling of alien mammals in Europe

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. In contrast, 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.