February 06, 2006
Disease may be devastating amphibian population
CARBONDALE, Ill. -- What is killing the world's frogs and salamanders?
Zoologist Karen R. Lips of Southern Illinois University Carbondale can't put the cuffs on just yet, but she says her research team has "narrowed the list of suspects" in what scientists call "enigmatic" declines — those that lack an obvious cause.
Her most likely culprit is a hugely infectious disease caused by a fungus. In just four months — from mid-September of 2004 to mid-January of 2005 — Lips and her colleagues saw more than half the amphibian population of El Copé, Panama, sicken and die from this disease.
It cut across 37 species in seven families of frogs and one species in a salamander family. All but three of the dead showed signs of moderate to heavy infection; the three exceptions had decomposed so badly the researchers could not diagnose them. The team's report appears in this week's issue of the journal Proceedings of the National Academy of Sciences.
The disease, which attacks the skin (likely making it impossible for amphibians to breathe or absorb water), is particularly deadly because it isn't choosy about its victims.
"Most disease affects only a few closely related organisms, but this seems to affect all species of amphibians to some degree," Lips said. "There are 6,000 species. Imagine losing 3,000 of them."
The disease is also astonishingly mobile, moving 30 to 40 kilometers — about 17 to 25 miles — each year. It has caused declines in at least 43 amphibian species in seven Central
American countries, including Costa Rica, where Lips did much of her early research, and researchers suspect it has been responsible for declines in 93 amphibian species around the globe.
That mobility adds to its virulence.
"This thing just keeps moving and moving," Lips said.
"The fungus can get into the soil or water to get to the next place where it can find another frog, and since one frog can carry it, they just infect each other."
Lips knows this fungus as well as anyone. Back in 1998, her team and some Australian researchers discovered they were seeing the same strange symptoms in frogs they were studying. Disease experts from the National Zoo in Washington, D.C., and the University of Maine, Orono confirmed that some as-yet unknown fungus was the cause.
"We got together, figured out what it was, and they put a name to it," she said.
Once they knew the fungus existed, they began to suspect it might help explain the plummeting amphibian populations that scientists had been seeing since the 1970s.
"People would come back to the same research sites year after year; maybe one year they'd see a couple of dead frogs, and then the next year they'd show up and there wouldn't be any frogs at all, and nobody knew what happened — it was a mystery," Lips said.
"There were no dead bodies to examine, no witnesses to ask, no smoking gun — that happened to us several times. That's what makes this study unique. We figured if it was the fungus, we could get ahead of it, collect all the data we needed (on the condition of habitat and species before the fungus arrived), and then when it got there, we could watch it. We would be able to say, ‘This is how things were, and one day, the fungus showed up, and this is what it did.'"
Lips, who saw her first population crash of Central American amphibians in 1993, knew from experience that the fungus was moving south and east.
"We moved just a couple of miles east of where we had been working (when the last population crashed) where everything was healthy and hoped it would show up," she said.
It took about six years, but on Sept. 23, 2004, Lips and her team found their first infected frog. By October, the fungus had infected more than 10 percent of 21 amphibian species at El Copé. By late December, the number of infected species had grown to 40.
The researchers eliminated predators, land-use changes, chemical contaminants and other diseases as the cause of the die-off they saw at El Copé, leaving the fungus as the most likely perpetrator. And because bioclimatic modeling suggests the fungus can survive in many other parts of the world, the researchers say they fear not global amphibian declines but global amphibian extinctions.
"Certainly loss of habitat is a huge problem, especially in the eastern United States and in Europe, but disease is also a huge problem — and not just for tropical frogs," Lips said.
"Most of the amphibians that live in the mountains of the Americas — North, Central and South — are missing or extinct because of this disease, and it's spreading. When things go extinct, they don't come back. There's no recovery."
In the near term, Lips and her colleagues predict the loss of more and more Central American amphibians, starting in the mountains east of El Copé.
"This thing is a wave that keeps going further and further east — the only place where there are still healthy frogs," Lips said.
Can anything be done to save these hapless creatures? Lips isn't sure.
"You can't just create a park to protect their habitat — we're losing huge numbers of amphibians in the parks," she said.
"You can't spray with fungicide because you will kill the fungi that are beneficial. And while you can vaccinate frogs in a lab, how can you do that in the wild? It's a tough issue — another mystery."
The report's authors include Forrest Brem, Roberto Brenes and John D. Reeve from SIUC; Ross A. Alford of James Cook University in Townsville, Queensland, Australia; Jamie Voyles, Cynthia Carey and Lauren Livo of the University of Colorado in Boulder; and James P. Collins, Arizona State University. The National Science Foundation and the Bay and Paul Foundation paid for the research.
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