Do vigour of introduced populations and escape from specialist herbivores contribute to invasiveness?

1 Plant species may become invasive due to a lack of natural enemies (e.g. herbivores) in their introduced range. Absence of herbivores may result in selection for the loss of costly herbivore-resistance traits, which are expected to show a trade-off with vigour or competitive ability (the evolution of increased competitive ability, or EICA, hypothesis). 2 We conducted a common garden experiment in Switzerland, in which we compared herbivore resistance and vigour of Senecio jacobaea plants exposed to the specialist flea beetle Longitarsus jacobaeae, for four populations originating within the native range (Europe), and four from regions where it had been introduced (New Zealand, USA) and was unaffected by L. jacobaeae. Our predictions were that, compared with plants from the native populations, plants from introduced populations would experience greater herbivory (due to loss of resistance traits), and exhibit more vigorous growth. 3 As expected, we found that introduced S. jacobaea grew larger, and had greater reproductive output, than plants from the native range. Larger plants experienced more feeding damage, and introduced plants were consumed more even when size differences were controlled. Introduced plants also exhibited a greater relative ability to reproduce after damage was sustained, i.e. higher tolerance to herbivory. 4 Contrary to predictions, however, plants from introduced populations had higher total pyrrolizidine alkaloid production (chemical defence against herbivores). 5 Although plants from introduced ranges exhibited life-history traits consistent with EICA predictions, similar phenotypes were common in one of the populations from the native range, suggesting that EICA may not fully explain the invasion success of S. jacobaea. 6 Our results imply that increased competitive ability (vigour) of invasive plants may be associated with changes in resistance as well as tolerance to herbivory, and both types of anti-herbivore defence may need to be examined simultaneously to advance our understanding of invasiveness.