Food web overlap among native axolotl ( Ambystoma mexicanum ) and two exotic fishes: carp ( Cyprinus carpio ) and tilapia ( Oreochromis niloticus ) in Xochimilco, Mexico City

Two exotic fishes, common carp ( Cyprinus carpio ) and tilapia ( Oreochromis niloticus ), were introduced more than 20 years ago into Xochimilco for aquaculture, and now dominate the system in terms of biomass and numbers. Over this same period, wild populations of the microendemic axolotl salamander ( Ambystoma mexicanum ) have been dramatically reduced, and it currently persists in isolated areas of this aquatic system, which is situated at the edge of Mexico City. In this study, we examine potential trophic interactions and niche overlap among two exotic fishes: carp and tilapia, and the native axolotl. Axolotl had more diverse diets and a higher trophic position compared to the exotics. Stable isotope analysis revealed substantial trophic niche overlap among axolotl and the exotics. The two exotics occupied a larger niche area than the axolotl, suggesting higher levels of omnivory and diet specialization. Current exotic fish removal efforts will further our understanding of interactions between the axolotl and exotic species, and are expected to benefit axolotl recovery efforts.