Meta-analysis of EICA hypothesis finds no trade-off between plant defense and competitive ability

Background/Question/Methods

The Evolution of Increased Competitive Ability (EICA) hypothesis makes specific predictions: that the release from the selective pressure of herbivory in the native range will allow invasive plants to evolve lower defenses and higher competitive ability. Since it was first proposed seventeen years ago, EICA has been tested in experiments that address the different hypotheses in a variety of invasive species using a range of techniques. We conducted a meta-analysis to assess existing evidence for EICA, using searches in Web of Science and a requirement for multiple populations within both native and invasive ranges and collected 30 publications of original studies that met these requirements. From these papers, we collected records to address the hypothesis that invasive plants have lower defences, altered reproductive potential and higher competitive ability than their native counterparts. 

Results/Conclusions

In congruence with EICA, plants from their native range were better defended than plants in their invasive range, with increases in quantitative leaf traits including secondary chemistry or leaf pubescence (n=47) and reductions in herbivore damage (n=75). Herbivores responded to native populations with reduced mass gain and slower developmental time in comparison to invasive populations grown under the same conditions (n=54). Under non-competitive conditions in common gardens, there was no significant difference between invasive and native ranges in growth metrics such as biomass (n=76). However, invasive plants had higher reproductive potential than native plants, producing more seeds, flowers, or ramets (n=39).  Only a few studies have directly (and robustly) tested the competitive ability of plants by comparing metrics of plants grown alone with either intraspecifc or interspecific competitors.  We conclude that these studies support EICA’s prediction that invasive plants have lower levels of defense against herbivory. However, the results do not support a tradeoff in plant allocation towards increased competitive ability, as growth metrics do not differ between invasive and native ranges, and there was insufficient evidence from direct tests of competition.  We recommend that future tests of the EICA hypothesis include explicit tests of competitive ability that manipulate plant density and include both intra- and interspecific competitors to conclusively determine if rapid evolution occurs following invasion to create more competitive invaders.