Excerpted from the article on Nature.com
When biologist Luis Zambrano began his career in the late 1990s, he pictured himself working miles from civilization, maybe discovering new species in some hidden corner of Mexico’s Yucatán Peninsula. Instead, in 2003, he found himself counting amphibians in the polluted, murky canals of Mexico City’s Xochimilco district. The job had its advantages: he was working minutes from his home and studying the axolotl (Ambystoma mexicanum), a national icon in Mexico and arguably the world’s most recognizable salamander.
In 1998, the first robust study to count axolotls estimated that there were about 6,000 of them per square kilometre in Xochimilco. Zambrano — who now is a professor at the National Autonomous University of Mexico (UNAM) in Mexico City — discovered in 2000 that the number had dropped to about 1,000 animals per square kilometre. By 2008, it was down to 100; today, thanks to pollution and invasive predators, there are fewer than 35 animals per square kilometre.
The axolotl is on the brink of annihilation in the canals of Mexico City, its only natural habitat. But although there might be just a few hundred individuals left in the wild, tens of thousands can be found in home aquariums and research laboratories around the world. They are bred so widely in captivity that certain restaurants in Japan even serve them up deep-fried.
“The axolotl is a complete conservation paradox,” says Richard Griffiths, an ecologist at the University of Kent in Canterbury, UK, who recruited Zambrano to the project. “Because it’s probably the most widely distributed amphibian around the world in pet shops and labs, and yet it’s almost extinct in the wild.”
This creates a problem for biologists. Thanks to its unique physiology and remarkable ability to regenerate severed limbs, the axolotl has become an important lab model for everything from tissue repair to development and cancer. But after centuries of inbreeding, captive populations are vulnerable to disease. And the loss of genetic diversity in wild axolotls — owing to their diminishing population — means that scientists lose out on learning all they can about the animal’s biology.
As lab scientists continue to study the captive animal and its large and complex genome, Zambrano and a handful of other researchers are doing their best to preserve the wild version. They are breeding and releasing axolotls into control ponds and canals in and around Xochimilco to see how they fare, and hopefully to retain some of their natural genetic diversity. The task of saving them is difficult, but should be doable given the animal’s hardiness — if the Mexican government would only engage with the process.
“I’ve seen that in other places in the world, these kinds of huge tasks are possible,” Zambrano says. “If they can do it, why can’t we?”
Zambrano says that to save the wild axolotl, policymakers must address its two primary threats. The first is non-native fish such as the common carp (Cyprinus carpio) and tilapia (Oreochromis niloticus). Ironically, these were introduced to Xochimilco in the 1970s and 1980s through programmes run by the Food and Agriculture Organization of the United Nations, with the aim of getting more protein into local diets. Zambrano says he has mapped the areas where axolotls still remain; he envisions a team of local fishers being paid to sweep them of fish on an ongoing basis. Although this wouldn’t remove all the fish, for a few hundred thousand dollars it might give the salamanders a window in which to re-establish themselves. His work has shown that axolotls are most vulnerable to carp when they are at the egg stage, and to tilapia when they are juveniles, but reveals that if they can grow beyond a certain size, they might still thrive5.
The second threat is trickier. Every time a powerful storm fills the city’s ageing sewer system, treatment facilities release human waste into Xochimilco, carrying with it ammonia, heavy metals and untold other toxic chemicals. Amphibians, which breathe in part through their highly permeable skin, are vulnerable to these regular pollution dumps. It’s a testament to the animal’s resilience that it exists in the wild at all.
These are complex issues, but they are not unsolvable. So far, however, there have been no efforts to save the wild axolotl beyond a few halfhearted outreach programmes and some photo opportunities. In 2013, CIBAC released a few thousand axolotls for a behavioural study; some of them survived and even seemed to breed the following year. This suggests that lab-bred salamanders might be able to thrive in the wild if they are raised in captivity to a certain size. But biologists caution that this doesn’t mean Mexico should start releasing them into canals.
“There’s probably not much point in doing releases into the wild until you can neutralize the threats,” says Griffiths. “You just might be increasing the fish population by just chucking out more fish food.”
When biologist Luis Zambrano began his career in the late 1990s, he pictured himself working miles from civilization, maybe discovering new species in some hidden corner of Mexico’s Yucatán Peninsula. Instead, in 2003, he found himself counting amphibians in the polluted, murky canals of Mexico City’s Xochimilco district. The job had its advantages: he was working minutes from his home and studying the axolotl (Ambystoma mexicanum), a national icon in Mexico and arguably the world’s most recognizable salamander.
In 1998, the first robust study to count axolotls estimated that there were about 6,000 of them per square kilometre in Xochimilco. Zambrano — who now is a professor at the National Autonomous University of Mexico (UNAM) in Mexico City — discovered in 2000 that the number had dropped to about 1,000 animals per square kilometre. By 2008, it was down to 100; today, thanks to pollution and invasive predators, there are fewer than 35 animals per square kilometre.
The axolotl is on the brink of annihilation in the canals of Mexico City, its only natural habitat. But although there might be just a few hundred individuals left in the wild, tens of thousands can be found in home aquariums and research laboratories around the world. They are bred so widely in captivity that certain restaurants in Japan even serve them up deep-fried.
“The axolotl is a complete conservation paradox,” says Richard Griffiths, an ecologist at the University of Kent in Canterbury, UK, who recruited Zambrano to the project. “Because it’s probably the most widely distributed amphibian around the world in pet shops and labs, and yet it’s almost extinct in the wild.”
This creates a problem for biologists. Thanks to its unique physiology and remarkable ability to regenerate severed limbs, the axolotl has become an important lab model for everything from tissue repair to development and cancer. But after centuries of inbreeding, captive populations are vulnerable to disease. And the loss of genetic diversity in wild axolotls — owing to their diminishing population — means that scientists lose out on learning all they can about the animal’s biology.
As lab scientists continue to study the captive animal and its large and complex genome, Zambrano and a handful of other researchers are doing their best to preserve the wild version. They are breeding and releasing axolotls into control ponds and canals in and around Xochimilco to see how they fare, and hopefully to retain some of their natural genetic diversity. The task of saving them is difficult, but should be doable given the animal’s hardiness — if the Mexican government would only engage with the process.
“I’ve seen that in other places in the world, these kinds of huge tasks are possible,” Zambrano says. “If they can do it, why can’t we?”
Zambrano says that to save the wild axolotl, policymakers must address its two primary threats. The first is non-native fish such as the common carp (Cyprinus carpio) and tilapia (Oreochromis niloticus). Ironically, these were introduced to Xochimilco in the 1970s and 1980s through programmes run by the Food and Agriculture Organization of the United Nations, with the aim of getting more protein into local diets. Zambrano says he has mapped the areas where axolotls still remain; he envisions a team of local fishers being paid to sweep them of fish on an ongoing basis. Although this wouldn’t remove all the fish, for a few hundred thousand dollars it might give the salamanders a window in which to re-establish themselves. His work has shown that axolotls are most vulnerable to carp when they are at the egg stage, and to tilapia when they are juveniles, but reveals that if they can grow beyond a certain size, they might still thrive5.
The second threat is trickier. Every time a powerful storm fills the city’s ageing sewer system, treatment facilities release human waste into Xochimilco, carrying with it ammonia, heavy metals and untold other toxic chemicals. Amphibians, which breathe in part through their highly permeable skin, are vulnerable to these regular pollution dumps. It’s a testament to the animal’s resilience that it exists in the wild at all.
These are complex issues, but they are not unsolvable. So far, however, there have been no efforts to save the wild axolotl beyond a few halfhearted outreach programmes and some photo opportunities. In 2013, CIBAC released a few thousand axolotls for a behavioural study; some of them survived and even seemed to breed the following year. This suggests that lab-bred salamanders might be able to thrive in the wild if they are raised in captivity to a certain size. But biologists caution that this doesn’t mean Mexico should start releasing them into canals.
“There’s probably not much point in doing releases into the wild until you can neutralize the threats,” says Griffiths. “You just might be increasing the fish population by just chucking out more fish food.”
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And to a magikarp 
