Understanding the behaviors underlying competitive abilities and the outcomes of species interactions across a range of densities are fundamental questions in basic ecology. These issues have taken on new importance and urgency in the field of invasion biology as competition between native and exotic species may result in species loss and community alteration. We investigated mechanisms influencing competitive effects of two ecologically similar snail species, Physa acuta and the highly invasive New Zealand mudsnail (Potamopyrgus antipodarum, (NZMS), on a native caddisfly, Brachycentrus sp. Using a combination of data from behavioral observations of foraging and activity, measures of resource consumption, and growth, we attempted to describe both the processes by which competition between invertebrate grazers occurs and the resulting effects. Overall, physid snails appear to exert the strongest effects on both other species and on themselves, primarily through high levels of foraging. Intraspecific competition between physids results in reduced per capita algal consumption, reduced uptake of δ15N, and reduced growth at higher conspecific densities. Physids have negative effects on the δ15N uptake of NZMS and mixed effects on NZMS growth that are consistent with the hypothesis that the impacts of physids on NZMS are density dependent. We found little evidence for an effect of intra or interspecific competition on Brachycentrus larvae. Notably, NZMS show no evidence of intraspecific competition. This final result may help explain the ability of NZMS to achieve much higher densities than physid snails, as they show no evidence of intraspecific competition at densities similar to those observed in an invaded stream.