What’s Killing All the Frogs?

According to a new study published in Proceedings of the National Academy of Sciences, nearly 200 species of frogs (or around 3.1 percent) have gone extinct since the 1970s, and hundreds more (an estimated 6.9 percent) may be at risk because of a pathogenic chytrid fungus known as Batrachochytrium dendrobatidis; it causes the often deadly disease chytridiomycosis, commonly known as chytrid.

The percentages are relatively small, but scientists are worried about the big picture. The Washington Post reports that the extinction rate for reptiles and amphibians is 10,000 times greater than for any other organisms. “If you project that forward for the next couple of centuries, you end up with a percentage which would resemble a mass extinction,” John Alroy, biologist and author of the study, told the Sydney Morning Herald.

The earliest case of chytrid dates all the way back to the 1930s, but the aquatic fungus itself was not identified until 1998. The origins are still unknown, but chytrid has been linked to mass deaths of amphibians on every continent except Antarctica. According to John Morton of the Kenai National Wildlife Refuge in Alaska, the disease

“occurs inside the cells of the outer skin layers that contain large amounts of keratin … with chytridiomycosis, the skin becomes very thick due to a microscopic change called ‘hyperplasia’ and ‘hyperkeratosis’.  These changes in the skin are deadly to amphibians because—unlike most other animals—amphibians absorb water and electrolytes like sodium and potassium through the skin and not through the mouth.”

Alroy notes that the largest loss of frog species caused by chytrid has occurred in Central America and Brazil, and that frogs have been dying elsewhere because of other factors, including deforestation, the introduction of invasive species, and, potentially, global warming. “Honest to goodness, I don’t know what’s going on,” Alroy told The Washington Post. “The pattern seems to be that there are more extinctions in the wet tropics, speaking very loosely, which is not too surprising because that’s where all the biodiversity is.”

As for solving the chytrid issue, there are ways to treat the disease with antifungal medications, but only in controlled environments, not in the wild. A 2009 study found that taking an antifungal bacteria known as Janthinobacterium lividum (which is found on some amphibians) and adding it to the skin of mountain yellow-legged frogs prevented chytrid-related deaths, but more research is needed to tackle the issue on a larger scale.