These remaining canals and their ecosystem was declared a World Heritage Site in 1987, with the purpose of saving them. An important part of this ecosystem is a willow tree called a “ahuejote” that is native to the shallow waters of the lake/canals. These stem erosion, act as wind breakers and favor the reproduction of a variety of aquatic species. Some of these endemic species include a freshwater crayfish called an acocil, and the Montezuma frog. However, the most representative animal from these waters is the axolotl (Ambystoma mexicanum). This amphibian was used as a medicine, food and ceremonial object during the Aztec Empire. It was considered to be an incarnation of the god Xolotl, brother of Quetzalcoatl. It has been studied due to its abilities to regenerate limbs and other body parts. It can also reach sexual maturity as a larva, which no other amphibian can do. While mostly aquatic, it does have limited ability to breathe air. As of 2003, there were only 600 axolotls known to exist in the wild. Most of the threat to the species is loss of habitat and pollution, but the introduction of non-native fish such as tilapia has also had disastrous effects on the population of this and other species. Conservation efforts include research and environmental education. The Grupo de Investigación del Ajolote en Xochimilco (GIA-X) is a nonprofit research group dedicated to the preservation of the axolotl, which is in danger of extinction. It works to better understand the creature as well as with the local community to protect what is left of its habitat. In addition to species that live in the area year round, the wetlands here host about forty percent of the migratory bird species that arrive to Mexico, roughly 350, use the wet areas around Xochimilco for nesting. Many of these come from the United States and Canada. However, much of this habitat has been urbanized. About 700 species have been found in the area overall. Some of the migratory species include pelicans, storks, buzzards and falcons.