Inducible defenses, generalist herbivores, and the success (and failure) of invasive plants: using replicated triplets to test ecological theories
Many theories attempt to explain the mechanisms by which certain plant species become invasive. In an extension of the Enemy Release and the Evolution of Increased Competitive Ability Hypotheses, the Shifting Defense Hypothesis suggests that escaping from specialist herbivores may allow exotic plants to optimize their defenses against generalist herbivores, gain a competitive advantage over native plants, and become invasive. Whereas digestion-disrupting compounds, which are often effective against specialist herbivores, are mostly constitutive, the toxins which are often effective against generalist herbivores can have large inducible components. In this study, four plant triplets, each containing three closely related taxa, one invasive, one exotic non-invasive, and one native plant species, were exposed to one of two treatments: browsing by white-tailed deer (Odocoileus virginianus) or protection from deer. Leaves from each treatment were then used in bioassays involving the generalist caterpillar Trichoplusia ni to assess constitutive and inducible defenses in each plant species. The replicated triplet approach allows direct test of whether or not differences observed are correlated with invasive ability rather simply native vs. non-native status, and by replicating triplets one can test for the generality of the phenomenon under study.
The expected trade-off between constitutive and inducible defenses was observed; however, invasive species did not consistently demonstrate strong inducible defenses, and there were no clear associations between invasive status and any particular pattern of inducible vs. constitutive defense. Though inducible defenses against generalists may increase invasive potential in some taxa, they are not essential to plant invasions in the eastern United States. Instead, different invasive species utilize different sets of adaptations to invade. The replicated triplet approach employed here allows rejection of the predictions that emerge from the Shifting Defense Hypothesis. This study supports a more pluralistic view of the factors promoting invasions by plants in these systems. Ancillary observations of cannibalism in other caterpillar species suggest that some ‘herbivorous’ species are not restricted to plant diets in reality. Given the frequent use of bioassays, future research should account for such variable feeding behaviors in species commonly assumed to be strictly herbivorous.