When frogs croak

Mountain yellow-legged frogs are disappearing.

Mountain yellow-legged frogs are disappearing.

Photo By Vance Vredenburg

A few decades ago at Tioga Pass, mountain yellow-legged frogs were so numerous that researchers reported, “it was difficult to walk without stepping on them.” Nowadays, the frogs in this and many other former habitats are gone.

Historically the most abundant frog in the high-elevation lakes and streams of the Sierra Nevada from Plumas to Tulare counties, mountain yellow-legged frog populations are overall down to less than 10 percent of their historical numbers.

One of the largest known populations, consisting of 2000 adults in 1996, was found to contain only two frogs in 1999. Remaining populations are “widely scattered and consist of a few breeding adults,” according to a court petition filed by the Center for Biological Diversity and the Pacific Rivers Council.

These two environmental groups are asking the U.S. Fish and Wildlife Service to list the Sierra Nevada mountain yellow-legged frog (Rana muscosa) and its habitat under the Endangered Species Act. These groups seek the same protection for the Yosemite toad (Bufo canorus), which shares a similar high mountain habitat.

“These particular amphibians are an indicator of the overall health of the environment,” said Mike Sherwood, an attorney for Earthjustice.

In other words, as the frogs go, so could go a full range of wildlife in the Sierra.

Exceptionally sensitive to environmental changes, these frogs and toads have porous skin that readily absorbs chemicals in the air or water. Sherwood said that endangered species listing “would release federal funds to study the situation and what can be done to prevent them from going extinct.”

A court order, resulting from a lawsuit filed by Earthjustice on behalf of the two environmental groups, requires U.S. Fish and Wildlife to decide on protected species listing by November for the Yosemite toad and January 2003 for the mountain yellow-legged frog, after missing the previous deadlines last March.

“That’s why the suit was filed,” said Jeff Miller from the Center for Biological Diversity, pointing to a backlog of threatened species on a waiting list to determine their status. “The agency is vastly underfunded. A few species have gone extinct while on this list.”

Deadlines were missed, said Peter Epanchin, a biologist with the U.S. Fish and Wildlife Service, because the “money was not allocated to work on either of these species. We’re often struggling with a limited amount of money and much of our work is directed by the courts. Anything without a court-ordered deadline is often put on the back burner.”

Ranging from a little over an inch up to 3 inches long, the yellow-legged frog can be colorful, mixing brown, yellow, gray, red and green-brown in few or numerous spots. Adults are usually found sitting on rocks along the shore of a lake or stream, while tadpoles seek warmer shallow water and refuge from fish that prey upon them.

Slow to mature, yellow-legged frogs remain tadpoles for as long as two to four years, requiring lakes and ponds that are at least 6 feet deep, that neither dry out during summer nor freeze through in winter, when both frogs and tadpoles hibernate under a layer of ice.

During the brief warm season, frogs must reproduce and consume enough insects to sustain them through a long hibernation.

Tadpoles and frogs are susceptible to a bacterium producing “red-leg disease” and the chytrid fungus, both of which can decimate populations. In addition, a multitude of predators feed upon them: garter snakes, blackbirds and nutcrackers, coyotes, black bears and particularly non-native trout.

Since predation rates are high and survival rates low, a population of even 20 adults is considered at risk of extinction. A U.S. Forest Service survey at White Cliff Lake in Toiyabe National Forest found “thousands of frogs hopping around” in 1990. In 1999, only three tadpoles and one young adult were found. Dozens of recent surveys document similar findings.

Deanna Spooner of the Pacific Rivers Council attributes the frog’s decline to “habitat alteration” from introduced non-native trout and chemical pollution.

“Historically, the High Sierra lakes were fishless,” said Epanchin. Fish stocking of these lakes began in the late 1800s and has been ongoing, other than in national parks, which quit the practice in the 1970s.

Spooner, a fly fisher who has “fished all up and down the Sierra Nevada,” said it is not a fish versus frogs issue. A solution is simply to “set aside certain lakes for the frogs.” Constant fish stocking every summer sets up an artificial environment where fish may not be able to survive, reproduce and sustain it population naturally over time.

“Creating a more natural system would not only help the frog, but also create better fishing.” Introduced fish also impact birds. Since “fish eat all the insects, some of the bird species prey on the frogs. You have to look at the whole system with regard to stocking fish,” she said.

Fish are not the only cause of waning frog populations. Formerly numerous in parts of Yosemite National Park in the mid-1970s, long after the introduction of non-native fish, the species has also declined in areas isolated from fish, such as meadow pools in Tuolumne Meadows.

Additional stressors on frog populations are increased ultraviolet (UV) radiation from the thinning ozone layer, and chemical pollution from pesticides and herbicides that drift from the Central Valley to the high ranges of the Sierra where residues have been found even in freshly fallen snow at 7000 feet.

Pesticide residue in water, sediment and aquatic vegetation can disrupt the frog’s endocrine system, delaying development or reducing breeding or feeding activity. This can be catastrophic, considering that frogs must breed and consume sufficient food for their hibernation.

Often insoluble in water, pesticides tend to concentrate on the surface. Frogs spend much of their time in this interface, moving from terrestrial to aquatic habitat and absorbing these chemicals through their skin.

“Water contamination, climate change and UV” are possibly contributing to weakened immune systems in these amphibians, said Epanchin, causing diseases such as the chytrid fungus and viruses to become more prevalent.

Livestock can also decimate the frog’s habitat, increasing erosion, widening stream channels and decreasing depth, in addition to polluting water with excessive nitrogen, leading to heightened bacteria levels.

Factors affecting frog populations hold true for the Yosemite toad, which also has experienced recent precipitous declines.

Similar in size to the frog (from 1.75 to 3 inches long) with smooth skin, the toad is native to high Sierra elevations. Females tend to be dark colored with irregular white-bordered blotches and larger than males, which are speckled with black spots on a yellow to olive-green background.

Preferring wet meadowland and lakeshores, generally within 100 yards of permanent water, toads live 15 years or more and must survive three to six years to become sexually mature. From October until May or June, they hibernate under the snow, in rodent burrows or rock crevices.

As tadpoles, they’re prey for dragonfly nymphs, birds and trout and are susceptible to red-leg disease and chytrid fungus. Grazing cattle can trample adults, tadpoles and their habitat.

Disturbed by the rapid amphibian decline and its ramifications for humans, Miller said the dramatic changes point to “ecosystem-level problems. Amphibians are the warning light on our dashboard because they inhabit both aquatic and terrestrial habitats. It indicates we’re having major problems with air and water quality.”

The decimation of these amphibians is “an indication that the ecosystem is in trouble,” Sherwood said. “In our own enlightened self-interest, we should be concerned.”