Variation in defensive traits against herbivores of native and invasive populations of Carpobrotus edulis

The Enemy Release Hypothesis (ERH) predicts that the escape from natural enemies, such as specialist herbivores, may explain the invasiveness of some invasive alien plants, maximizing their investment in growth and reproduction. This release from natural enemies might decrease the investment in expensive defense mechanisms (i.e., digestibility reducers) against the attack of specialist enemies, whilst increasing the investment in defenses (i.e., cheap toxins) and tolerance against the attack of generalist herbivores, as exposed by the Shifting Defense Hypothesis (SDH). To test this, we conducted a greenhouse experiment to compare morphological and physiological traits of Carpobrotus edulis, collected in its native (South Africa) and introduced (Chile and Spain) ranges, attacked by the generalist spittlebug Philaenus spumarius and the specialist scale insect Pulvinariella mesembryanthemi. Our results do not support the ER and SD hypotheses. We found that C. edulis plants collected from native and introduced ranges showed no significant differences in growth and defensive compounds studied for both controls and those plants under attack by the generalist P. spumarius (i.e., showing no increase in biomass or changes in biochemical defenses). In contrast, the specialist herbivore P. mesembryanthemi induced the production of total phenols and tannins and reduced the growth and survival of C. edulis plants. Overall, we found strong evidence that C. edulis is negatively affected by the attack of its specialist herbivore, but not by that of generalist herbivores, regardless of origin. The observed tolerance to generalist herbivores suggests the intriguing possibility of trait selection, allowing C. edulis plants to tolerate generalist herbivores more than specialists.