Traits and impacts of introduced species: a quantitative review of meta-analyses

Despite significant growth in the information on introduced species (NIS), agreement on their comparative fitness and effects on native (NAT) communities is heterogeneous. Hypotheses aimed at explaining their fitness and impacts are as often supported as challenged. In order to investigate whether origin is associated with fitness or impact, we reviewed 72 meta-analyses of trait differences between NAT and NIS, and impacts of NIS on NAT and the environment, covering terrestrial and aquatic organisms. Interpretation of the trends was based on overall meta-analysis conclusions, and on numbers of point estimates (individual effect sizes) within and across studies. Mixed results were found in 44% of the studies. Higher NIS fitness and significant impacts were found in 31%, and 25% concluded that significant outcomes are largely absent. Point estimates showed that non-significant outcomes (57%) are more common than significant ones. Two thirds yield mixed trait differences and impacts, or do not support the concept that NIS are more fit or have negative effects on NAT. When trait differences or impacts are significant, NAT are usually less fit, or are negatively impacted, but this trend is influenced by the fact that most surveys are based on the most damaging invasive species.