The jewel-like lakes of the High Sierra in Yosemite National Park are awe-inspiring sights. But for more than a hundred years they’ve also been biologically disrupted, stocked each year with non-native fish, which in turn destroyed the population of Sierra Nevada Yellow-legged frogs that once covered their shores and filled their depths.
With that loss, the entire ecosystem shifted. The frogs had once been an important part of the summer diet of not only bears, coyotes and snakes but also multiple bird species, including the Clark’s Nutcracker and the Gray-crowned rosy finch.
Then the few frogs that survived were almost wiped out by the arrival of the dreaded amphibian chytrid fungus, which killed them off in the few fish-free lakes that remained.
“It was a double whammy that almost wiped out the species,” said Roland Knapp, a research biologist at the University of California, Santa Barbara who has been studying them since 1995.
Then sometime remarkable happened.
For the last 30 years, Knapp and a tireless group of biologists have been scouring the few fish-free lakes left for remnant populations of the once iconic frog. Not only did they find a few, but over decades of observation they realized that in rare cases, the frogs were evolving resistance to the chytrid fungus.
After years of research and voluminous requests to every federal and state entity involved, they reintroduced these fungus-resistant frogs to 12 lakes and watched the populations rejuvenate.
“The lakes are alive again, completely transformed,” said Knapp.
The research included scientists from the University of Tennessee, Colorado University and Yosemite National Park and was published this week in the journal Nature Communications.
“You literally can look down the shoreline and see 50 frogs on one side and 50 on the other and in the water you see 100 to 1,000 tadpoles. It’s a completely different lake,” he said.
The fish came, the tadpoles got eaten
The story of the demise of the Sierra Nevada Yellow-legged frog goes back to the days of the California gold rush, which began in 1848. Suddenly tens of thousands of miners, many from Europe, were hiking deep into the backcountry and mountains.
They found not only gold but glorious vistas, so beautiful that Yosemite National Park was founded in 1890 to preserve them.
They also discovered more than 1,500 alpine lakes in the high Sierra that were crystal clear and bursting with life ‒ but not with fish.
“There were all these people showing up looking for something to eat, many of them from Europe which had alpine lakes where they’d been stocking them with fish for 400 years,” said Knapp.
To fill the lakes, stocks of rainbow trout, golden trout, brown, brook and cutthroat trout, along with Atlantic salmon and grayling were brought in. For decades they restocked simply by carrying buckets of live fish to the shore and tossing them in.
“It had a huge effect. We have lots of reports of people who were in the high country and watched these changes take place. This frog that was the most abundant amphibian around. Within a few years of fish stocking it’s gone,” said Knapp.
The yellow-legged frogs remained abundant in only about 20% of the most remote, unstocked lakes. But even that changed after World War II.
“All these pilots are coming back with very, very highly developed targeting abilities,” Knapp said. They were eager to put their skills to work using planes to drop fish into the lakes. “Once aerial fish stocking was in play, there was no lake that was too remote to stock.”
Only the tiniest sliver of lakes remained fish free and frog full.
The National Parks ended fish stocking in the 1990s out of concern for native species but it turned out yearly-restocking had never been necessary. The fish populations in the lakes continued to thrive.
A fungus among us
In 1992 Knapp set about to see if the frog population could be saved by removing non-native fish. He got permission to use gill nets to clear fish from a small number of lakes and were hopeful of a comeback. “For the most part, removing the fish provided the opportunity for immediate recolonization by the frogs”
Other entities, including California Department of Fish and Wildlife the National Parks Service and U.S. Forest Service, began doing their own fish eradication programs and all began seeing the frogs come back.
Then disaster struck.
In the early 2000s research showed that the amphibian chytrid fungus – a global scourge – had arrived in the Sierra Nevada and began wiping out many of the last of the remnant frog populations.
First detected in Australia and Central America in the 1990s, the fungus caused mass mortality and population declines of frogs and other amphibians around the world.
“Those were really dark days in terms of trying to recover the species. It felt like for about ten years we were just describing the extinction,” Knapp said.
The Sierra Nevada yellow-legged frog was added to the endangered species list in 2014.
But then the scientists saw a ray of hope.
Frog populations crashed in lake after lake in the high Sierra as the deadly fungus moved across the landscape. But then they began to see some of the populations slowly increase.
“It sure looked like they had evolved some degree of resistance,” said Knapp.
This only happened in fish-free lakes, because only those had enough frogs and enough genetic variation to evolve resistance.
So in 2006 Knapp and other researchers set about to see if the frog population could be saved in fish-free lakes. After long and intense study and preparation they began to reintroduce the fungus-resistant frogs into the fish-free lakes. “They’re literally transported in Ziploc containers that you’d buy in the supermarket. Our only modification was to drill holes so they get oxygen.”
It worked.
Today, Knapp can sit at the edge of numerous high country lakes and see then transformed. Early explorers described lakes whose shores teamed with Yellow-legged frogs sunning themselves at the edges. As they walked up “there would be a shower of frogs jumping back into the water,” said Knapp.
That frog shower, which he’d only ever read about it, is now something he’s seen himself.
Though for those who make the hike to the lakes, set 7,000 to 10,000 feet above sea level, don’t expect to hear a chorus of frogs.
Sierra Nevada yellow-legged frogs don’t have vocal sacks, so they don’t call out during their springtime mating seasons. “If you hear frogs at one of these lakes, it’s the Pacific tree frog,” Knapp said.
But that doesn’t mean that they’re not making sound – you just have to stick your head under water to hear it.
“They grind their teeth together to make their mating calls,” he said. As someone who has indeed stuck his head under the freezing-cold water to listen, he tried to describe it.
“The best description I can provide is that it sounds like a wet hand on a balloon,” he said. “It’s this very squeaky sound. It’s very loud underwater.”
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