Are alien plant species superior to natives, and is this determined by performance measure and study design? A meta-analysis

Biological invasions are a major threat to biodiversity because of invasive alien species’ high population growth rates and spread in new ranges. The inherent superiority hypothesis states that particular characteristics of alien species cause them to perform better than native species. Using a meta-analysis of 127 studies and more than 900 experimental observations comparing alien and native plant performance, we investigated, whether: (1) studies comparing alien and native performance generally support the inherent superiority hypothesis; (2) the direction and magnitude of superiority depend on the choice of performance measure; and (3) it depends on other aspects of the study design or species. We found that the inherent superiority hypothesis was overall supported, although the strength of this result depended on the chosen measure of effect size (a significant effect for the standardized mean difference SMD (Hedge's g) but not for the log response ratio LRR). Alien plant species were more likely to be found superior compared to natives if performance was measured in terms of growth, reproduction or response to natural enemies. Measuring survival or abundance was less likely to result in alien superiority, while for measurements of physiology and response to mutualists results were mixed. Furthermore, aspects of experimental design, selection and number of study species played an important role. We thus quantitatively showed across a broad range of conditions how choice of performance measure and experimental design affect the direction and magnitude of alien superiority found in small-scale studies. Furthermore, our review pointed out a lack of studies that assessed population growth as a direct determinant of true superiority. Conducting studies using performance measures relevant for superiority, while also considering other potentially important factors such as residence time, will shed more light on how common true alien superiority is and in which contexts it is to be expected.