Background/Question/Methods
Exotic species have been hypothesized to successfully invade new habitats by virtue of possessing novel biochemistry that repels evolutionarily naïve, native enemies. Despite the pivotal long-term consequences of invasion for native food-webs, to date there are no experimental studies examining directly whether exotic plants are any more or less biochemically deterrent than native plants to native herbivores. In a direct test of this hypothesis, we conducted herbivore feeding assays with chemical extracts from 19 invasive plants and 21 co-occurring native plants. We tested for an increase in deterrence with time since invasion, and tested the relationship between deterrence and plant abundance in the field. We also examined morphological defenses and growth-related traits of the plant species, explicitly considering influence of phylogenetic relatedness.
Results/Conclusions
Invasive plant biochemistry is no more deterrent (on average) than extracts from native plants. There was no relationship between extract deterrence and length of time since introduction, suggesting that time has not mitigated putative biochemical novelty. Moreover, the least deterrent plant extracts were from the most abundant species in the field, a pattern that held for both native and exotic plants. Analysis of chemical deterrence in context with morphological defenses and growth-related traits showed that native and exotic plants had similar trade-offs among traits. Overall, our results suggest that particular invasive species may possess deterrent secondary chemistry, but it does not appear to be a general pattern resulting from evolutionary mismatches between exotic plants and native herbivores. Thus fundamentally similar processes may promote the ecological success of both native and exotic species.