Trophic segregation underlies the coexistence of two piranha species after the removal of a geographic barrier

The processes that determine the coexistence of phylogenetically close species are very complex, particularly when species introduction leads to sympatry among species that did not co-evolved. We evaluated possible differences in delta C-13 and delta N-15 signatures between two piranha species (Serrasalmus marginatus and S. maculatus), in a system where S. marginatus invaded 30 years ago (floodplain ponds in the Upper Parana River). We predicted that carbon and nitrogen stable isotope values would not differ between piranha species. Additionally, we evaluated the abundance (CPUE) of both piranha populations along the years (1986-2015). Native and non-native Serrasalmus species have different delta C-13 signatures, likely exploiting different energy pathways on the food web. Overall, native and non-native piranhas have similar delta N-15 values and occupy the third trophic level in the food web. Regarding the two piranha population fluctuations, there was an inversion of dominance after the non-native species establishment, where S. marginatus became dominant over S. maculatus (after 1988). Our results showed that trophic niche dimension (revealed by trophic segregation) is not the reason of the observed inversion in the dominant species, and this could be a primary factor driving the persistence of the native species in the ecosystem.