A global synthesis of the ecological effects of co‐invasions

Abstract Globally, the majority of ecosystems are being invaded by multiple co‐occurring alien species. While most of the quantitative syntheses have focused on single invasive species impacts, a global‐scale quantitative synthesis on the interaction effects of species co‐invasions across multiple taxa and levels of ecological organization remain largely unexplored. We address this knowledge gap by synthesizing quantitative information from 256 effect sizes published in 36 studies, that reported the individual and interaction effects of co‐occuring alien plant and animal species. Although, our meta‐analysis showed that the mean interaction effect of co‐invasions across a range of variables was antagonistic (i.e. combined effect less than the sum of individual effects), the vote‐counting revealed additive interaction effects (i.e. combined effect equal to the sum of individual effects) to dominate individual observations. Also, the frequency of interaction effect types varied significantly with response variables, ecological organization, ecosystems, continents and study design, thereby indicating the role of these ecological characteristics in shaping co‐invasion interaction effects. Synthesis . Overall, our synthesis highlights the need for incorporating local ecological complexities in future co‐invasion impact research to better predict their interaction effect on biodiversity and ecosystem functioning across spatio‐temporal scales. Looking ahead, we call for concerted research efforts on the ecological impacts of co‐invasions to unravel the causes of variability in interaction outcomes.