Exploitation of shared resources often mediates the impacts of invasive species on native species. In a field experiment, we compared the ability to graze periphyton and the genera of diatoms removed by the invasive New Zealand mudsnail, Potamopyrgus antipodarum, a native caddisfly larva (Brachycentrus sp.), and a native mayfly nymph (Ephemerella sp.) over 1 week. P. antipodarum removed as much or slightly more periphyton than the native grazers, depending on whether chlorophyll a or ash-free dry mass was used to measure periphyton biomass. When we examined the diatoms in the periphyton, P. antipodarum altered the diatom assemblage more than the native grazers. Effective grazing of periphyton by P. antipodarum may impact native grazers by consuming shared algal resources. In particular, because Ephemerella sp. were also effective grazers, these mayflies may compete for periphyton with P. antipodarum in the western United States. Taken together, these results suggest that ability to procure food resources may contribute to the invasion success of P. antipodarum.

To understand the impacts of an herbivorous invasive species on native herbivores, it is critical to quantify the relative impact of the invasive and the native species on shared resources. In a field experiment, we compared grazing efficacy of periphyton by the invasive New Zealand mudsnail, Potamopyrgus antipidarum, and 3 native macroinvertebrate grazers. Depending on the measure of periphyton biomass, P. antipodarum removed as much or more periphyton than any of the native grazers. When we examined diatom genera individually, P. antipodarum also suppressed the relative abundance of the greatest number of diatom genera and suppressed those diatoms more than the native grazers. As a result, P. antipodarum should compete strongly for periphyton with native grazers. In particular, because Ephemerella mayflies were the second most effective grazers and grazed many diatom genera similarly to the invasive snails, these mayflies may be competing with P. antipodarum in the introduced range. Overall, grazing ability may contribute to the invasion success of P. antipodarum.