by Merrik Bush-Pirkle
As a killer pandemic sweeps hundreds of amphibian species into oblivion, an SF state biologist battles to give imperiled frogs a fighting chance
Vance Vredenburg didn't expect the frogs to die.
Sheltered in a basin of sky-blue lakes so remote it takes two days of hardscrabble hiking to reach, the growing population of mountain yellow-legged frogs Vredenburg had worked so long to save was thriving.
Annual field surveys revealed that the San Francisco State biologist's campaign to eradicate non-native trout from this pristine pocket of the Sierra Nevada Mountains was paying off. A devastating predator, this introduced sport fish had reduced the amphibian from tens of thousands of individuals to small isolated populations. But years of hard work were paying off. The frogs of the 60 Lakes Basin were repopulating their fish-free habitats in record numbers.
Still, Vredenburg worried over a new threat lingering on the fringes of this federally protected wilderness. Around the world, a mysterious lethal fungal disease called chytridiomycosis was decimating amphibians at an alarming rate, pushing dozens of species to extinction.
Caused by a common microscopic aquatic fungus called Batrachochytrium dendrobatidis, chytrid was unusual in its transformation from harmless decomposer to virulent infectious pathogen. Scientists were mystified that some infected species experienced 100 percent mortality while others merely carried the disease. Equally unsettling was its ability to jump from species to species, host to host, without discrimination. How it caused death was unknown, but scientists surmised that since chytrid colonized the skin, it likely interfered with oxygen and nutrient intake.
"I knew cases were showing up in the lower elevations," recalls Vredenburg. "But I kept thinking, 'This remote little jewel of an area is going to survive.'"
But when he arrived in the basin, he found total devastation. Slipping off his backpack and hiking boots, he stepped into the first lake he came to and tried to count the mass of upturned bodies bobbing in the wake of his movements.
There were hundreds.
That was summer 2004. Today, says Vredenburg, the epidemic has pushed nearly 400 amphibian species to extinction and reduced the mountain yellow-legged frog population by 98 percent.
"I spent the last five years watching chytrid sweep through the basin, lake by lake, and wipe them out," says Vredenburg. "The disease is unprecedented in human history. It hits a population, and boom! Everything dies. It makes the black plague look like the flu."
The pandemic is likened to the extinction of the dinosaurs, and biologists estimate that of the 6,000- plus recognized amphibian species -- which include frogs, salamanders and wormlike caecilians -- one in three are at risk of extinction within our lifetime. At 3 million years old, amphibians are not only the oldest and most abundant living land vertebrate, they are also the most tenacious, having survived the planet's past five mass extinctions. Their sudden fragility, argue scientists, is just one more sign that Earth is in the midst of its sixth extinction event.
"They are the canary in the coalmine of our biosphere," warns Vredenburg. "And they're telling us we're in trouble."
Last year, Vredenburg and his mentor, University of California, Berkeley Professor Emeritus David Wake, published an article in the Proceedings of the National Academy of Sciences pointing to human induced environmental changes such as global warming, invasive species, and habitat loss as catalysts for the extinction phenomenon.
"It's death by a thousand cuts," says Wake. "This disease is simply the final blow."
Scientists, policy makers and conservation organizations are in battle mode, collaborating on a scale never before seen in the scientific community. Researchers -- including Vredenburg -- are sharing preliminary findings, engaging in cross-country pilot studies and pulling the plug on slow-moving modes of information exchange in order to arrest this environmental cataclysm.
"We're not always waiting for data to appear in journals before sharing it," says Vredenburg.
As proof, Vredenburg cites one breakthrough shared a year before it was published. "A group of microbiologists quickly developed a technique that allowed us to catch frogs in their habitat, swab their skin, and release them."
Thirty-four thousand swabs later, Vredenburg is revealing much about this infectious disease.
Funded by the National Science Foundation, his current research focuses on the ecology of the disease, and the potential of beneficial bacteria and fungicides to stave off population collapses.
He's found that along with high elevation species, amphibians hosting lower concentrations of beneficial bacteria on their skin seem most susceptible to the disease. Survivors, on the other hand, have higher concentrations of these antimicrobial compounds, which fight off invading cells. Preliminary data on a collaborative project with Reid Harris from James Madison University shows that yellow-legged frogs treated with beneficial bacteria and then infected with chytrid have a 100 percent survival rate. Introducing beneficial bacteria into populations of both disease-plagued species and areas not yet hit with the disease, like Madagascar, he says, is a promising new line of defense. A more immediate tactic, he says, involves treating frogs with antifungal compounds.
A pioneer in the method, Vredenburg began treating yellow-legged frogs almost immediately after chytrid entered his basin. He began by soaking a handful of beleaguered frogs in a tub of antifungal solution, then tracking their status by inserting transponders, or pit tags, under their skin. Once treated, the frogs were returned to the lakes, fungus-free, and although they became re-infected, many survived, with lower infection levels. Not surprisingly, Vredenburg has found that there is a distinct threshold of infection that leads to extinction, so reducing virulence may help populations persist. What began as an almost desperate stopgap measure has evolved into a major endeavor, involving thousands of yellow-legged frogs.
Of all his basin lakes, the only two left with surviving populations are those with treated frogs. Among them are many of the first frogs he bathed in fungicide.
When he returned to the basin this summer, he felt hope.
Walking toward the edge of the lake, he was greeted by a cascade of startled frogs leaping their retreat back to water.
"They didn't just survive," he says enthusiastically, "they are thriving. They're fat and they're breeding." Sitting behind his desk, surrounded by posters of exotic amphibians -- many of them now extinct -- he allows himself to daydream a little. "Maybe that whole basin really will be filled with frogs again. One day..."
Learn more on the National Science Foundation's new science video series: www.nsf.gov/news/special_reports/science_nation/disappearingfrogs.jsp
Share this story: