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. 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.