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article imageConservation efforts for a Spanish toad may become the cause of its extinction

By Bart B. Van Bockstaele     Sep 23, 2008 in Environment
Efforts to save the Mallorcan midwife toad and to reintroduce it into its natural environment, may be causing its extinction because of a fungal infection that was unknown at the time of the reintroduction.
Midwife toads are called that, because the male of the species carries the eggs while they are developing. The Mallorcan midwife toad, Alytes muletensis, was once a species that was thought to be long extinct, so it was only logical that it caused quite a stir when it was discovered alive and well in 1977. It was then described as a "living fossil". Its near-extinction is now thought to have been caused mainly by the introduction of several species that are not native to the Balearic Island of Majorca in the Mediterranean Sea, the only place where this toad is endemic, and more specifically a green frog, Rana perezi, and a viperine snake, Natrix maura.
Once the toad were discovered, an island-wide search was conducted and a monitoring and conservation program was started to preserve the approximately 500 breeding pairs in existence. A breeding and reintroduction program was also started by the Durell Wildlife Conservation Trust in Jersey, UK. Toads were reintroduced in the early 1990s, and the whole program became one of the great success stories of conservation.
But now Current Biology reports that the declarations of success may have been premature. In 2004, Matthew Fisher, a fungus specialist of the Imperial College London, discovered a chytrid fungus, Batrachochytrium dendrobatidis, in a dead Mallorcan midwife toad.
Further research showed that there was a real epidemic in two limestone gorges, that it was rare in two other gorges and that the infection had not yet spread to 17 other sites. This provided a clue that the infection was recent, since it had not (yet) been able to spread. This was further corroborated by the finding that the fungal specimens had a similar genetic makeup, evidence for a single source of infection.
The evidence led Fisher to speculate that captive-bred, reintroduced toads could be the cause of the infection. He was able to examine a number of toads that had died during the captive breeding program and that had been preserved by the researchers of Durell. Fungal DNA was found in three of the dead Mallorcan midwife toad and in a dead South African frog. While still alive, several of these South African frogs were kept in the same room as the Mallorcan midwife toad colony. It is suspected that they were the original source of the fungus that infected the midwife toads. In 1991, shortly after their arrival, 23 Mallorcan midwife toads died, probably from an infection by the fungus.
Some of the captive midwife toads were released into one of the gorges where the fungus is now rampant. Unfortunately, the fungal DNA in the preserved captive-bred toads was too degraded to determine if it was similar to that found on Mallorca.
It is easy to blame the people at Durell for the spread of this infection. However, let's take into account that this would be most unfair. At the time, no-one even knew that this fungus existed, let alone that it could cause problems.
This fungus was only first described in 1998, it was previously unknown. Amphibians worldwide are facing a mass extinction crisis and precisely this fungus is generally recognized as the main driver of the dramatic declines. There is an ongoing debate with respect to the cause of the spread of this fungus. Some blame invasion whereas others point at the mother of all that is bad: global warming. Whatever the cause, the infection is so bad that OIE, the World Organisation for Animal Health, has declared this fungus a notifiable disease.
There are very few studies documenting anthropogenic (= caused by humans) spread, and this is the first study presenting really strong evidence that anthropogenic movement of amphibians is spreading chytridiomycosis caused by Batrachochytrium dendrobatidis. It also makes a strong case for the necessity of ensuring that breeding programs do not become the cause of disease transmission by making sure that species do not have any infectious agents before they are released in nature.
Fortunately for the Mallorcan midwife toads, the fungus has not (yet) been as deadly as it is to amphibians in the rest of the world.
More about Mallorcan midwife toad, Alytes muletensis, Chytrid fungus
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