Declines in mountain yellow-legged frog (Rana muscosa and Rana sierrae) populations were first recognized in the 1970s. Since then, according to National Park Service, the population size has decreased 92%, and in 2013 they were listed under the U.S. Endangered Species Act. Mountain yellow-legged frogs thrive in the lakes, rivers and streams of the Sierra Nevada.
Roland Knapp, biologist at UCSB’s Sierra Nevada Aquatic Research Lab, began studying mountain yellow-legged frogs in 1995 when the decline in population size had increased rapidly. Knapp referenced Joseph Grinnell, former professor at UC Berkeley, and one of those who noticed and studied the mountain yellow-legged frog decline far before its mainstream recognition.
“They’re really the poster child for a number of decades now of the whole amphibian decline situation,” Knapp said.
According to Knapp, a fungus named chytridiomycosis, or B.D. for short, has been taking its toll on these especially sensitive frogs. Chytrid fungus was discovered in 1998, though it’s been around for much longer than that. Symptoms vary with the species, but the typical signs include reddening and excessive shedding of the skin. Some species, however, aren’t bothered at all. Knapp referenced American bullfrogs, who live side-by-side with the mountain yellow-legged frogs, but who don’t seem to have an issue with B.D. They contract it, but they aren’t affected by it, unlike their more sensitive (and arguably much cuter) neighbors.
“We know that there are certain aquatic amphibians that live in habitats that seem relatively ideal for B.D. that don’t in fact succumb to the disease. They just don’t have the pre-adaptation to deal with this pathogen that other frogs might have,” Roland said.
Carlos Haslam, a frog specialist at the Berkeley Vivarium, described how weather conditions may affect fungus populations, in turn affecting frog populations.
“The general notion is that, with the shifting weather patterns in the Sierras where we haven’t had a snowpack in years, you’re starting to see a grand shift in the overall environment. Not just in ground temperatures but also in soil humidity, so things like fungi which were dormant are now starting to come to the surface,” Haslam said.
Haslam explained that while this population is indeed suffering, it’s not solely because of the fungus.
“Yellow legs aren’t any more susceptible than any other frogs, but their population is already in decline, so I think it’s more documented,” Haslam said.
Haslam emphasized the importance of non-native species like kingfishers, crows and red-eared slider turtles, which play a major role in the frog’s decline. The continued release of these invasive species exaggerates the drop in numbers for these frogs.
“In traditional Chinese culture, there are certain New Year’s rituals where you write a prayer or a wish on the back of a turtle and release it,” Haslam said.
In areas where the population shows no signs of recovery, researchers working with the San Francisco, San Diego, Los Angeles and Oakland zoos are doing their best to cure the frogs by airlifting infected populations, flying them to the zoos and keeping them in quarantine for treatment. They refer to this as “head-starting” the animals. The fungus-ridden frogs are first cleared of the infection, having been bathed in antifungal drugs. Once they are no longer infectious, they can be raised alongside other amphibians at the zoo, later to be released into the wild when fully grown. Knapp admitted that head-starting the frogs can be an extreme measure.
“It’s expensive and time intensive, but in these places where we’re really seeing a severe risk of losing these frog populations, it seems like an important avenue to pursue,” Knapp said.
On the bright side, amphibians are incredibly adaptable and they’ll probably be fine. As Haslam said, “Their genome is basically meant to adapt. That’s what makes amphibians, amphibians.”
A version of this story appeared on p. 14 of the Thursday, Jan. 21 print edition of the Daily Nexus.