Abstract 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 co...