Herbivore defense syndromes differ between native and invasive plant species

In their non-native ranges, invasive plant species may escape specialist herbivores and thus experience exclusive pressure from generalists. This is predicted to shape the defense strategies of successful non-native species, as proposed by the Shifting Defense Hypothesis. However, most research to date has quantified only limited sets of defense traits of non-native invasive plant species, and thus our understanding of their defense syndromes, which encompass suites of both physical and chemical traits, remains poorly understood. We evaluated a comprehensive array of leaf physicochemical traits, including quantitative structural defenses, qualitative chemical defenses, and nutritional properties, for 15 phylogenetically matched pairs of non-native species and native congeners co-occurring in abandoned farmland in China. We coupled this trait screening with assessments of herbivore damage in a common garden and laboratory larval growth assays using a generalist herbivore (Spodoptera littoralis; Lepidoptera). Non-native species experienced 50.8% less herbivory and reduced larval growth rate by 25.7%, relative to native congeners. This increase in resistance corresponded with a distinct defense syndrome: non-native species allocated more to broad spectrum, apparently qualitative, chemical defenses, exhibiting 43.1% higher phenolic concentrations, but less to quantitative structural defenses than natives. Hierarchical partitioning revealed that phenolics were the primary predictor of lower Spodoptera growth on non-native species, whereas leaf C content was the best predictor for lower Spodoptera growth on natives. We found differences in defense syndromes and strategies between non-native species and natives, and our findings were consistent with predictions of the Shifting Defense Hypothesis.