Invasive plants are widely assumed to gain an advantage over native plants by possessing novel biochemistry that renders them distasteful to native herbivores. However, this perception is based largely on indirect lines of evidence including library surveys showing that exotic plants can contain chemical compounds that are absent from native plants. There is little direct evidence that exotic plants are any more or less chemically noxious or nutritious than native plants despite the potential long-term consequences for native food-webs. Here, we describe a series of assays with native herbivores feeding on chemicals extracted from over 100 native and exotic plant species from both freshwater and terrestrial ecosystems. We also conducted tissue analyses of plant ecophysiological traits that can be indicative of a plant’s defensive or nutritional characteristics, including leaf toughness, C:N:P content, trichome density, and secondary chemistry.
Results/Conclusions
We found little direct evidence that exotic plants are any more or less chemically noxious or nutritious than native plants. In fact, in freshwater systems exotic plants are often less noxious than their native counterparts. These results make evolutionary sense because exotic defenses may be biochemically novel but ineffective against evolutionarily novel enemies, particularly when compared to the native plants that have long been selected by their co-occurring enemies to evolve effective defenses. In terrestrial systems, exotic plants often had higher trichome densities than native plants, but softer leaves, similar nitrogen content, and higher phosphorous content. High P content can be indicative of a fast-growth, low defense strategy, and phosphorous is often a critical limiting nutrient to insect herbivores. Thus, our results to date suggest that exotic plants are actually less chemically deterrent and more nutritious than their native counterparts, and instead of spreading via herbivore deterrence, exotic plants may succeed by growing fast and tolerating herbivory.
