Known for its pale pink body and Mona Lisa-like smile, the Mexican axolotl (Ambystoma mexicanum) is a figure of intrigue. Affectionately nicknamed “water monsters”, axolotls are eerie creatures that have burrowed their way into the hearts of many people.1 Their protruding frill-like gills and lidless eyes have been known to captivate and repulse people alike, but their mutant-like ability to perfectly regenerate limbs can be accredited to their rise to fame, both in history and in contemporary times (See Figure 1).
Axolotls first gained recognition during the time of the Aztecs. While building their city Tenotchtitlan in modern-day Mexico City, the Aztecs cooperated with the natural wetlands and canals, shaping them to facilitate the growth of their city while still maintaining their natural features. Axolotls thrived during these times, and featured prominently in Aztec myths. One of the most notable legends is that of Xolotl, the god of fire and lighting, as well as the god of sickness and deformities, and how he chose to transform himself into an axolotl to escape being sacrificed by the other gods. Although his endeavor failed and he ultimately perished, the axolotl was named in his honor.2 Today, the axolotls are most famous for their unique quality of regenerating their tissues. They can grow back limbs, organs, spinal cords, and even parts of their brains without a single trace of scarring. This ability makes them incredibly valuable in the world of medical research. Discovering what aspects of their biology grants them this miracle would lead to significant leaps in human medicine. 3However, despite these accolades, this species is in peril. According to the International Union for Conservation of Nature Red List of Threatened Species, the number of wild axolotls has decreased significantly within the past decades, and their numbers range from 50 to 1,000 individuals in the wild.5
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.9The rate of deterioration of these habitats significantly increased within the past decades. Between 1950 and 1975, Mexico City tripled in size. Such a significant boom in population and urbanization drained the water bodies of the city, placing undue stress on the wetlands of Xochimilco. The water table and fertile soils supporting Mexico City’s ecosystems and people used to be maintained by underground springs and rivers. However, these were demolished with the expansion of the city. As the supply of fresh clean water drastically reduced, Xochimilco’s water system gradually became “alkaline, salty, and polluted”.11
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.15Although the axolotl is in a perilous situation, scientists and local citizens are working to preserve this species. Refuge programs have been implemented; researchers have teamed up with local farmers to create refuges for axolotls which consist of semipermeable barriers placed at the entrances of the canals. They block the entrance of invasive fishes like the tilapia and carp and reduce the amount of sediment that passes through. Axolotls are captured and relocated to areas within these “refuges” inside Xochimilco. The axolotls are safe from their predators and the cleaner water provides a safer environment for their survival. Additionally, the cleaner water in this region has been noted to benefit farmers and their crops as well, which serves as motivation to continue with the program. Another method has been supporting farmers to utilize organic, ecologically friendly fertilizers. However, these efforts have far to go. Governmental policy in Mexico hasn’t restricted urbanization in areas surrounding Xochimilco, which endangers any progress made forward. Long-term actions are often deemed too expensive and not conducive to economic growth, and this limits the survival of the axolotl species.17 If the axolotl is to survive in its native habitat, it will need stronger conservation efforts to ultimately save it from extinction.