The axolotl (Ambystoma mexicanum) is a unique type of salamander found exclusively in the Xochimilco canal system near Mexico City. This amphibian exhibits neoteny, meaning it retains its larval characteristics into adulthood, remaining fully aquatic with external gills and a tail fin. Despite its remarkable regenerative abilities, the axolotl is listed as critically endangered in the wild by the IUCN. Its precarious existence is due to factors that have made its native waters increasingly hostile.
Invasive Species as Primary Threats
The greatest biological threat to the axolotl’s survival comes from introduced non-native fish species. The Mexican government introduced large fish, primarily tilapia (Oreochromis niloticus) and carp (Cyprinus carpio), into the Xochimilco system in the 1970s and 1990s as a food source. These invasive species have flourished, creating a dangerous environment for the native salamander.
Tilapia and carp are highly efficient predators of the most vulnerable axolotl life stages: eggs and newly hatched juveniles. Axolotl eggs are laid on submerged vegetation and have no defense against these large fish. Invasive fish also aggressively compete with the axolotl for limited food resources like aquatic invertebrates and small crustaceans.
These non-native species thrive in the degraded water quality of the modern canals, out-competing the axolotl for space and oxygen. The sheer biomass of invasive fish can be staggering, often making up the majority of the aquatic animal mass. This introduced predation pressure has severely impacted the wild population.
Native and Avian Predators
Historically, the axolotl faced pressure from native wildlife, though it had few natural predators in its original lake system. Wading birds, such as herons and egrets, are the primary natural predators of adult and sub-adult axolotls. These birds hunt from the air or the shoreline, using their eyesight to spot amphibians before striking with their beaks.
Axolotls lack movable eyelids, making them susceptible to detection when resting near the surface or in open water. The delicate early life stages are also preyed upon by smaller, native aquatic organisms. Aquatic insects, such as the larvae of water beetles and dragonflies, consume axolotl eggs and larvae.
Native predators were historically part of an ecological balance. However, their impact is more pronounced when environmental factors force axolotls out of protective cover and into open, shallow water.
The Role of Habitat Loss in Vulnerability
Habitat degradation increases the success of both native and invasive predators. Urbanization from the expanding Mexico City has led to the draining and pollution of the Xochimilco wetland system. The remaining canals suffer from high contamination, including sewage runoff and agricultural waste, to which the axolotl is highly sensitive.
Poor water quality stresses the axolotls, making them slower, less alert, and more susceptible to disease, which makes them easier targets. The destruction of the natural environment has also eliminated the dense aquatic vegetation that once provided camouflage and shelter. Axolotls prefer to hide their eggs and themselves among these plants and in muddy substrates.
The lack of suitable cover leaves the axolotl exposed, turning the canals into an open hunting ground. Habitat loss acts as a multiplier by reducing hiding places and stressing the animals with pollution. This intensifies the predation risk from every species present.