Europe's top 10 invasive species: relative importance of climatic, habitat and socio-economic factors

Using a representative set of 10 of the worst invasive species in Europe, this study investigates the relative importance of climatic, habitat and socio-economic factors in driving the occurrence of invasive species. According to the regression models performed, these factors can be interpreted as multi-scale filters that determine the occurrence of invasive species, with human degradation potentially affecting the performance of the other two environmental filters. Amongst climate factors, minimum temperature of the coldest month was one of the most important drivers of the occurrence of Europe's worst freshwater and terrestrial invaders like the red swamp crayfish (Procambarus clarkii), Bermuda buttercup (Oxalis pes-caprae) and Sika deer (Cervus nippon). Water chemistry (alkalinity, pH, nitrate) determines the availability of habitat and resources for species at regional to local levels and was relevant to explain the occurrence of aquatic and semi-aquatic invaders such as the brook trout (Salvalinus fontinallis) and Canada goose (Branta canadensis). Likewise, nitrate and cholorophyll-a concentration were important determinants of marine invaders like the bay barnacle (Balanus improvisus) and green sea fingers (Codium fragile). Most relevant socio-economic predictors included the density of roads, country gross domestic product (GDP), distance to ports and the degree of human influence on ecosystems. These variables were particularly relevant to explain the occurrence of the zebra mussel (Dreissena polymorpha) and coypu (Myocastor coypu), species usually associated to disturbed environments. The Japanese kelp (Undaria pinnatifida) was generally distributed much closer to ports than the other two marine organisms, although insufficient information on human impacts prevented a correct assessment of the three marine species. In conclusion, this study shows how socio-economic development is associated with the presence of the top 10 worst European invasive species at a continental scale, and relates this fact to the provision and transport of propagules and the degradation of natural habitats that favour the establishment of invasive species.