, wild axolotls were near extinction due to urbanization in Mexico City and consequent water pollution, as well as the introduction of invasive species such as tilapia and perch. They are listed as critically endangered in the wild, with a decreasing population, by the International Union for Conservation of Nature and Natural Resources (IUCN) and as an endangered species by the IUCN's CITES treaty. Axolotls are used extensively in scientific research due to their ability to regenerate limbs. Axolotls were also sold as food in Mexican markets and were a staple in the Aztec diet.