Fighting for the Frogs

Researchers hope to keep amphibians from disappearing in High Sierra

Carl T. Hall / SF Chronicle 4dec00

Perhaps the most disturbing evidence of ecological trouble in the stunning backcountry of the High Sierra is the dead silence in watersheds once alive with frogs.

Mark Twain's famed jumpers of Calaveras County favor lower foothills and could never last in the highest elevations, a harsh environment where retreating Pleistocene glaciers helped carve out 4,000 lakes and 11,000 miles of crystal-clear streams.

But the hardy Sierra native known as the mountain yellow-legged frog managed to do very well even way up here. Frog populations would rise and fall,

and nobody ever bothered to carefully count noses, but the amphibians were still a ubiquitous presence until the 1970s.

Now, the yellow-legged frog, Rana muscosa, is being proposed for the endangered species list, wiped out from about 80 percent of its historic range.

Declining amphibian populations are considered a worldwide problem - a mystery with no single culprit and no clear solution in sight. It's a different story in the High Sierra.

Prompted by a petition filed by environmentalists, a one-year review of the yellow-legged frog's status is under way at the U.S. Fish and Wildlife Service.

And while some argue that listing is not needed, there appears to be growing consensus that something needs to happen soon if the frog is to be rescued.

"They're not dead yet, but certainly the species has been severely wounded, " said Roland Knapp, a veteran frog researcher at the University of California at Santa Barbara.

It may take only a short drought, or an exceptionally long winter, for the frogs to disappear forever from its last vulnerable redoubts, wildlife officials say.

"The danger is we will start to see different watersheds systematically blink out, one after another," said Jason Davis, a biologist in the Fish and Wildlife Service's endangered-species division in Sacramento.

It's a problem that extends far beyond the water's edge.

The yellow-legged frog is a major food source for the Clark's nutcracker, one of the most common bird species in the High Sierra. The birds spend their winters in the mountains, living off stores of whitebark pine seeds the nutcrackers cache all around their range.

The birds always leave enough seeds scattered around to suit the reproductive needs of the whitebark pine, a particularly hardy species that is sometimes the only tree found in the Sierra's higher elevations.

Without the birds' help, whitebark seeds would fall straight to the ground, lacking the structural features of pine species that rely on wind propagation.

So fewer frogs means fewer birds, which in turn means fewer trees. And there's little doubt that there are a lot fewer frogs these days.

"You can hike the Sierra for days and weeks and not see a single frog anywhere," Knapp said. "Clearly something has happened."

But just when extinction began looming, scientists have found reason to hope that the frogs might yet have a chance to survive.

After a decade of close scientific scrutiny, experts believe they know exactly what has led to the yellow-legged frog's demise.

It's the fish.

A Taste for Tadpoles

Only about 20 high-country lakes and basins ever contained native Sierran golden or Lahontan cutthroat trout. Today, after widespread recreational stocking that began early in this century, trout can be found throughout the range.

The newcomers have a voracious appetite for defenseless tadpoles and mature frogs. Evidence includes laboratory studies in which trout have been observed feasting on frogs, but the smoking gun has been uncovered in field research.

Knapp, a researcher at the UC-affiliated Sierra Nevada Aquatic Research Laboratory, has found that the yellow-legged frog's few remaining holdouts are almost exclusively in national parks, such as Yosemite and Kings Canyon- Sequoia, where fish-stocking in most places ceased after 1977.

Right next door to the national parks, such as here in the seemingly pristine John Muir Wilderness, stocking continues. And there are hardly any frogs.

"Trout, from the frogs' point of view, are sharks - very, very effective predators," Knapp said.

Direct predation by fish isn't the only cause of the frogs' problems. Another factor is loss of migration routes for the amphibians. With so many fish-stocked lakes and waterways, frogs are essentially hemmed in - unable to move into friendly habitat whenever conditions become untenable in nearby areas.

Not all Sierra fish are outsiders.

Some native golden trout can still be found in the Kern River drainage, and some native cutthroat remain farther north. But native trout were extremely rare in the High Sierra until they started being brought in by people, who carried them in buckets in the old days, and are now dropping them in by air.

This meant the Sierran native frogs evolved in an essentially fishless world, and so evolved none of the protective tricks - fish-repelling toxins, for example, or hiding behaviors - found in some other amphibian species.

As for other frog-killing factors, studies have all but ruled out possibilities such as ultra-violet radiation, suspected of being on the increase due to human-caused changes in the atmosphere and which may be causing damage to frog reproduction cycles elsewhere.

Pollution Studied

Traces of pesticides and other airborne contaminants waft up from the populated, intensively farmed valleys in the Sierra's long shadow, but concentrations are very low in the thin air surrounding the highest peaks.

Air pollution research continues, but so far there is no evidence that it's a significant problem for the mountain-dwelling amphibians that took hold in the high country. However, pollution is still considered a possible key factor in some of the lower elevations on the western slope of the range, where species such as the mountain red-legged frog are also fighting for survival.

In the land of the yellow-legged frog, the research has evolved a new focus,

where the aim is not so much to discover the cause of problems, but rather how a solution might work out.

Can the frogs recover in a natural Sierra mountain lake when fish are removed?

The answer, it seems, is yes.

A case in point can be found in a high basin, 11,400 above sea level, about 15 miles off the John Muir Trail in a small unnamed lake deliberately kept out of the public eye.

Seldom visited by anglers, researchers selected it as a test case for a simple frog-recovery plan: They hauled out all the fish in nets, then waited to see what happened.

The fish were removed in 1997 when annual counts of the frog population began. The first count showed only 20 adult frogs and 20 tadpoles. In September, the field crew counted 120 adults and 1,300 tadpoles.

Similar results are being recorded in three other field tests.

"You remove the fish, and if there are frogs nearby, they move back and start repopulating by leaps and bounds," Knapp said. "Removing the fish turns out to be a very effective method of frog recovery."

It's not necessary - or practical, from either the budgetary or political point of view - to try to remove all the introduced trout from the Sierra. Just a few lakes would help, Knapp said, leaving plenty of fish around.

In Defense of Fish

Some anglers are suspicious.

"A balance needs to be struck of some sort," said Randy Van Vliet, 42, of Thousand Oaks, who has been fishing the Sierra since he was 13 years old. "What I am saying is the fish have a place also."

Officials at the California Fish and Game Department insist that nobody plans to eliminate fish. Instead, the issue is whether stocking needs to continue as it has outside the national parks, and whether fish may need to be taken out of some critical frog habitats.

Researchers are now looking at how an effective amphibian rescue might be attempted by selectively returning certain lakes to their original, fish-free state.

"There will certainly have to be a compromise," Knapp said. "Right now we're trying to figure out just what the frog needs."

An avid fly-fisherman, Knapp insists that he is not the "trout anti-Christ" some anglers suspect. He noted some studies show that once fish are established in a watershed, populations reach an equilibrium, so that further stocking merely adds short-lived competition for scarce nutrients.

As for removing the fish, Knapp maintains it can be done without stepping on too many toes.

"Anglers don't need to be too concerned that the Sierra Nevada will become a huge fishless preserve," he said. "We just need carefully planned fish removals from areas that could benefit the most."

AN ECOSYSTEM IN PERIL

High in the Sierra Nevada, the yellow-legged frog has been wiped out from nearly 80 percent of its historic range. The decline is rippling through the rarified mountain ecosystem, threatening populations of the Clark's nutcracker and, in turn, the health of whitebark pine forests. Now, after a decade of close scientific scrutiny, experts believe they have found the culprit in the yellow-legged frog's demise. It's the fish.

Rainbow Trout

Non-native trout, which have been stocked in Sierra lakes since the 1950s, are devastating yellow-legged frogs, which evolved without the protective tricks of other amphibian species.

Yellow-Legged Frog

Characteristics: Dark and rough-skinned, usually with dark speckles or mottling; yellow on underside of hind legs

Habitat: Mountain streams, lakes and ponds

Diet: Aquatic and terrestrial invertebrates, insects, tadpoles of other species

Yellow-legged frogs are a significant food source for the Clark's nutcracker.

Clark's Nutcracker

The birds aid in the reproduction of whitebark pine trees by scattering their seeds.

Sources: Audubon Western Birds Handbook; A Field Guide to Western Reptiles and Amphibians; The Audubon Society Field Guide to North American Trees; Chronicle research

E-mail Carl T. Hall at carlhall@sfchronicle.com