Invasive plant species are believed to decrease biodiversity and local abundances of native species. Examining the mechanisms of invasive species impacts can help direct restoration efforts, especially where species employ multiple mechanisms to increase their relative dominance and negatively affect native species. Ranunculus ficaria is an invasive species in many temperate deciduous forests in the northeastern United States, and is especially dense in highly disturbed urban riparian habitats. This species may prevent establishment of native species in invaded areas directly through competition or allelopathy. Alternatively, its success may simply be a consequence of a modified disturbance regime. We tested for direct effects of R. ficaria on the growth of a native riparian grass phytometer, Elymus riparius. We also examined three potential mechanisms by which R. ficaria could directly impact native plants: nutrient competition, light/space competition, and allelopathy. We tested for nutrient competition by adding fertilizer to the soil in selected plots. To test for allelopathy, we added a tea made from R. ficaria leaves to selected plots. We also quantified plant community responses to R. ficaria removal based on volunteer seedling abundances. We found 99% higher phytometer biomass in removal plots than in invaded plots. Removal plots also had 493% more volunteer seedlings. There was no evidence for nutrient competition or allelopathy. Ranunculus ficaria is directly responsible for reducing volunteer sprout abundance and the biomass, but not necessarily diversity, in our study sites, probably through competition for space and/or light. This suggests that R. ficaria has some role in driving ecosystem change, and that removal of these populations will help restore some native species.