Mechansims of invasion and native species coexistence: Interactions and implications in the Holcus lanatus invasion

Despite extensive research and wide interest, the mechanisms that drive plant invasions are poorly understood. These mechanisms underlie all interactions between the introduced plant and its newly invaded native community, including both competitive displacement and coexistence. To advance our understanding of invasion mechanisms and their role in the local distribution of native plant species, I researched enemy release and plant-soil feedback mechanisms in the Holcus lanatus (Poaceae) grass invasion in the California coastal prairie. Using field and greenhouse experiments, I explored (1) the effects of enemy release on H. lanatus and native competitors by manipulating insects and aboveground fungal pathogens, (2) the effects of plant-soil feedback from invaded soils on competition between H. lanatus and displaced and coexisting native plant species, and (3) the effect of plant-soil feedback from invaded soils on performance of displaced and coexisting native competitors in the field.;In summary, Holcus lanatus invasion success is, at least in part, due to enemy release, competitive ability, and positive to neutral plant-soil feedback. Aboveground enemies and soil biota and nutrients, all impacted the performance of H. lanatus' native competitors, and influenced their competitive interactions with H. lanatus. For example, soil manipulations showed that displaced species preferred uninvaded soil and thus competed badly with H. lanatus, whereas coexisting species were either insensitive or positively influenced by soil invaded by H. lanatus and were also able to compete well with H. lanatus . Therefore, mechanisms promoting the H. lanatus invasion differentially affect native species in a manner that explains competitive displacement and coexistence.;As a compliment to my dissertation on invasive plant ecology, I participated in an interdisciplinary group project on the role of the horticulture and aquarium trades as vectors of invasive and potentially invasive plants and fish. After surveying nurseries and aquarium stores in the San Francisco Bay region, we found that (1) aquarium stores stock potentially invasive fish species and (2) store owners and the public need to be better educated on their role in prevention of non-native species invasions.