Are alien plants more competitive than their native conspecifics? A test using Hypericum perforatum L.

The evolution of increased competitive ability hypothesis predicts that introduced plants that are long liberated from their natural enemies may lose costly herbivore defense, enabling them to reallocate resources previously spent on defense to traits that increase competitive superiority. We tested this prediction by comparing the competitive ability of native St John's wort (Hypericum perforatum) from Europe with introduced St John's wort from central North America where plants have long grown free of specialist herbivores, and introduced plants from western North America where plants have been subjected to over 57 years of biological control. Plants were grown in a greenhouse with and without competition with Italian ryegrass (Lolium multiflorum). St John's wort from the introduced range were not better interspecific competitors than plants from the native range. The magnitude of the effect of ryegrass on St John's wort was similar for introduced and native genotypes. Furthermore, introduced plants were not uniformly larger than natives; rather, within each region of origin there was a high variability in size between populations. Competition with ryegrass reduced the growth of St John's wort by >90%. In contrast, St John's wort reduced ryegrass growth <10%. These results do not support the contention that plants from the introduced range evolve greater competitive ability in the absence of natural enemies.