global synthesis of the effects of biological invasions on greenhouse gas emissions

AIM: Evidence is mounting that biological invasions profoundly alter the capacity of ecosystems to regulate or mitigate greenhouse gas (GHG) emissions – a crucial ecosystem service in a changing climate. However, the growing literature has revealed different, even contradictory results and the general pattern over large spatial scales remains obscure. This study synthesizes the effects of invasions by different alien taxa on major GHG emissions. LOCATION: Global. METHODS: A structured meta‐analysis of 68 case studies was performed to determine the generality of the effects of biological invasions on emissions of three GHGs and assess the extent to which the heterogeneity of effects can be explained by recipient ecosystems, invasive taxa, functional traits, climate and methodological aspects. RESULTS: Invasive alien species increased N₂O emissions but promoted carbon sequestration. Effects on CH₄ emissions remained inconclusive. Given the general trends, effects differed by ecosystems, with greater N₂O emissions in invaded forest and higher increase in carbon stock in invaded grassland. Invasive taxa also mediated the effects of invasions: invasive plants enhanced carbon storage whereas animal invaders consistently showed negative effects. Focusing on exotic plant invasions, N‐fixing species caused greater N₂O emissions than non‐N‐fixing species, and for carbon stock, N‐fixing and woody plants exerted stronger positive effects than non‐N‐fixing and herbaceous plants, respectively. Moreover, climatic factors explained the variation in effects of exotic plant invasions but not those of animal invasions. The effects of plant invasions on carbon content varied nonlinearly with climate, with more pronounced effects where temperature and precipitation were extremely high or low. MAIN CONCLUSIONS: This meta‐analysis reveals the overall magnitude and direction of the effects of biological invasions on major GHG emissions, demonstrates that the effects vary substantially by GHGs, biological and environmental factors and proposes avenues for future research. These results highlight the importance of considering species traits and local and climatic conditions in assessing and managing biological invaders.