Evolution of defence and herbivory in introduced plants - testing enemy release using a known source population, herbivore trials and time since introduction

The enemy release hypothesis is often cited as a potential explanation for the success of introduced plants; yet empirical evidence for enemy release is mixed. We aimed to quantify changes in herbivory and defence in introduced plants while controlling for three factors that might have confounded past studies: using a wide native range for comparison with the introduced range, measuring defence traits without determining whether they affect herbivore preferences, and not considering the effect of time since introduction.
The first hypothesis we tested was that introduced plants will have evolved lower levels of plant defence compared to their source population. We grew South African (source) and Australian (introduced) beach daisies (Arctotheca populifolia) in a common-environment glasshouse experiment and measured seven defence traits. Introduced plants had more ash, alkaloids and leaf hairs than source plants, but were also less tough, with a lower C:N ratio and less phenolics. Overal...