There is a plague ripping through the amphibian species of the world. It’s caused by fungus that’s invisible to the naked eye and spreads easily by many means. It kills by disrupting the way these creatures breathe through their skin, essentially suffocating frogs and salamanders.
The disease is called chytridiomycosis, and according to a landmark study published in the journal Science, it’s even worse than we thought.
Scientists once estimated that about 200 species of frogs and salamanders have been harmed by the disease, but the new study concludes that chytrid fungus has contributed to declines in at least 501 amphibian species. Ninety of the species are thought to have gone extinct because of the epidemic. Populations in tropical Australia, Central and South American seem to be hit hardest, though Africa, Europe, and North America are also affected. According to this new accounting, the epidemic has caused the biggest loss of biodiversity of any recorded disease.
“It’s a staggering thing to consider,” said Jonathan Kolby, one of the study’s authors and a herpetologist specializing in conservation and wildlife diseases at James Cook University in Australia. “We’ve never before had a single disease that had the power to make multiple species extinct, on multiple continents, all at the same time.”
Carly Muletz Wolz, a molecular pathogen scientist at the Smithsonian National Zoo’s Center for Conservation Genomics, said she wasn’t surprised by the findings, but that it was important to finally see the scale of the problem quantified.
“This is the newest, best estimate on where things stand,” said Muletz Wolz, who was not involved in the study. “And it’s probably even worse than this, because you only know what you know.”
Scope unprecedented
The scope of the chytrid fungus onslaught on the amphibian world, which began in the 1980s, is unprecedented. West Nile virus, which is now a well-known pathogen, affects 23 bird species. The fungus laying waste to bat populations across North America, white nose syndrome, is attacking six species.
“You’ve got hundreds if not thousands of frog species that could go extinct, and they’re getting much less attention from a global health perspective,” said Kolby, who helped create the Honduras Amphibian Rescue and Conservation Center.
Scientists suspect that the chytrid fungus lineage that has been ravaging frog populations became more virulent in the 1980s after it hybridized with another lineage. And when more people transport infected amphibians across the world for the pet trade, there is more opportunity for spread of the fungus and additional hybridizations.
“There’s nothing preventing hybridization from happening again, and if it happens again, who knows what that hybrid offspring will act like?” said Kolby. “We could have another global wave of disease, which could be similar, different, or even worse than the one we’re facing now.”
In 2013 scientists discovered a species of chytrid that goes after salamanders, called Batrachochytrium salamandrivorans. This species has not made its way into the United States, and in 2016, the U.S. Fish and Wildlife Service banned imports of 201 high-risk species of salamander in an effort to prevent future outbreaks.
There are no such restrictions regarding trade in frogs, however.
“I think Canada really one-upped us,” said Muletz Wolz. “They just said you can’t import any amphibians into Canada anymore. They just totally banned it.”
Kolby argues the United States and other countries need to enact stricter biosecurity measures for several reasons. It’s already been shown that frogs and toads can harbor the fungus that afflicts salamanders, so only banning certain salamander species may not be enough to protect native species.
Slipping through cracks
In many ways, Kolby said, the U.S. already does a good job of preventing the spread of diseases that affect crops and livestock, thanks to agencies like the U.S. Department of Agriculture and the Centers for Disease Control and Prevention. But chytrid fungus may have slipped through the cracks because it doesn’t immediately affect humans.
“What concerns me is there’s going to be a next time,” he said. “By not using this as a learning experience about what happens when we aren’t being careful, it almost undoubtedly ensures that the wildlife trade is moving other pathogens right now, be it for mammals, birds, fish. You name it.”
It is also unclear how a global decline in amphibians will disrupt the ecosystems they inhabit. Many species improve water quality by clearing waterways of vegetation when they’re in the tadpole stage of development. Similarly, other frog species play a crucial role in keeping mosquito numbers in check by feasting on their larvae. Other species rely on frogs as a food sources.
There are glimmers of hope. Kolby and his co-authors found population trends for 292 frog species that have been thinned by chytrid, and of those, 60 species have shown evidence of recovery, possibly because they are evolving resistance to the fungus. The scientists note that these are mostly recoveries of individual populations, rather than rebounds for the species at large.
“It at least gives us something to look at in terms of why some species are recovering and why others are not,” said Muletz Wolz.
Kolby said he can understand why the fungus may be disheartening.
“I still look at this as just one huge opportunity,” he said, “to learn how to not let this happen again.”
