Plant-herbivore interactions or colonization history: what drives changes in plant chemical defenses after invasion?

Biological invasions offer a good opportunity to study the changes in plant-herbivore interactions at ecological and evolutionary levels, because introduced plants encounter a complete novel biotic environment. Once in the novel range, plants generally experience a decreased herbivore predation due to a loss of specialist herbivores from the plant native area. This lower herbivory has been associated to changes in plant chemical defenses at population level. With no enemies associated, plants assigning more resources to growth and reproduction and fewer to chemical defenses would be favorable selected increasing the species invasion capacity, as suggested by the Evolution of Increased Competitive Ability (EICA) hypothesis. However, levels of secondary metabolites are highly determined by plant genetics, and thus changes in chemical defenses after invasion could be a result of the plant colonization history. To determine whether changes in plant chemical defenses after invasion are a result of an evolutionary process driven by herbivores or simply determined by the plant invasion pathways, we have conducted an study using Senecio pterophorus (Asteraceae) as a model system. A broad biogeographical survey was conducted covering all areas where S. pterophorus had been previously reported, including the native range (South Africa), the expanded range (Western Cape) and two introduced regions (Australia and Europe). Levels of in situ herbivory were determined for 640 individual plants. Additionally, leaves and seeds were collected to analyze genetic neutral markers (AFLPs) and chemical defenses (pyrrolizidine alkaloids). S. pterophorus from the invaded areas (Australia and Europe) suffered lower herbivory compared to plants from the native area (South Africa), and levels of pyrrolizidine alkaloids were higher in Australia and lower in Europe compared to plants from South Africa. We discuss whether the phylogeographical origin or, in contrast, the decrease in herbivore predation in the introduced populations can explain the observed differences in the type and concentrations of pyrrolizidine alkaloids after invasion