Ecological and evolutionary responses of native species to exotic community members
Biological invasions can have dramatic impacts on the ecology of native communities by altering population dynamics, species interactions, and community composition. Because of these strong ecological effects, invasion by exotic species may also provoke striking evolutionary responses in native species. Here I investigate how the presence of an exotic plant competitor and an exotic insect herbivore interact to affect both the ecology and the evolution of a common native plant species.
Chapter One demonstrates that invasion by the exotic plant Medicago polymorpha has strong direct competitive effects and strong indirect effects, mediated by the exotic herbivore Hypera brunneipennis, on the co-occurring native plant Lotus wrangelianus. These results suggest that exotic plants and exotic herbivores can interact to exacerbate the ecological consequences of invasion. Furthermore, rather than limiting the success of exotics, introduced herbivores that feed on exotic plants may provide additional, indirect avenues through which exotic plants can reduce the fitness of native species.
Chapter Two investigates the mechanisms underlying potential evolutionary responses of native species to biological invasions and demonstrates that patterns of natural selection on native plant traits vary dramatically, depending on the presence or absence of novel species. Here I show that the exotic plant M. polymorpha and the common exotic herbivore H. brunneipennis interact to produce strong novel selection pressures on the competitive response and anti-herbivore defenses of the native L. wrangelianus .
Chapter Three explores whether exotic plant competitors and exotic insect herbivores provoke evolutionary responses in native plants. L. wrangelianus has failed to respond evolutionarily to the serial invasion of M. polymorpha followed by H. brunneipennis. However, when herbivory from H. brunneipennis is experimentally reduced, Lotus appears to exhibit an evolutionary response to invasion by M. polymorpha. Thus, the presence of an exotic insect herbivore counteracts adaptation by the native L. wrangelianus to invasion by the exotic plant M. polymorpha, suggesting that serial invasions by many exotic species may limit the ability of natives to respond evolutionarily to the novel and frequently changing selection pressures that arise with each new invasion.