The success of invasive plants has often been attributed to their rapid evolution at the introduced range. In particular, release from native enemies has been suggested to select for an evolutionary shift in resource allocation patterns from herbivore defence to increased size. Such evolutionary processes can take place not only between the native and invasive ranges but also within the invasive range over time, but this premise has been very seldom studied. In this study, we examined the potential for post-invasion evolution in two traits hypothesized to facilitate plant invasion success, that is herbivore resistance and allelopathic ability. We studied these traits in the invasive plant Impatiens glandulifera by comparing plants from its native populations and from populations across its invasion chronosequence. Results of common-garden experiment and chemical analyses revealed that plants from native populations or older populations within the invasive range show greater resistance to the generalist herbivore, Deilephila elpenor, coupled with greater production of the secondary defence compound 2-methoxy-1,4-naphthoquinone glycoside. In contrast, no differences were found between populations in their allelopathic effect on the germination of the co-occurring neighbour, Urtica doica. Finally, results from a field survey suggested that older populations within the invasive range incur greater attack rates from local herbivores compared to more recently established populations. Synthesis. Our findings support the idea that the selection pressure of enemy release at the introduced range might attenuate over time, leading to the evolutionary recovery of enemy resistance. This study emphasizes the importance of incorporating the effect of time since introduction when examining evolutionary or ecological processes of plant invasions.
Bibliographical noteFunding Information:
We wish to thank Magdalena Sannwald for her assistance in glasshouse experiments and chemical analyses; Sara Tomiolo, Nicola Lechner, Dominik Reiner, Lara Braun and Christoph Otto for their assistance in the seed collection and for sampling herbivore load in European populations; Dorothee Gross, Elena Spöri and Nadine Bihler for their technical support in the laboratory; and Oliver Bossdorf for stimulating discussions on the experiment. We are grateful to Hammad Hassan, Abdul Wahid Jasra and Kanwal Waqar for providing I. glandulifera seeds from Pakistan; Neeraj Sharma for providing seeds from India; and Almut Kelber for providing eggs of D. elpenor. We thank two anonymous referees for helpful suggestions that substantially improved the manuscript. This study was supported by a grant of the German Research Foundation (DFG) to M.G. (GR4325/1-1).
© 2016 The Authors. Journal of Ecology © 2016 British Ecological Society
- Deilephila elpenor
- Impatiens glandulifera
- Urtica dioica
- chemical defence
- evolution of increased competitive ability
- invasion ecology
- novel weapons hypothesis