Ecological effects of non-native species in marine ecosystems relate to co-occurring anthropogenic pressures

Predictors for the ecological effects of non-native species are lacking, even though such knowledge is fundamental to manage non-native species and mitigate their impacts. Current theories suggest that the ecological effects of non-native species may be related to other concomitant anthropogenic stressors, but this has not been tested at a global scale. We combine an exhaustive meta-analysis of the ecological effects of marine non-native species with human footprint proxies to determine whether the ecological changes due to non-native species are modulated by co-occurring anthropogenic impacts. We found that non-native species had greater negative effects on native biodiversity where human population was high and caused reductions in individual performance where cumulative human impacts were large. On this basis we identified several marine ecoregions where non-native species may have the greatest ecological effects, including areas in the Mediterranean Sea and along the northwest coast of the United States. In conclusion, our global assessment suggests coexisting anthropogenic impacts can intensify the ecological effects of non-native species.