Frog-attacking fungus flourished before mass extinctions

Mountain yellow legged frogs of the Sierra Nevada might join the list of species that have disappeared due to the fungal pathogen B.d. / Flickr: USFWS Pacific Southwest Region

By John Upton

Scientists had never encountered anything like it.

Discovered in the late 1990s by researchers trying to figure out why frog populations were disappearing around the world, Batrachochytrium dendrobatidis, or B.d., has already wiped out perhaps 200 to 300 amphibian species. It has shaken ecosystems, starving birds and other frog eaters and allowing insects to run rampant.

“We have something the world has never seen before,” Vance Vredenburg, a biology professor at San Francisco State University who specializes in amphibians, told me earlier this year. “It’s jumping from species to species to species.”

The recent discovery that chytrid was present in New England in the 1960s, three decades before the disease’s effects were noticed, points to the alarming possibility that the die-offs are the result of worldwide environmental degradation.

B.d. is a type of chytrid — a member of the most primitive division of fungus: Chytridiomycota. Toadstools, molds and all of the other forms of fungus evolved from chytridiomycotes. Chytridiomycotes today are the smallest and simplest type of fungus, but these wily grandfathers still pack one helluva punch.

Before a frog is infected, it is hunted down by swarms of tiny chytrid zoopores that propel themselves through the water by flapping their tail-like flagella. The pathogen changes shape and burrows into the frog’s skin, which it consumes while it creates more zoospores, causing the frog to grow more layers. Many species can tolerate the parasites at low doses, but once a frog’s skin is infected with enough of the fungus it will go into cardiac arrest, its electrolyte levels thrown out of balance and its tiny heart stopped. (Weirdly, some species, including the American bullfrog, appear completely immune.)

“We find literally hundreds, and tens of thousands — I’ve seen it myself — dead animals on the shorelines of lakes,” Vredenburg said.

Research led by University of California, Berkeley ecologist Jamie Voyles helped explain how the fungus kills. What’s less clear is why it apparently started to cull amphibians all around the world at about the same time.

B.d. under a microscope / Flickr: AJC1

Kathryn Richards-Hrdlicka, a doctoral candidate at the Yale School of Forestry and Environmental Studies, took samples from 10 species of amphibians preserved in formalin at the Peabody Museum of Natural History. She reported Tuesday in the journal Methods in Ecology and Evolution that the fungus was infecting frogs in New England as long ago as 1968, the year in which one of the oldest of the studied specimens was collected.

“It’s possible dieoffs did happen back then and no one noticed, although die-hard herpetologists around here tell me someone would have noticed,” Richards-Hrdlicka told me. “I think it’s possible that when B.d. came to New England, it may have wiped out the more susceptible lineages or gene pools and what we’re left with today are those gene-pool winners.  That may explain why I can pick up 10 frogs here and three to four of them will be infected, with light zoospore loads, and show no signs of infection.”

Alternatively, rampant world trade could have brought two chytrid strains into contact that merged to spawn a super pathogen, as other B.d. researchers have hypothesized.

But Richards-Hrdlicka also said that the changing environmental conditions facing all species around the world right now might simply make frogs more vulnerable to the chytrid than they were in the past.

And there’s the rub. When frogs were disappearing without explanation in the 1990s, many speculated that the chordata class amphibia was acting like a canary in a coal mine, dying off before other types of animals because they are so sensitive to their environment.

The discovery of B.d. muted that suspicion, but now we know that B.d. was present long before these prominent collapses. Previous studies showed it was lurking in Africa in the 1930s.

Fungus is the great decomposer. When animals and plants are stressed, their defenses weaken and they can be eaten alive by fungus, which treats its prey as though it is dead flesh awaiting decomposition. So perhaps the mystery of the disappearing frogs really can be traced back to just about everything that is out of whack with our environment: Climate change, pesticides, habitat loss, water diversions, water degradation, air pollution, you name it.

Scientists had never encountered anything like B.d.

Until 2006.

That’s when bats in a cave in New York started dropping dead from white nose syndrome. Since then, the disease, which is caused by a type of soil fungus that chews through the mammals’ wings, has spread rapidly west, killing an estimated 7 million hibernating bats in just six years.

“That’s really similar,” Vredenburg said.

2 thoughts on “Frog-attacking fungus flourished before mass extinctions”

  1. could you have put a sadder portrait of a frog? Is that a frog on his B.d.’ed death bed?

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