THE COMEBACK KID

“We live in a world of unintended consequences,” says Roland Knapp, PhD—renowned research biologist, and, as it turns out, articulate historian.

For nearly three decades, the focus of Knapp’s career has been the Sierra Nevada yellow-legged frog. But to understand that species and its story spanning the past half-century—one which Knapp describes as a “roller coaster”—we are impelled to start at the beginning.

According to Knapp, the mountain yellow-legged frog has called the Sierra Nevada home for a million years, give or take. They were then pushed out of that home by the same glaciers that eroded the landscape into its shapes of horns and cirques and U-shaped valleys before recolonizing the range following glacial retreat.

“But they’re here,” says Knapp, “despite everything that, in those days, Mother Nature was throwing their way.”

And then came the Gold Rush, and with it, the Euro- Americans with their own ideas of what the Sierra Nevada ought to look like, says Knapp. 

“They thought the Sierra Nevada should look like the Alps, where the Europeans had been stocking non-native trout into their lakes for 400 years … which are also historically fishless,” explained Knapp.

These stocking efforts—the earliest of which sometimes occurred by the transfer of one coffee can-full of trout at a time, from one watershed to the next—was viewed, initially, as an innocuous boon for fishermen and recreationists. 

But there were other implications, too. Less immediate. But with no lack of severity.

“Of course—no blame intended—all of this was being done without any understanding, or I suspect any thinking, of what the consequences would be of introducing a very effective predatory fish into these lakes, which have their own assemblage of native species.”

What initially began as a pioneer effort was later formalized by the California Department of Fish and Wildlife (CDFW) through their aerial stocking program—a practice which continues to this day.

Fish stocking in front-country waters, where lakes are easily accessible, is straightforward. Trucks drive up, and fish are poured into the lakes. However, in the backcountry, the logistics are not so simple. Historically, fish were transported by hand, horse or mule. Now, airplanes are used to stock those furthest-removed lakes, often located within National Parks or designated wilderness areas. These regions have received high levels of protection, some specifically aiming to maintain natural processes, yet these protections come with carve-outs that allow ongoing fish stocking activities.

Reconciling these conflicting mandates—protecting native ecosystems and the species that team within them (i.e., the mountain yellow-legged frog) while maintaining recreational fisheries—presents a time-old challenge. 

Outside National Parks, explains Knapp, the US Forest Service manages habitats, while the CDFW oversees wildlife, including fish—two government entities that often have overlapping jurisdictions and differing agendas.

The Sheet: So the left hand doesn’t know what the right hand is doing?

Knapp: Or, even if they know what the other hand is doing, each point to their own mandates.

In the ‘90s, however, scientific research—much of it conducted out of the Sierra Nevada Aquatic Library (SNARL)— began to highlight the substantial impacts of introduced trout on aquatic ecosystems. The findings, in revealing that non-native trout adversely affect endangered amphibians and the overall aquatic ecosystem, helped actuate the CDFW to reduce the intensity of fish stocking in the region, and exclude certain lakes from stocking altogether, to prioritize native species’ habitat.

“Take Rock Creek for example,” says Knapp. “in that basin, probably 80% of the water bodies still contain non-native trout. So the small fraction that have been set aside—that’s a judgment call. How many fishless lakes do the frogs and the other species need? How many lakes do the anglers need to keep everybody happy? It’s an amazing juggling act that everyone has to come to consensus over.”

(Inside National Parks like Sequoia, Kings Canyon, and Yosemite, where CDFW has no jurisdiction, it’s a different story. In the early ‘90s, the CDFW actually decided to halt fish stocking altogether within the parks. Still, many fish populations within these parks are self-sustaining—a lasting and irreparable legacy of earlier stocking efforts.)

And while the cessation of stocking did not eliminate fish from all lakes, it allowed many water bodies to revert to their natural, fishless state—a state vital to the survival of the yellow-legged species and the other natives of its ecosystem.

But by the end of the 20th century, just as strategies were being developed to mitigate the impact of non-native fish, a pathogenic threat emerged. 

In 1999, a team of Australian scientists published a paper on a new fungal pathogen called amphibian chytrid fungus, which reported cases on their home continent, the US, and Central America. 

“It was the first chytrid fungus found to attack a vertebrate host,” said Knapp, who also recounted that his own mentor, David Bradford, had earned his PhD studying the physiology of the mountain yellow-legged frog in Sequoia National Park back in the ‘70s. According to Knapp, Bradford had been witness to his test subjects dying off at exorbitant rates while conducting his research.

“It turns out that almost certainly what he was witnessing, 20 years prior, was the beginning of the invasion of the amphibian kitchen fungus into the Sierra Nevada.”

Since then, the amphibian chytrid fungus has spread worldwide, likely originating in Asia and spreading through the pet trade. According to National Geographic, the fungus has driven the decline of approximately 500 amphibian species. Of these, 90 are presumed to have gone extinct.

Combining the effects of fish introductions and the chytrid fungus, then, created a dire situation for the species—already vulnerable and spatially fragmented due to the nature of the region’s topography.

By 2000—5 years into Knapp’s research on the species—it appeared as though the species was nearing extinction.

“But I wasn’t willing to give up on them,” recalls Knapp. “This is a creature that was by many accounts, the most abundant vertebrate in the Sierra Nevada… We can’t ignore—we can’t pretend—that when these frogs go away, the mountains will look as they always have.”

By 2005, 80% of the frogs Knapp had made contact with at the start of his research in 1995 had been lost. But the key piece, he says, in the species’ recovery, was that certain individuals had evolved in response to the fungus in a way that made them less susceptible to its effects—an inability to thermoregulate the body and respire due to destruction of the skin’s epidermal layer, according to the Wisconsin Department of Natural Resources). 

For these individuals, the symptoms resulting from fungal infection, while extensive in their impact, are not necessarily fatal. 

And that’s rare.

“You have to remember,” says Knapp, “that when a fungus shows up for the first time in a completely naive population, the chances are, that population of frogs is going to get wiped out completely—not a single individual left. But in about 5% of cases, the population declines dramatically, stays at that low level without going extinct, and then, if you’re lucky, within 15 or 20 years, begins to recover.”

And says, Knapp, more than 40 years after his mentor saw the first signs of the fungus’s impact, the Sierra Nevada Yellow-Legged Frog is one of those outlier cases, recovering on its own.

Well, mostly.

“That evolutionary change was key to everything we’ve done since,” says Knapp. “And when I first saw that, I immediately thought, can we use frogs in those recovering frog populations that are more resistant [to the fungus] to perform reintroductions?”

Reintroductions—the intentional translocation of individuals in a species to parts of its natural habitat where it has suffered decline as to restore new, viable populations—begin with identifying healthy donor populations. 

“We’ve been following these populations for decades, knowing which ones are still chytrid-free,” Knapp says. The selection of donor frogs from healthier populations is critical to ensure genetic diversity and the potential for recovery in those populations that are suffering.

Once a donor population is identified, the team collects a sample of frogs—maybe about 50, of which each gets weighed, measured, and swabbed to analyze their infection levels and establish a baseline prior to reintroduction.

Each individual is then tagged with a small glass-encased Passive Integrated Transponder (PIT) tag, which allows Knapp and his team of field researchers to track each frog, monitor its survival, and observe “recruitment of new individuals,” Knapp explains.

And, with no lack of irony, much like those first fish-stockers traversed by foot into the backcountry to bring non-native life into its waters, Knapp and his team travel in much the same way to bring home its native species.

“We often have to backpack them into remote sites because roads don’t always get us there,” Knapp notes. 

Each frog, then, is placed in a ventilated plastic cup, which is then secured in a larger container to prevent crushing. This container is surrounded by snow-filled dry bags to maintain a suitable temperature for the amphibian. The team then hikes to the reintroduction site, with each person carrying up to 20 frogs on his or her back. Distances vary, from a one-hour hike to a full day’s trek. 

Alongside frogs, more recent expeditions have transported tadpoles or egg masses—raised by the Oakland and San Francisco Zoos—in ice chests with aerators.

Upon arrival, and following 30 minutes of rest on the shoreline to recover from the “ruckus,” the frogs are released into their new lake habitat. From there, the team will revisit the site once every year, for the next 10 years, to monitor the reintroduced individuals.

Says Knapp: “It’s quite a thing to see.”

Knapp—who will be conducting his 100th reintroduction this week—acknowledges that the road to recovery for the mountain yellow-legged frog is long. Due to both habitat loss and the decimation wrought by disease, the species has disappeared from more than 90% of its historical localities.

“We’ve got a long way to go,” he says. “But we can use those recovering populations to speed up recovery with more reintroductions. Frog populations are beginning to rebound.”

Knapp says that the first field trip of the season will be to visit a population that has been under the ice since last October—frozen in the water, in suspended animation. It’s a population which the scientist fears has been hit, significantly, by the fungus.

The Sheet asked if he was nervous heading into the trip; if he felt an emotional connection to these amphibians that have become the mainstay of his career.

“I unfortunately have seen so many populations disappear,” he said. “It’s a horrible feeling. But now, I know the other part of the story—which is that, maybe, in a few years that frog population will actually recover. Maybe, there’s still some hope here.”

He recalled the first time he reintroduced frogs into Yosemite’s lakes—that those landscapes felt weirdly quiet; that there was a piece missing. 

“But,” says Knapp, “over 20 years, I’ve watched these frog populations reestablish themselves. Birds and garter snakes are feeding on them again. There are tadpoles swimming in the shallows of the water… It’s amazing to see the cycle of population crash and recovery; the whole web of life…

… So when I get to that lake in a couple of days, I’m sure I’ll have a little bit of a pit in my stomach… But what really helps me is knowing that, as far as I can tell, those lakes probably look a lot like they did 150 or 200 years ago. That is, they are so alive.”