Threatened and invasive reptiles are not two sides of the same coin

Aim: The ‘two sides of the same coin’ hypothesis posits that biological traits that predispose species to extinction and invasion lie on opposite ends of a continuum. Conversely, anthropogenic factors may have similar effects on extinction and invasion risk. We test these two hypotheses using data on more than 1000 reptile species. 

Location: Global. 

Methods: We used hierarchical Bayesian models to determine whether biological traits and anthropogenic factors were correlated with whether a species was: (1) listed as Threatened versus Least Concern on the IUCN Red List, and (2) successful versus unsuccessful at establishing a viable population once introduced outside of its native geographical range. The ‘two sides of the same coin’ hypothesis predicts that model coefficients for each trait should be opposite in sign between these two models. 

Results: Seventy-three per cent of model coefficients describing 10 aspects of a species’ life history, ecology, biogeography and environmental niche breadth were opposite in sign between the two groups; however, most effect sizes for variables that showed contrasting relationships were small and/or uncertain. The only exception was body size: larger-bodied species were more likely to be threatened, whereas smaller-bodied species were more likely to be invasive. As predicted, human population density across a species’ native geographical range was positively correlated with both threat and invasion probabilities. Other anthropogenic variables did not have strong analogous effects. 

Main conclusions: The assumption that threatened and invasive species lie on opposite ends of a continuum, while consistent with life-history theory, appears to be an oversimplification. Our results do suggest, however, that anthropogenic variables can be important predictors of a species’ fate, and should be more routinely incorporated in trait-based analyses of extinction and invasion risk.