An exotic herbivore reinforces competition between exotic and native plants

Despite increasing evidence that herbivore-mediated indirect effects play a major role in plant competition, it is unclear how and when they contribute to plant invasiveness. The outcomes of herbivore-mediated indirect effects are primarily dependent on the environment and have complex interactions with the direct interactions between plants. We evaluated the herbivore-mediated indirect effects of Solidago altissima (Asteraceae) on other co-occurring native Asteraceae species at multiple sites in a native range [the United States (US)] and in an introduced range (Japan) using common garden experiments. We examined the effect of S. altissima on herbivore damage, above-ground vegetative and reproductive traits of the co-occurring Asteraceae species at sites with different densities of a herbivore, the lace bug, Corythucha marmorata, which is native to the US and introduced in Japan. We observed increased lace bug herbivory in plants grown with S. altissima in Japan but decreased lace bug herbivory in the US. We did not find consistent effect of S. altissima on aboveground vegetative production of co-occurring plants in either ranges. Flower production decreased in plants grown with S. altissima in Japan because of both direct competition and increased herbivory. Direct competition and apparent competition via lace bug herbivory interacted to produce a strong negative effect of S. altissima on co-occurring plants only in Japan. Synthesis. The findings imply that evolutionary history and the local environment jointly affect herbivore-mediated indirect effects, which play a large role in shaping the pattern of herbivory and plant competition. Another implication of our results is that exotic herbivores may reinforce the negative effects of exotic plants on native plants under high herbivore density environments.