Invasions by non-native species are increasingly detected all over the World, as a consequence of globalization. Successful establishment implies the presence of unexploited niches and/or competitive advantage of invader vs. native closely related species. As a consequence, invasion also impacts functional diversity and trophic relationships in ecosystems. Feeding niche is a relevant component of ecological niche; it can be investigated by means of C, N stable isotopes analyses (SIA). Here we present results of a study in which feeding niche of the invader Rutius rutilus is compared with those of Coregone lavaretus and Alosa agone. in a deep, subalpine lake (Lago Maggiore, Italy ). By applying SIA we quantified seasonal shifts in pelagic vs. littoral diet sources and in their percentage contributions for the three species. Feeding plasticity, namely spatial and temporal displacement, allowed for a peaceful coexistence of the invader and the other two potential competitors.