A meta-analysis of the evolution of increased competitive ability hypothesis: genetic-based trait variation and herbivory resistance trade-offs

Abstract

Non-native organisms are an abundant component of almost all global ecosystems. A prominent framework to explain the success of non-native plants is the evolution of increased competitive ability (EICA) hypothesis. EICA predicts that plants escape from co-evolved herbivores after introduction into a non-native habitat. Assuming limited resources, a relaxation in selection pressures for resistance traits against the co-evolved specialist herbivores allows plants to allocate increased resources to traits related to fitness and/or competitive ability. Despite the prominence of the EICA hypothesis in the literature, empirical evidence has been mixed. We conducted a meta-analysis on 30 studies that focused on genetic-based trait variation and the trade-off between resistance traits and fitness to assess support for the EICA hypothesis. We found general support for EICA across studies. Performance of herbivores was higher on non-native plant populations than on native populations of the same species. Fitness trait values were higher in non-native populations, relative to native, and we found evidence for trade-offs between herbivore performance and plant fitness traits. Support for EICA was strongest when we focused on direct measurements of herbivore performance, and weakest when we assessed resistance traits, highlighting the complex and often unknown relationship between resistance traits and particular herbivores in many plant–herbivore systems.