They used to be the most abundant amphibians in the Sierra Nevada, so common that hikers had to take care to avoid squashing them in creek and lake shorelines in warmer months. But now, hibernating deep beneath ice and snow, mountain yellow-legged frogs are spiraling toward extinction, scientists say.
The population of the small, garlic-scented frogs has collapsed so severely — probably by more than 90 percent, according to biologists — that the California Fish and Game Commission will meet next week to consider listing the species as threatened or endangered. Environmentalists hope the vote will help prompt the federal government to protect the frogs as well.
“This species clearly qualifies on all counts,” said Jeff Miller, a conservation advocate at the nonprofit Center for Biological Diversity, which has been fighting in the courts since 2000 to force the federal and state governments to better protect the frogs. “The evidence is overwhelming.”
The three-inch frogs are masterfully evolved to survive the harsh mountain winters and fleeting summers, spending as much as nine months beneath ice. But they have few defenses against a hypervirulent fungus disease that is wiping out amphibian populations around the world, or against nonnative trout that were introduced to their creeks and lakes.
Pollution from the Central Valley, habitat loss, cattle grazing and wildfires are also believed to be taking a toll on the species, which is found only in the Sierra Nevada.
If the commissioners vote to list the frogs as threatened or endangered, killing or removing one of the frogs without permission from the state would become a crime. Additionally, state agencies would be required to start considering how industries that they regulate, including lumber harvesting and home building, could affect the frogs before they issue permits.
Miller said he expected the federal government to list the frogs as endangered within two years, which would force the agency to develop a detailed plan to help the frog population recover.
Despite their low profile, scientists say, the frogs play critical natural roles in the mountains, providing food for birds, bears and raccoons and keeping pests in check.
“They have an enormous effect on the ecosystem, but it’s mostly unnoticed by humans,” said Vance Vredenburg, an assistant biology professor at San Francisco State University. “They were the most abundant vertebrate out there and they consume enormous amounts of insects.”
Vredenburg said rainbow trout and a fungus disease have taken the heaviest toll on the frogs. Research is under way to determine the effects on the frogs of fertilizers and pesticides sprayed over Central Valley farms, he said.
Trout were introduced in recent decades to mountain streams and lakes, which the fish could not reach on their own. The trout eat frog eggs and tadpoles, as well as insects that normally provide food for the frogs. Under pressure from environmentalists, California has been sharply reducing the number of trout it stocks in waterways for recreational fishing in recent years.
The fungus affecting the frogs, Batrachochytrium dendrobatidis, is commonly called chytrid. It was discovered in the 1990s and has quickly swept through the world, driving an estimated 200 to 300 species of amphibians to extinction.
“It’s like no other pathogen we’ve ever seen, in the sense that it’s jumping from species to species to species,” Vredenburg said. “In order for it to grow, it needs to continually reinfect the host. It’s not like a virus that gets into our body and then replicates like crazy. This is an infection of the skin.”
That means amphibians that spend much of their time close to one another in the water, including the mountain yellow legged frogs, are most vulnerable to the disease.
Amphibians’ skin plays a critical role in regulating the levels of electrolytes and other substances inside their organs.
“As they get progressively sicker and sicker, they have problems with the electrical functioning of their hearts,” said Jamie Voyles, a researcher at the University of California, Berkeley, and an expert on fungus diseases. “The skin breaks down and ultimately they suffer from cardiac arrest.”
Listing the species as endangered would not directly protect it from the disease, but Vredenburg said it would help researchers secure government money to investigate the pathogen and a potential bacteria-based treatment.
Department of Fish and Game officials urged their commissioners to vote to list the species as threatened during the meeting in Sacramento next week, noting in a report that remaining populations “are generally very small.” They also suggested listing a closely related Southern California species as endangered.
“These low abundances are in stark contrast to historical accounts that describe MYLF (mountain yellow legged frogs) in the Sierra Nevada as extremely abundant,” the officials wrote.