It is common knowledge that invasive species cause worldwide ecological and economic damage, and are nearly impossible to eradicate. However, upon introduction to a novel environment, alien species should be the underdogs: They are present in small numbers, possess low genetic diversity, and have not adapted to the climate and competitors present in the new habitat. So, how are alien species able to invade an environment occupied by native species that have already adapted to the local environment? To discover some answers to this apparent paradox I conducted four ecological genetic studies that utilized the invasive species Brachypodium sylvaticum (Hudson) Beauv. to determine mechanisms contributing to adaptation and success in the novel habitat. The first study used simulations and experiments to test the hypothesis that genetic purging, the process where genetic load is reduced by selection against the recessive deleterious alleles expressed in the homozygous state, promotes invasive range expansion. I found that homozygous populations on B. sylvaticum's range periphery displayed lower inbreeding depression compared to heterozygous populations near introduction sites. Empirical tests with B. sylvaticum further demonstrate that purging of genetic load is a plausible scenario promoting range expansion during invasion. Next, I explored how the interaction between population genetic diversity and the environment contributed to the establishment and spread of Brachypodium sylvaticum. I found that nitrogen application increases both final size and shoot biomass for B. sylvaticum individuals from source populations with low HS levels to levels found in individuals from populations with high HS. A coefficient of relative competition intensity index (RCI) displayed reduced effects of interspecific competition on B. sylvaticum biomass in high nitrogen plots. Results show that elevated nitrogen deposition is a factor that increases establishment of introduced species with historically small effective population sizes. Thirdly, I investigated phenotypic differentiation during the establishment and range expansion of Brachypodium sylvaticum. Utilizing a novel approach, unique alleles were used to determine the genetic probability of contribution from native source regions to invasive regions. These probabilities were integrated into QST-FST comparisons to determine the influence of selection and genetic drift on twelve physiological and anatomical traits associated with drought stress. Phenotypic divergence greater than neutral expectations was found for five traits between native and invasive populations, indicating selective divergence. Results from this study show that the majority of divergence in B. sylvaticum occurred after introduction to the novel environment, but prior to invasive range expansion. The final chapter of my dissertation investigates the adaptive role of genetic differentiation and plasticity for Brachypodium sylvaticum invasion. Plasticity was measured across treatments of contrasting water availability. Linear and nonlinear selection gradients determined the effect of directional and quadratic selection on plasticity and genetic differentiation. Invasive trait divergence was a consequence of post-introduction selection leading to genetic differentiation, as there were no plastic responses to contrasting water availability for any measured traits. Genetic divergence of invasive plants was not consistently in the direction indicated by selection, suggesting limitations of selection that may be a consequence of physical constraints and/or tradeoffs between growth and abiotic tolerance. Results suggest that selection, rather than plasticity, is driving phenotypic change in the invaded environment. The combined volume of these studies contributes significantly to the field of invasion and plant biology by providing novel insights into the processes underlying range expansion, adaptation, and ultimately, evolution of introduced species.

The invasibility, or susceptibility of an ecosystem to biological invasion is influenced by changes in biotic and abiotic resistance often due to shifts in disturbance regime. The magnitude of invasive propagule pressure interacts with an ecosystem's invasibility to determine the extent of a biological invasion. I examined how propagule pressure, forest community structure and disturbance interact to influence the invasibility of temperate Pacific Northwest forests by the newly-invasive grass, Brachypodium sylvaticum. My goal was to identify which of these factors is most instrumental in enabling the shift from establishment to population growth in B. sylvaticum at the edge of its expanding range. Both observational and experimental studies were employed to identify the many ecological components of this problem. Ecological sampling methods were used to identify trends in B. sylvaticum habitat preference and signs of habitat disturbance. In addition, an experimental study was performed to test the effects of soil and vegetation disturbance on B. sylvaticum seedling propagation. I found that while soil disturbance did not have a significant effect on seedling propagation, vegetation disturbance was implicated in B. sylvaticum spread. Higher propagule pressure and coniferous forest type were also strong predictors of increased B. sylvaticum seedling propagation and survival within established sites. My study demonstrates how propagule pressure and plant community dynamics interact to shift the invasibility of Pacific Northwest forests and facilitate the transition from establishment to spread in the invasion of B. sylvaticum.