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On Thursday, 41 scientists published the first global analysis of a fungal epidemic that has been destroying frogs for decades. The devastation turns out to be much worse than everyone else has done before.
By writing in the journal Science, the researchers conclude that populations of more than 500 species of amphibians have declined dramatically due to the epidemic – including at least 90 presumed extinct species. This figure is more than twice the previous estimates.
"It's pretty seismic," said Wendy Palen, a biologist at Simon Fraser University and co-author of the commentary that accompanies the study. "He now deserves the nickname of the most lethal pathogen known to science."
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Scientists first noted in the 1970s that some frog populations were declining rapidly; in the 1980s, some species seemed to be extinguished. The losses were disconcerting because the frogs lived in preserved habitats, free from pollution or deforestation.
In the late 1990s, researchers discovered that Australian and Panamanian frogs were infected with a deadly fungus, which they called Batrachochytrium dendrobatidis – Bd, for short.
The fungus has appeared in other countries, but studies on its DNA suggest that Bd originates from the Korean peninsula. In Asia, amphibians seem insensitive to BD, but when it arrived in other parts of the world – probably through the international trade of pet amphibians – the pathogen has reaches hundreds of vulnerable species.
Amphibians are infected with Bd by contact with other animals or by spores floating in the water. The fungus invades the cells of the skin and multiplies. The skin of an infected frog will begin to peel off as the animal becomes sluggish. Before dying, a frog can move to a new stream or pond, spreading the fungus.
In 2007, researchers hypothesized that Bd could be responsible for all known frog declines with no other apparent cause – about 200 species. For the most part, however, scientists have studied Bd at the local level, examining its impacts on particular species in particular places.
"We knew that frogs were dying all over the world, but no one had come back early and had really evaluated the impact," said Benjamin Scheele, an environmentalist at the Australian National University and lead author of the new study. .
In 2015, Scheele and his colleagues collected data from more than 1,000 Bd articles and traveled the world to meet experts and hear their unpublished observations.
The team not only analyzed data on live amphibians, but also museum data, where scientists discovered that Bd DNA was incorporated into preserved specimens stored in wardrobes.
The new study has shown that some amphibians are more exposed than others.
The fungus grows in cool, wet conditions. As a result, frogs living in mountain cloud forests have been particularly affected.
Big frogs are also at greater risk, perhaps because they do not multiply as fast as small ones.
Dr. Scheele and his colleagues identified 501 species in decline, far more than the previous estimate of 200. Some factors, once considered responsible for decimating frog populations – such as climate change and deforestation – do not constitute not the biggest threat, according to scientists.
"Many of these assumptions have been discredited," said Dr. Scheele. "And the more we know about the mushroom, the more it matches the pattern."
It turns out that Bd has eliminated some species well before its discovery. It is only by returning to museum specimens that scientists have been able to estimate the balance sheet. "It's scary that so many species can disappear without our knowledge," said Dr. Scheele.
Researchers found that the decimation of frogs peaked in the 1980s, a decade before the discovery of Bd. Today, 39% of species that have experienced population decline are still in decline. Twelve percent show signs of recovery, probably because natural selection favors resistant animals.
Although it turned out that the results of the study were disappointing, Dr. Scheele is optimistic about future outbreaks of wildlife. The element of surprise may have been largely due to Bd's devastating success.
"It was not planned or planned, so it took a lot of time for the research community to catch up," said Dr. Scheele.
In 2013, researchers discovered that a related fungus was attacking fire salamanders in Belgium. Called Batrachochytrium salamandrivorans (Bsal for short), it seemed about to make salamanders what Bd did to frogs.
But this time, things are going differently.
The researchers discovered the outbreak and quickly identified Bsal. They immediately started experimenting to understand the threat this posed. Thanks to the trade barriers in place, Bsal has not yet threatened another species, anywhere.
"We have learned and we are doing better," said Dr. Scheele. "I guess the question is always whether we're doing enough? And it's debatable."
There are still many reasons to worry about future outbreaks. The Bd has not yet reached New Guinea or Madagascar, which are home to a wealth of amphibian species that are found nowhere else on Earth.
If a Bd infected frog arrived at one or the other place – through the pet trade or as an accidental clandestine passenger – the fungus would have a large number of bugs. vulnerable hosts to attack.
"It could be a collapse of the ecosystems out there," said Daniel Greenberg, a graduate student at Simon Fraser University and co-author of the Science commentary.
The loss of frogs can alter whole ecosystems.
Without tadpoles to engulf the algae, the flowers can smother the streams. Without frogs to eat insects, some disease vector species may become more common. Birds and other predators that eat frogs must find alternatives.
Scientists do not even rely on intact species released from the Bd assault. Another strain of Bd, or a different mushroom species, could prove even more lethal.
"This is only Russian roulette, with pathogens moving around the world," said Dr. Scheele.
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