Above- and below-ground community responses to a mutualism-disrupting allelopathic invasive plant

To understand the dynamics of biological invasions, mechanisms of invaders’ impacts must be assessed across scales of ecological organization, and under different biotic and abiotic conditions. Traits of invasive species that generate successful competitors such as high fecundity, efficient resource acquisition, or novel chemical weapons allow them to rapidly increase in abundance and negatively affect native communities. Mycorrhizal mutualism disruption is a common and well-supported mechanism of impact of invasive plants, where allelopathic chemicals indirectly impact native plant species via disruption of mycorrhizal soil resource uptake. However, not all native plant species depend on mycorrhizal fungi, creating opportunities for allelopathic plant invaders to have disparate impacts across plant communities. Therefore, direct impacts of allelopathic plant invasion on mycorrhizal fungal taxa and the subsequent indirect effects on native plant species are not expected to be universal. For my research, I use a model for allelopathic plant invasion, Alliaria petiolata (garlic mustard). My dissertation expands mechanistic understanding of impacts of allelopathic plant invasion, knowledge of the consequences of complex species interactions and allows for predictions of mutualism disruption across environments. I do this by 1) quantifying community change in mycorrhizal mutualists in plant roots exposed to garlic mustard in the field, 2) scaling up known physiological- and population-level effects of mutualism disruption to the plant community, and 3) understanding the impacts of garlic mustard and mutualism disruption across heterogenous environmental conditions and invader abundance. My results show that garlic mustard increased rare AM fungal taxa in plant roots, had a disproportionate negative effect on the mycorrhizal plant community relative to non-mycorrhizal plant community members, the negative effects of mutualism disruption on the native plant community are intensified at high garlic mustard abundance, and are evident across regional-scale despite heterogeneous environmental conditions. These results provide strong support for the mutualism disruption hypothesis as an important theory for understanding causes and consequences of allelopathic plant invasion and build links between the above- and below-ground community consequences of mutualism disruption. Mechanistic drivers of invader impact can scale across levels of ecological organization and doing so enhances our predictive ability of the impacts of invasive species.