Heads you win, tails you lose: Life-history traits predict invasion and extinction risk of the world's freshwater fishes

Freshwater fishes have the distinction of being both a highly imperilled taxonomic group as well as one that has produced many invasive species with widespread ecological impacts. Faced with the difficult task of identifying those species with the greatest need for management action, ecologists have turned to using predictive suites of ecological and life-history traits to provide reasonable estimates of fish invasion and extinction risk. Whether traits associated with invasiveness are the inverse of those associated with imperillment, known as the ‘two-sides-of-the-same-coin’ hypothesis, remains unclear. A global trait analysis (including maximum total body size, longevity, size at maturation, age at maturation, fecundity and egg size) for 6293 freshwater fishes was conducted to examine the trait correlates of species proneness to invasion or extinction. A meta-analytical procedure was deployed using univariate and multivariate trait analyses that accounted for the effects of shared phylogeny. Mean trait differences (measured as Hedges' d effect size) were found between invasive and threatened species when compared with native species, thus supporting the two-sides-of-the-same-coin hypothesis for freshwater fish. Invasive species were characterized by larger body size, greater longevity, delayed maturation and higher fecundity than threatened species. Furthermore, invasive species were found to display greater trait variability compared with threatened species, suggesting that different traits may be selected at different stages of the invasion process (from pathway entrainment to establishment) whereas more specific trait combinations may predispose species to higher extinction risk. The present study demonstrated a strong trait basis to global-scale invasion risk and extinction vulnerability for freshwater fishes. Given that both time and resources are too limited for detailed species-by-species assessments, the results suggest that trait correlates provide a reasonable estimate of invasion and extinction risk that can inform more targeted and proactive conservation strategies.