Impacts and Implications of Co-occurring Invasive Plant Species

The anthropogenic spread of species is a potent form of global change that impacts the population dynamics of native species, the composition of native communities, and the functioning of ecosystems. As the reorganization of species around the globe continues unabated, there is an increasing likelihood that habitats will contain co-occurring invaders. In this dissertation, I emphasize the need to study co-occurring invasive plants by juxtaposing the relative occurrence of multiple versus single invasive plants in important conservation habitats to the relative occurrence of published studies that consider the impacts of single versus multiple invasive plants. I found that over two-thirds of conservation habitats are multiply invaded while less that one-third of studies consider the impacts of co-occurring invaders and only 6% of studies focused on invasive plant interactions. To address this conservation-research mismatch, I use an observational study of the above- and belowground impacts of two co-occurring invasive woody plants among plots containing both shrubs, each species singly, or lacking both species. I found that subdominant invasive plant richness in plots with both invaders was twice as high as in plots with either invader singly and that β [beta]-glucosidase activity, a carbon-degrading extracellular soil enzyme, was three times greater than in control plots. These findings indicate that co-occurring invaders can have additive and non-additive effects compared to when they are found singly. Next, using a greenhouse experiment, I asked how interactions within native and nonnative plant communities affected their response to species gains and losses. I constructed phylogenetically paired native and nonnative plant communities that varied in species richness and measured above- and belowground productivity and seedling establishment of woody species. I found that native and nonnative plant communities differed in their overall biomass allocation patterns, the mechanisms driving community response to species losses, and the receptivity of communities to species gains. Overall, my work implies that the impacts of co-occurring invasive plant species are not necessarily predictable based upon single-invader impacts or interactions of closely related native species.