The role of different enemy regimes in the evolution of North American versus European Verbascum thapsus

Background/Question/Methods

Escape from coevolved specialist enemies is hypothesized to be a central driver of invader success. Evidence for the importance of enemy escape is substantial, leading to the important question: do invaders evolve to become locally adapted to these different enemy regimes? In plants, trade-offs between defenses against specialist versus generalist enemies should result in evolution that reflects the relative composition of those enemies. To test this prediction, we used a reciprocal transplant experiment to compare 22 non-native populations of Verbascum thapsus from North America to 16 native populations from Europe. We planted two common gardens: one in Colorado (where generalist leaf feeders are dominant) and one in the Czech Republic (where specialist leaf chewers and root borers are dominant). In each garden, half of the plants were treated with insecticide to remove insect feeding, allowing us to investigate between-range differences in plant response to herbivory. By comparing patterns between gardens we can also infer whether observed differences in performance are the result of evolutionary change or current environmental differences between the native and non-native range.

Results/Conclusions

European and North American populations of V. thapsus perform differently in both locations. Within both control and insecticide-treated plants, differences in seed quality and quantity provide evidence that different selective pressures have acted upon these populations since their initial geographic separation. However, predicted patterns of local adaptation only become apparent when comparing plants within the control treatment. When local insect communities were allowed to eat V. thapsus grown in Colorado, North American populations produced more, and larger, seeds than European populations, while the opposite was true for plants grown in the Czech Republic. Because local adaptation appears when comparing plants experiencing herbivory, our results suggest that differences in selection arise from differences in local herbivore communities. In addition, North American populations are more attractive to specialists, which supports our prediction that North American and European populations have become locally adapted to differences in enemy regimes.