Frogs leaped into the air and swam for cover as Paul Johnson strolled toward a stream crossing, a Jeep crawling slowly behind him. As the SUV climbed into the Griswold Hills from the San Joaquin desert, it crossed the creek another five times. Each time, Johnson, a wildlife biologist with the National Park Service, got out and shooed away dozens of frogs before the vehicle splashed through.
Clear Creek, about 100 miles southeast of San Jose, is one of the few remaining strongholds of the Central Coast population of the foothill yellow-legged frog. Once found in streams from the Bay Area to Fresno County, the species was placed on the federal endangered species list in late August — which explains Johnson’s extreme caution.
Johnson and his colleague at the Jeep’s wheel, Mike Westphal, a Bureau of Land Management biologist, are part of a small band of researchers hatching an ambitious amphibian comeback. Eventually, they hope that frog eggs from the creek can be used to re-establish populations of the frog in suitable habitats within its historic range.
The foothill yellow-legged frog, typically only a couple of inches long, is named for the bright color on the underside of its legs. Viewed from above, a mottled gray-brown to reddish body helps it blend into the rocky stream beds that it inhabits from Southern California to the northern Sierra Nevada.
The frogs have faced many threats, including habitat destruction, pollution and loss of eggs, which are often washed away when dams release large volumes of water after heavy rains.
But the biggest problem, according to Andrea Adams of UC Santa Barbara’s Earth Research Institute, has been a highly infectious fungus — the chytrid fungus — that has spread across the globe since the 1970s, decimating amphibian populations with a deadly skin disease.
After interviewing older naturalists, poring over field notes pulled out of garages and attics, and sampling for the fungus in museum specimens, Adams found that the fungus caused the frog to almost completely disappear from Southern California in less than 10 years in the mid-20th century.
Foothill yellow-legged frogs are also preyed on by snakes and bullfrogs, which spread the deadly fungus but are resistant to infection. At one stream crossing, Westphal spotted a common garter snake curled up on a rock and rushed over.
“This is a very healthy garter snake,” he said, feeling its guts. But it didn’t seem to have recently feasted on an endangered frog. “He doesn’t have any big frogs in him,” Westphal said.
Johnson and Westphal explained their hopes for the amphibians’ future as they traveled through the dry, hilly landscape, dotted with cattle ranches and juniper trees. In the past, the frogs thrived at nearby Pinnacles National Park in sunny streambeds with little vegetation. But they haven’t been seen there since at least 1965.
In 2022, the team began estimating the number of frogs in Clear Creek. Westphal says that they have surveyed 15 to 20 miles of creek so far, and the team will soon use computer modeling to generate an estimate of the number of adult frogs in the creek in the BLM’s Clear Creek Management Area.
The biologists hope eggs from the healthy population at the creek can be used to re-establish the frog at Pinnacles. But roadblocks may arise if the frog’s old habitat at Pinnacles is no longer hospitable. “Maybe even if the frogs were there historically, something has changed that’s beyond our control, and it doesn’t make sense to bring them back,” said Johnson.
Reintroductions are unlikely to spread the chytrid fungus, since it has already gone through amphibian populations throughout the state, and eggs cannot transmit the disease.
Westphal and Johnson aren’t the only scientists with big plans for the species. A re-establishment project is already well underway for the South Coast population of the frog, which historically was found from Monterey County to Baja California. It also received a federal endangered listing in August.
Only two robust groups of the South Coast population remain. One is thriving in a remote area of the Los Padres National Forest, north of San Luis Obispo. The other, smaller population is just a short hop away on Fort Hunter Liggett, in the far south of Monterey County, where U.S. Army Reservists train over an area of approximately 165,000 acres.
In advance of the federal endangered listing, the U.S. Fish and Wildlife Service began to collaborate with Adams and other researchers to study the populations of the frog in Los Padres National Forest and at Fort Hunter Liggett, and to identify potential sites for re-establishment. As at Pinnacles, researchers need to check for threats that could keep frogs from thriving in their historical habitats.
In 2022, the team began a pilot study to boost the frog population at Liggett, working with Department of Defense wildlife biologists. They moved some egg masses from the stream at Liggett to a second, frog-free stream on base, placed them in enclosures made from modified laundry baskets to protect them from predators, and fed the hatched tadpoles with algae. Typically only one out of every 1,000 frog eggs will make it to adulthood, but sheltering tadpoles in baskets significantly boosted survival. This year, they started moving eggs from the Los Padres population to Liggett.
The success of this pilot study is good news for the team at Pinnacles and Clear Creek. However, the South Coast team still isn’t moving frog eggs from Los Padres National Forest or Fort Hunter Liggett to other areas of the state. Because the populations are fragile, the researchers first need to ensure their methods are sound before attempting reintroductions to former habitats — after all, the survival of the species may depend on it.
“You literally don’t want to put all your eggs in one basket,” Adams said.
