Testing the Evolution of Increased Competitive Ability hypothesis using three Rumex species : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

An important goal in the research of introduced organisms is the understanding of the traits and mechanisms that contribute to the long-term ecological success of non-native plants. The evolution of increased competitive ability (EICA) hypothesis states that introduced plants escape natural coevolved enemies (pests, pathogens and herbivores) in their new environment, allowing them to reallocate resources from producing expensive chemical defences towards a higher growth rate and competitive ability. To date numerous studies have tested the predictions of EICA, but empirical support is mixed. This lack of consensus may exist because studies usually only assess competitive ability and do not examine all the predictions of EICA. Furthermore, most studies assess competitive ability against a heterospecific, which could confound competitive effects with species-specific interactive effects. This thesis addresses the shortcomings of previous studies by conducting the most comprehensive EICA test to date. To do this, I use three Rumex species (R. conglomeratus, R. crispus and R. obtusifolius), which have their native range in the United Kingdom and Europe and were introduced to New Zealand. I conducted field surveys in both the native (United Kingdom) and introduced (New Zealand) ranges of the study plants with the aim of assessing the abundance and diversity of insect herbivores, as well as the damage they cause. I found that in the introduced range specialist insect herbivores are absent, there are fewer chewing insects and less herbivore damage on the leaves and roots of plants. These findings show that Rumex plants in the introduced range have all the necessary conditions for the evolution of increased competitive ability and the resulting shifts in defences. To test for increased competitive ability, I conducted an intraspecific greenhouse competition experiment in New Zealand to see if plants from the introduced provenance have evolved to be better competitors. However, contrary to expectations, this experiment did not show any evidence of greater competitive ability in introduced provenance plants. In order to assess any evidence for shifts in chemical defences, I investigated if the quantitative chemical defences of plants arising from seeds from the introduced range are different from plants grown from seed from the native range when grown under similar glasshouse conditions. Here it was found that R. conglomeratus plants from the introduced provenance had lower levels of percentage ellagitannins, but no differences between ranges were detected for any of the other quantitative defences measured. For the other two study species, the quantitative defences measured did not differ between ranges. The results of this thesis provide convincing evidence that EICA is not a key mechanism for explaining the success of Rumex species in New Zealand, although all three study species experience less herbivory in the introduced range. The results also suggest that EICA should be considered on a case-by-case basis and cannot be generalized across species. This thesis also shows that an accurate assessment of EICA requires a comprehensive test of all its predictions, which is something that future studies should take into consideration.