Toiling against a deadly disease to save a threatened frog
SEQUOIA AND KINGS CANYON NATIONAL PARKS, Calif. — From the summit of Bishop Pass in the Sierra Nevada, elevation 11,972 feet, all you can see are miles of granite peaks against the sky. There is no traffic and no pollution. The natural world seems pure and unspoiled.
But appearances are deceiving. Over the last decade, disaster has struck in the form of chytridiomycosis, or chytrid, a deadly fungal disease that has driven at least 200 of the world’s 6,700 amphibian species to extinction. One-third of the world’s frogs, toads and salamanders are threatened. Forty percent are declining. Chytrid’s arrival has laid waste to the indigenous Sierra Nevada yellow-legged frog, Rana sierrae.
In Dusy Basin, a remote glacial valley in Sequoia and Kings Canyon National Parks a few miles west of Bishop Pass, Vance Vredenburg, a professor of biology at San Francisco State University, is conducting an experiment he hopes will help preserve what remains of these once abundant creatures. Vredenburg and his colleagues are inoculating chytrid-infected frogs with a bacteria, Janthinobacterium lividum, or J.liv, that does not prevent infection with chytrid but can help frogs survive.
Vredenburg, Reid Harris of James Madison University in Harrisonburg, Va., and colleagues found the symbiotic bacteria on several amphibian species. Lab experiments last year showed that J.liv produces a metabolite, violacein, that is toxic to the chytrid fungus. Vredenburg wants to see how effective the treatment will be in the wild.
Even before chytrid arrived, the Sierra frog population had been severely reduced by the California Department of Fish and Game’s practice of seeding high-elevation lakes with hatchery-raised fingerling trout for the sport fishing industry. Chytrid has hastened the destruction. Vredenburg and colleagues counted 512 populations scattered among the thousands of mountain lakes in the park in 1997. In 2009, 214 of these populations had gone extinct. A further 22 showed evidence of the disease. It is a far cry from the early 1900s, when frogs in the region were so common that lakeside visitors reported trampling them underfoot.
Vredenburg, 41, has been doing frog research in the Sierra since the mid-1990s. He chose frogs as research subjects because he wanted to do “basic science that could be applied toward solving some real-world problems, like the biodiversity crisis. Once your study animals start dying, believe me, you pay attention!” At the time, he said, “I saw many scientists as living and working in a bubble. Besides,” he added, “I like catching frogs.”
Vredenburg himself was “speechless” when the park service granted permission to carry out the J.liv experiment in Dusy Basin. “Then I had to start planning,” he said.
Vredenburg chose Dusy Basin for his experiment because chytrid is just arriving here. Unlike Sixty Lake Basin several miles to the south, where frogs went extinct within four years of the arrival of chytrid, Dusy Basin still has frogs. Biologists do not know what first brought chytrid to the Sierra. But Vredenburg’s research showed that chytrid spreads in a linear wave across the landscape, an infection pattern like that of human epidemics. Infection levels start out light, then increase to very high. Then there is a mass die-off.
In July, Vredenburg and his students captured and tagged 100 frogs, apparently the last remaining here, with transponder tags. They weighed and measured frogs, and they recorded the tag numbers using an electronic reader. The experimental group contained 80 frogs; 20 were designated controls. Vredenburg and his students placed experimental frogs in plastic containers for an hourlong bath in cultured J.liv — long enough for J.liv to colonize on frogs’ skins. They released the frogs into the ponds and streams where they had been captured.
Early in September, Vredenburg made his last trip of the season to Dusy Basin to see how the frogs were faring. At this elevation, snowfall often starts right after Labor Day, and the lakes start to freeze over. The yellow-legged frogs hibernate beneath the ice for eight months of the year.
Vredenburg spent two days catching frogs, weighing them, checking to see if they were experimental or control animals, and treating them. He found several that had not been tagged.
He found one untagged female in a puddle beside a lake. He held her outstretched in his hand and pointed to her yellow underbelly and the bright yellow-orange of her legs. “I can smell garlic on her,” Vredenburg said. “Many frogs species have defensive compounds that smell like garlic.” He swabbed her belly, her thighs, her toes and the webbing between her feet. “This gets a layer of skin where I can detect both microbes,” he explained. Later in the lab, he would extract DNA from the swab to verify the presence of the protective J.liv bacteria.
By Sept. 2, Vredenburg had caught 43 frogs: 33 that had been treated with J.liv previously, one control — previously tagged but untreated — and eight brand new, which he treated. Frogs inoculated earlier in the summer were surviving.
Several weeks later, Vredenburg had results from the lab: all the frogs caught in early September were infected with chytrid. The inoculated animals had the lowest levels of infection.