Stable isotopes reveal mild trophic modifications in a native-invasive competitive relationship.
Temporal or evolutionary changes in the effects of invasive competitors on native species have not been studied in great depth. In this study, we explored possible modifications in the trophic shifts of native and invasive tadpoles in a set of ephemeral ponds with different amounts of time elapsed since the invasion, i.e., with different degrees of naiveté of the native species to its invasive competitor. Using stable isotopes analysis, we found that the native (Epidalea calamita) and invasive (Discoglossus pictus) species always segregated in their trophic position within ponds. Furthermore, the isotopic signature of the tadpoles was affected by the composition and diversity of the surrounding vegetal and animal communities. The amount of time elapsed since the invasion did not influence the magnitude of the trophic differences between the species, but it did affect the nature of this segregation. Segregation at the trophic level occurred most frequently during the first stages of invasion, with the invasive species occupying higher trophic levels. However, segregation was progressively attained through the consumption of different items within the same trophic level when the amount of time since invasion increased. Thus, our results point that the native species no longer uses a lower trophic level after several generations of coexistence. In contrast, changes in the trophic niche width of either species during the invasion process were largely undetected.