Global Analysis of Factors Affecting the Outcome of Freshwater Fish Introductions

As humans move species around the globe, biotic homogenization decreases diversity. It is therefore crucial to understand factors influencing invasion success at a global scale. I analyzed factors predicting establishment of freshwater fishes transferred internationally, based on 1424 cases of first introductions from one country to another as reported in FishBase, an encyclopedic database of finfish biology. I used multiple logistic regression to explore four classes of predictor variables: species traits, environmental traits, match between species and environment, and propagule pressure. The best regression model, selected by Akaike’s information criterion, included four factors that explained variation in establishment of 789 cases for which complete information was available. Establishment was higher for fishes in families with small body size and for omnivores. Establishment rose with endemism of the recipient country’s fish fauna. Species were more likely to establish when humans intended their establishment (384/506 = 76%) rather than when fish were cultivated or used with no explicit desire for naturalization (524/918 = 57%). Three factors proved unrelated to establishment: latitudinal difference between countries of origin and introduction, native species richness of the recipient country (adjusted for area), and date of first introduction. Classification-tree analysis correctly classified (success vs. failure) 60% of 789 cases, based on the same predictor variables as in multiple logistic regression. My significant results reinforce current ideas that invasion risk is high for rapidly reproducing, generalist species introduced into isolated environments. The success of intentional introductions implies that higher propagule pressure generates riskier invasions. Both models were tested by applying them to 445 cases not used for model development. With the regression model, invasion fate was correctly predicted for low-risk (only 40% established) and high-risk (90% established) introductions. With the classification tree, 67% were correctly classified. These analyses could be used to guide quantitative risk assessment of future international transfers of freshwater fishes. The overall high rate of establishment (64%) and strong influence of propagule pressure, however, suggest that few planned introductions of freshwater fishes have low establishment risk.