An unassuming fungus that dwells in lakes and damp soil has proved to be the most potent killer of a large group of species ever documented. The victims are members of at least 501 species of frogs and other amphibians that have succumbed to a disease inflicted by Batrachochytrium dendrobatidis, or Bd.
In a report published last week in Science, dozens of researchers calculated the number of species harmed by Bd. Their findings are devastating: 90 amphibians driven extinct and another 124 species hanging in precarious fate. Only a quarter of the near 500 amphibians impacted show signs of recovery, the authors report.
The scale of devastation makes the fungus historic.
“The biggest impact of any pathogen that has been recorded yet by science,” said Dan Greenberg, a biologist at Simon Fraser University who did not participate in the research. “It seems like Bd's pretty special.”
When scientists first began reporting frog declines around the world in the 1980s, their cause was a mystery.
“One day frogs would be there, and you'd come back a couple weeks later and there'd be nothing,” says Greenberg. “For the longest time, there weren't even bodies.”
Far-fetched theories sprang up to explain the deaths.
“People thought that the ozone was weakening and frogs were getting fried by UV radiation—all sorts of things,” Greenberg said. It wasn’t until 1998 that researchers identified the killer. From what is normally a benign type of fungi, two species had morphed into lethal parasites (a second was discovered in 2013).
More recently, researchers determined that the disease-causing strains originated on the Korean peninsula and likely spread across the globe through the amphibian pet trade, making them particularly lethal in environments far from their origin, like the Americas.
Death by Bd isn’t pretty. Tiny fungal cells swim through water and enter amphibians through their skin, where they multiply. Frogs’ skin will redden and thicken. They’ll suffocate or die of starvation. But not all frogs are so badly affected.
“Some species are pretty tough as nails,” Greenberg said.
As a whole, amphibians have a pretty bleak outlook: more than a third are threatened with extinction from habitat loss and impacts from climate change on top of disease.
“This is not the end of the extinction story that we’re going to hear,” Greenberg predicted.
But he takes the long view, noting that amphibians – as a group – have survived other dramatic environmental changes. We can only hope they’ll manage to do the same this time.
Web content created by Lexi Krupp.