Invasive, exotic plants are currently a major threat to natural areas because they can spread through communities and replace native vegetation. Invasive species are believed to be second only to habitat loss in terms of their impact on native diversity, but the mechanisms that allow plants to become invasive are still poorly understood. We propose to investigate the mechanisms promoting invasiveness in Solidago gigantea, a species native to North America and an invasive exotic in Europe. We plan to test the Evolution of Increased Competitive Ability (EICA) hypothesis, which holds that because specialized natural enemies may be absent from the introduced range, exotic plants may evolve to invest less in anti-herbivore defenses and thereby gain a competitive advantage over native plants. In a series of experiments, we will compare European and US goldenrods in terms of their defensive chemistry, their performance in the presence and absence of herbivores, and their ability to support growth of both a specialist and a generalist insect. The EICA hypothesis predicts that European goldenrods should invest less in secondary compounds, and therefore should support better insect performance and have a growth advantage in the absence of herbivores. If tolerance to herbivory, or the ability to recover following herbivore damage, has associated fitness costs as suggested by some recent studies, then European goldenrods would also be expected to have reduced abilities to compensate for damage, compared to US goldenrods.
