The life cycle of plant invasions: a study of the drivers and impacts of dominant non-native grassland species

  • Andrew O'Reilly Nugent

    Student thesis: Doctoral Thesis

    Abstract

    Non-native plant species that become invasive can impact native communities and
    exclude native species. Understanding what drives the ongoing success of dominant non-native
    species is essential to mitigating the impacts of non-native species and predicting
    where mitigation will be successful. However, the key processes that determine invasion
    outcomes are context dependent and change throughout the life cycle of an invasion.
    Consequently, predicting invasion success and non-native species impact remains
    challenging. In this thesis I aim to identify dominant non-native species having significant
    negative impacts on grassland communities. I present four studies that span the major
    stages of an invasion: the spread and establishment of non-native species, the impacts of
    non-native species following establishment and the persistence of these impacts in the
    long term.
    A common theme throughout is the need to disentangle the relative importance of site
    suitability and competitive interactions between non-native and resident species. We
    present a framework for understanding the landscape effects on the spread of non-native
    species. Long-distance dispersal events, coupled with the distribution of sites suitable for
    establishment, are the key determinants of non-native species spread. We also show that
    non-native species impact is determined by the conditions under which dominant non-native
    species maintain competitive superiority.
    We develop a modelling approach for quantifying the impacts of dominant non-native
    species in a community context. We find that non-native species impact varied
    significantly under different conditions of resource availability and disturbance.
    Experimentally validating this analysis, we test the role of environmental and
    competitive conditions in determining these impacts. We also show that many non-native
    species can persist long-term. Our findings suggest that rehabilitation can be effective,
    but that interventions should target communities where the conditions facilitating non-native
    species impact are not expected to change.
    Overall, this thesis highlights the difficulties in separating the different processes
    underlying invasion success and non-native species impact. Because environmental
    and competitive drivers are both important, the relative importance of different processes
    can be confounded in observed patterns of species abundance. Combining observational
    and experimental data is essential, as neither approach is enough to conclusively identify
    which processes are most important at determining success and impact at different
    stages of the invasion lifecycle. The studies in this thesis demonstrate when models can
    fail and highlight the need for experiments that are both general and robust. The need to
    compare model and experiment is especially acute in ecology, where natural settings are
    often far from controlled, but managing the impacts of dominant non-native species
    requires action under uncertainty.
    Date of Award2020
    Original languageEnglish
    SupervisorRichard Duncan (Supervisor), Bernd Gruber (Supervisor) & Lizzie Wandrag (Supervisor)

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