Evaluating the Impacts of Invasive Plants on the Forest-floor Food Web

The proliferation of invasive plants into local communities is a major threat to species diversity on a global scale. To offset the effects of invasive plants on biodiversity, a mechanistic understanding of how plant invaders alter ecological networks is essential. This dissertation evaluates the changes plant invasions can elicit to forest-floor food webs. The objectives of this research are threefold, each accomplished with a separate study. First, I use a meta-analysis of the primary literature to evaluate the impacts of invasive plants on the trophic structure of terrestrial ecosystems. Next, a natural field study was performed to investigate how the North American invader, Alliaria petiolata (garlic mustard), impacts soil food webs in woodland ecosystems via reduction of soil fungi. Lastly, I employed a manipulative field experiment using garlic mustard to mechanistically evaluate how it alters the structure of soil food webs. Findings from the meta-analysis indicate that invasive plants have negative effects on primary consumers in wooded ecosystems, which are less numerous in both brown and green food webs of invaded systems. Furthermore, plant invasions increased the abundance of secondary consumers in brown food webs of woodlands, while causing no changes to secondary consumers in green food webs. The natural field study revealed that uninvaded plots had 70% more fungi, 40% more fungivores, and no differences in predators compared to garlic mustard-invaded plots. Fungal community analyses and structural equation modeling (SEM) show that garlic mustard effects on fungivores were associated with changes in fungal resources, suggesting an indirect effect on fungivores. Moreover, fungivore and predator densities were highly correlated. In the manipulative experiment, there were no differences in primary and secondary consumers in the first year of garlic mustard’s life-cycle. However, the adult stage of garlic mustard alters the density of fungivores. Garlic mustard’s effects on fungivores were greatest in non-invaded soils, whereas the native plant comparison (Lactuca floridana) had two times more fungivores in similar non-invaded soils. Overall, the findings from this dissertation show that invasive plants can cause considerable alterations to soil food-web structure in woodland ecosystems, but the effects are more apparent for primary consumers.