Saving the Mountain Yellow-legged Frog

Photo: Steven Sadro

Photo: Steven Sadro

Mountain yellow-legged frogs in the California Sierra Nevada are disappearing at an alarming rate, primarily due to a virulent fungus. UCSB scientists Cheryl Briggs and Roland Knapp are racing to understand why some populations of frogs succumb and others survive, with the aim of not only saving the frogs but also gaining knowledge of how and why organisms develop resistance to virulent pathogen attack.

“Frogs all over the world are suffering declines,” so it’s critical to find out what’s happening in the Sierra, says Briggs, a professor of ecology, evolution and marine biology.

With concern about emerging infectious diseases that affect humans, like SARS, HIV and Avian influenza, it’s vitally important to understand how this pathogen is transmitted and why some frog populations are able to withstand the attack while others can’t, Briggs explains.

Moreover, with an alarming number of species on Earth disappearing, finding the key to the frogs’ survival could help scientists save other species.

Briggs, who graduated from UCSB, was on the faculty at UC Berkeley for 10 years before returning to UCSB in 2007. In 2008, Briggs and co-principal investigator Knapp received a five-year, $1.9 million National Science Foundation grant to continue the frog study she began at Berkeley.

Mountain yellow-legged frogs (Rana muscosa) are found primarily in high-elevation lakes and ponds in the Sierra, and these highly aquatic animals typically spend up to nine months of the year under ice. Once one of the most abundant species of vertebrates in the Sierra, R. muscosa has disappeared from most of its historic range over the past several decades and is under consideration for endangered species listing. A major cause is the virulent amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd), which is moving like a wave eastward through the Sierra.

The fungus causes Chytridiomycosis, which was first described in the late 1990s. No one knows where Bd came from, but it has been devastating to the mountain yellow-legged frog populations throughout the Sierra. In fact, museum specimens show Bd infection since at least the 1970s. The fungus spores infect the skin of the subadult and adult frogs and the mouth parts of larval stages. Bd has little or no negative effect on tadpoles, and effects on post-metamorphic animals is highly variable among species, with individuals of some species dying within weeks of infection and others suffering little or no effect.

So Briggs and her team are testing four hypotheses:

• Host extinction vs. persistence is the result of density-dependent disease dynamics.
• Host extinction vs. persistence is the result of differences in Bd virulence.
• Host extinction vs. persistence is the result of differences in frog susceptibility.
• Host extinction vs. persistence is the result of differences in environmental conditions.

“It’s still early to say, but I honestly think that part of it is population dynamics,” Briggs says. People try to focus on one process or another to explain the collapse, but it’s probably a combination of several things happening at once with the frog population and the fungus, she explains.

In the early 1990s, scientists started to notice that trout planted in higher-elevation Sierra lakes were killing off the native mountain yellow-legged frog. About the time they decided to take steps to stop seeding the lakes with fish, they realized the fungus was also killing the frogs to varying degrees in lakes throughout Mammoth and in parts of Yosemite.

Field studies are examining how the populations in different areas are affected, and why some seem to collapse and others seem to resist the fungus enough to survive it (as in parts of Yosemite). One thing they do know is some species are more susceptible than others, Briggs says. For example, bullfrogs and Xenopus lab frogs, which are used around the world, have the fungus but aren’t sickened or killed by it. Because frogs have two stages of development – tadpoles (which feed on algae) and adults (which eat insects), infections in the Sierra – and elsewhere – have the potential to threaten and/or affect greater portions of the ecosystem, Briggs explains.

While Knapp and co-researcher Vance Vredenburg from San Francisco State University do most of the field work, Briggs focuses on experimentation and mathematical modeling of the disease. “Disease modeling has been very successful in understanding and dealing with outbreaks,” she says.

Treatment is possible, and in summer 2009 Briggs and her team will begin treating some remote watershed populations in Kings Canyon/Sequoia National parks by dunking individual frogs into buckets of malachite green, a compound that kills the fungus. They will also temporarily remove tadpoles from some lakes and then return them once the fungus goes through so the frogs are better able to survive the onslaught. Scientists are also looking into the use of probiotics and other natural bacteria solutions.

“It feels like just now we’re starting to know enough about the fungus to do something about it,” she says. “This is not just a problem in the Sierra; it’s worldwide.”