Wild axolotl populations began to be monitored in the 1990s. They had decreased from 6,000 individuals per square kilometer in 1998 to 100 individuals per square kilometer in 2008.⁶ This number then dropped to 35 individuals per square kilometer in 2015. This drastic reduction in wild axolotls is concerning, especially considering they are an endemic species, meaning they can only be naturally found in one spot in the world. In the decades since these revelations, three main threats to axolotl populations have been identified: the urbanization of their natural habitat, the decline of water quality, and the introduction of exotic, predatory fish.⁷

The axolotl’s native habitat is the wetlands of Xochimilco in Mexico City, an area of about 40 square kilometers. It consists of canals interwoven among chinampas, small rectangular island-like pieces of land traditionally used to grow crops by farmers (See Figure 2). These chinampas formed part of the traditional Aztec style of farming, and the axolotl coexisted alongside them. However, in the centuries after the fall of the Aztec empire, these aquatic habitats began to be dismissed in favor of more contemporary styles of farming and urbanization.⁸ The wetlands were drained of their water and replaced with buildings and roads. Previously home to wild axolotls, Lake Chalco in Mexico City was drained completely to expand the city and assuage worries of flooding among citizens.⁹

Today, agricultural runoff from the chinampas and nearby farms, which includes pesticides and fertilizer, inundates these wetlands, creating a eutrophic environment. Eutrophic means that the environment is so nutrient-heavy that plant growth is overwhelming and absorbs all the oxygen, suffocating native animal species like the axolotl.¹² As if this weren’t enough, pollution from Mexico City often also enters the wetlands. Overflows from the sewage system are common and they can be catastrophic to the animal populations in Xochimilco, particularly the axolotl. Heavy metals, ammonia, oils, and other toxins common in runoff from urban areas can penetrate their permeable skin and cause significant damage as exposure grows. ¹³

Axolotl eggs and young are particularly vulnerable, as their development is negatively impacted by these toxins. The contamination in the water has been found to lead to methemoglobinemia, a condition that lowers the activity levels of the axolotls and interferes with their natural behaviors. Pollution obstructs their sense of smell, thereby interfering with their hunting habits. Although deformities in axolotls have not occurred yet, researchers believe that if contamination rates continue, it might only be a matter of time until deformities are documented.¹⁴

Not only have the urbanization and decrease in water quality affected the axolotls, but invasive fish species have been introduced to these wetlands, displacing the role of the axolotls in their ecosystem. Nile tilapia (Oreochromis niloticus) and common carp (Cyprinus carpio) were initially introduced into Xochimilco to provide more fishing opportunities, but competition with the axolotl soon led to trouble. These invasive fish species devour the eggs, larvae, and juveniles of axolotls. Not satisfied with merely eating axolotls, they also compete with them when hunting for prey. Studies have found that axolotls catch less prey in the presence of tilapia and carp due to the competition, as well as the fact they spend valuable energy avoiding the path of these predators. The presence of these fishes has not only affected the number of axolotl individuals but also their size and growth rate, since their dietary habits have been disrupted.¹⁵