Evolution of increased competitive ability and shifting defence hypotheses

Müller C. Evolution of increased competitive ability and shifting defence hypotheses. In: Jeschke JM, Heger T, eds. Invasion biology. Hypotheses and evidence. CABI invasives species series. Vol 9. Wallingford: CABI; 2018: 103-123.This chapter focuses on two of the various hypotheses that take into account the role of biotic interactions in invasion biology, namely the evolution of increased competitive ability (EICA) hypothesis and the shifting defence hypothesis (SDH). Both hypotheses mainly consider changes in concentrations of chemical defences in plant individuals from native vs exotic populations and are studied independently of the novelty of the chemical metabolite in the invasive range. The EICA hypothesis predicts that chemical defences should be lower in plants of invasive populations because enemy pressure is reduced in the exotic range. The SDH predicts a shift in chemical defences by discriminating between qualitative and quantitative defences. Qualitative defences (toxins) are cheaper to produce and expected to increase in invasive populations because they are needed as defence against generalists, whereas expensive quantitative defences (digestibility reducers) may be reduced in plants of the invasive range owing to the overall lower enemy pressure. Methodological issues are pointed out that should be considered when testing the differences in plant traits between native and invasive populations. A literature review on qualitative and quantitative defence traits, which were compared between plants of native and invasive origin grown under standardized common-garden conditions, revealed 37 studies, comprising 22 plant species. The results of the review infrequently support predictions for defence distributions of the EICA hypothesis, whereas predictions of the SDH are supported by somewhat more traits. The definition of qualitative vs quantitative defences has shortcomings, however. In particular, actual costs of these defences are difficult to estimate but should be investigated in future studies. Furthermore, instead of focusing on individual defences, multiple defences should be considered in plant species that are invasive to gain a more complete understanding of resource-allocation patterns