Geographical variability in propagule pressure and climatic suitability explain the European distribution of two highly invasive crayfish

Aim We assess the relative contribution of human, biological and climatic factors in explaining the colonization success of two highly invasive freshwater decapods: the signal crayfish (Pacifastacus leniusculus) and the red swamp crayfish (Procambarus clarkii). Location Europe. Methods We used boosted regression trees to evaluate the relative influence of, and relationship between, the invader's current pattern of distribution and a set of spatially explicit variables considered important to their colonization success. These variables are related to four well-known invasion hypotheses, namely the role of propagule pressure, climate matching, biotic resistance from known competitors, and human disturbance. Results Model predictions attained a high accuracy for the two invaders (mean AUC0.91). Propagule pressure and climatic suitability were identified as the primary drivers of colonization, but the former had a much higher relative influence on the red swamp crayfish. Climate matching was shown to have limited predictive value and climatic suitability models based on occurrences from other invaded areas had consistently higher relative explanatory power than models based on native range data. Biotic resistance and human disturbance were also shown to be weak predictors of the distribution of the two invaders. Main conclusions These results contribute to our general understanding of the factors that enable certain species to become notable invaders. Being primarily driven by propagule pressure and climatic suitability, we expect that, given their continued dispersal, the future distribution of these problematic decapods in Europe will increasingly represent their fundamental climatic niche.