Frogs Fall Silent
A decline in frogs and toads
is a vexing issue that may signal greater ecological downturn. One scientist
studies possible ties to pesticides.
by Mark Grossi
The Fresno Bee, June 24, 2001
YOSEMITE NATIONAL PARK -- Sitting around a muddy bog at 9,000 feet where mosquitoes
hunt their evening blood meal, Gary Fellers just might be in a biologist's heaven.
He barely notices a startling, iridescent sunset that gives way slowly to a
darkened panorama, outlining Yosemite's granite in soft starlight. He's not
looking -- he's listening.
"That's a squirrel," says the federal scientist, identifying the nighttime
call. "That's a thrush. I love it when the sun goes down. But I was hoping
to hear frogs by now."
He clicks on an audio tape of frogs that he recorded at Lake Tahoe. A motivational
tape, he calls it. He's trying to coax a Pacific treefrog into a breeding song
so he can capture the creature for study.
Fellers would eventually collect specimens at other ponds on this June night,
but not here, where he has found numerous amphibians before.
His bad luck on this research safari is part of a noticeable trend since 1991,
when he began studying frogs and toads at Lassen Volcanic, Yosemite and Sequoia
national parks where there should be plenty of these animals.
There's no doubt he is seeing a decline in frogs and toads in the Sierra, says
Fellers, a herpetologist in the Biological Resources Division of the U.S. Geological
Survey at Point Reyes National Seashore. What's more, he believes he knows one
of the possible causes -- pesticides from farming in the San Joaquin Valley.
Fellers and two other scientists will drop a bombshell next month on Valley
agriculture when their study tentatively linking pesticides and the amphibian
decline is published in an international journal, Environmental Toxicology and
"We need to alert the public that there is a major decline going on,"
The Sierra decline is part of a vexing worldwide die-off in amphibian populations,
and it is a wake-up call, scientists say. Many experts say the loss of amphibians
may signal a global downturn in environmental conditions caused by humans.
The problems, which include atmospheric warming and ultraviolet radiation from
the thinning ozone layer, threaten to turn on humans, causing oceans to rise,
disease to increase and life expectancy to plummet. Indeed, the frog may be
the "canary in the coal mine," some scientists believe.
The eventual question in California: How much pesticide damage is being caused
to humans in the Valley if amphibians 90 miles away in the Sierra are being
Right now, Fellers and his cohorts are trying to collect evidence to demonstrate
the link between pesticides and the amphibian decline in the Sierra.
The scientists already have found residues of common agricultural pesticides,
so-called organophosphates, known as chlorpyrifos and diazinon.
According to the California Department of Pesticide Regulation, farmers statewide
applied a combined total of more than 1,500 tons of the two chemicals in 1999.
Farmers in Fresno County, the No. 1 county for pesticide use in California,
used a total of 235 tons of the two chemicals that year.
How did the pesticides get from Valley fields to high Sierra ponds? Previous
studies show the prevailing winds in the Valley carry these chemicals into the
It is a critical time to build on the previous research. The foothill red-legged
frog is listed as threatened under the U.S. Endangered Species Act. The Center
for Biological Diversity last month filed a federal lawsuit to force protection
for two high Sierra amphibians, the Yosemite toad and the mountain yellow-legged
The Pacific treefrog is not dwindling as fast as the other two high Sierra amphibians
are, but its population is dropping, too.
"Some important pieces of the amphibian puzzle are coming in," says
ecologist Carlos Davidson, a California State University, Sacramento, professor
who co-wrote a study that tentatively connects farming areas to foothill areas
where red-legged frogs have declined. "People have been trying to find
answers for years, and we're still working on it."
Back in the mountains, Gary Fellers works on the puzzle, wearing hip waders,
peering through binoculars at a scummy pond near Mono Lake and counting amphibians.
His dip net yields a yellow-legged frog tadpole that appears to be the victim
of hungry dragonfly larvae.
"It's a second-year tadpole," Fellers says, explaining that yellow-legged
frog tadpoles take two or three years to evolve into a frog. "I counted
500 frogs here a few years ago. Last year, I found two. So far, we've found
just this tadpole. It's not very healthy."
The tadpole appears to be suffering from chytrid fungus, a disease that began
spreading in the mid-1990s after it appeared in Central America and Australia.
The fungus and viruses are among many suspects in the worldwide die-off of amphibians.
Ultraviolet radiation, acid rain, global warming and non-native fish that eat
tadpoles also figure into the equation. The factors could be working in concert
to cause the declines.
"It's a hard thing to nail down," Fellers says. "If your grandmother
falls, breaks her hip and dies after contracting pneumonia while she's bedridden,
what killed her?"
Whatever the amphibian killers are, they appear to be effective. Mountain yellow-legged
frogs can lay hundreds of eggs, but he finds only a few more tadpoles in the
small drainage near Mono Lake.
He later pulls off the hip waders and dunks them in bleach to kill off any of
the chytrid fungus on the boots so he won't infect the next pond.
Fellers is counting only adult frogs, toads and tadpoles today. The night before,
near 9,945-foot Tioga Pass in Yosemite, he captured 10 adult amphibians and
froze them in liquid nitrogen at 84 degrees below zero for shipping to testing
Should he be removing any amphibians if the populations are dwindling? Fellers
says the losses will not make a big dent and, in any case, they are acceptable
in pursuit of a problem affecting frogs and toads in a 400-mile-long mountain
Though the amphibians are scattered throughout the range, frogs and toads often
can be found in the same ponds because they seem to co-exist peacefully, he
says. But a male's spring singing sometimes will become lower and menacing when
another male moves too close to his territory, which is an important resource
for attracting mates.
The male frogs sometimes will butt heads and kick with their hind legs over
the territory. Fellers, a California native who earned his doctorate at the
University of Maryland, is fascinated with such territorialism in nature.
"Nobody really gets hurt in frog confrontations, compared to ants,"
Fellers says. "One group of ants will raid another's nest for eggs, and
they fight to the death. You might see an ant carrying the head of another ant.
When I used to hear the rustle of leaves in the forest on the East Coast and
there was no wind, I would check it and find an ant war going on."
A political turf war with Valley agriculture over pesticides would seem inevitable
if the research results continue in the same direction. Pesticides keep the
multibillion-dollar farm industry productive, agricultural officials say. And
the pesticides are regulated and monitored to protect human
Earlier this year, trout fishing interests raised a political fuss when Kings
Canyon National Park announced an experiment to remove trout from some isolated,
backcountry lakes to protect amphibians. The trout are not native in the high
The confrontation with agriculture has been coming since the early 1990s, when
studies established that chlorpyrifos, malathion, diazinon and other pesticides
were in water samples and precipitation from Lake Tahoe Basin and Sequoia National
In 1996, Fellers and zoologist Charles Drost published research revealing a
collapse in amphibian populations that had been thriving in the early part of
The frogs, toads and tadpoles are particularly susceptible to chemicals in the
air and water because they respire through their skin when underwater and through
lungs when out of water.
Now, Fellers and fellow federal biologist Donald Sparling are not only counting
amphibians but also examining their bodies -- specifically, levels of cholinesterase,
an enzyme vital to an animal's nervous system.
Chlorpyrifos and diazinon are designed to repress the enzyme in insects, which
causes many problems, including respiratory failure.
In other words, the chemicals appear to do the same thing to frogs, toads and
tadpoles as they do to any unwanted pests in cotton fields southwest of Yosemite
in Fresno County.
Fellers and Sparling found diazinon in more than 50% of the Pacific treefrogs
at Yosemite sites. At coastal sites, no diazinon was reported, and only 9% of
the frogs had chlorpyrifos residue. It is strong evidence that pesticide use
in such places as Fresno County and other Valley locations is reaching amphibians.
But farmers defend their use of pesticides, saying they are already making changes
and cutting back.
Chlorpyrifos usage dropped 20% from 482,300 pounds in 1996 to 386,336 pounds
in 1999. The amount of diazinon applied to fields was cut almost in half from
165,576 pounds in 1996 to 86,272 pounds in 1999.
"We're not trying to pollute the environment," says Paul Betancourt,
president of the Fresno County Farm Bureau. "We're pressing technology
for better answers. We have the strictest rules on the planet, and we're among
the most innovative farmers in the state."
Betancourt, who farms cotton, tomatoes and almonds on 765 acres near Kerman,
says he does not spray heavily and even skips applications. He is experimenting
with cotton that is genetically engineered to resist infestations. He also anticipates
using new chemistry to kill pests and spare beneficial insects such as ladybugs
But Betancourt often cannot produce as much as does a competitor who uses more
pesticides. He also is taking a huge risk if he decides not to use a pesticide
and he is wrong. His crop could be decimated by an infestation.
"If there's an alternative and it works financially, we'll be among the
first to try it," Betancourt says. "You can understand both sides
of this issue. But if you have a family business on the line, like many in farming,
people can get defensive when you get on this subject."
Deep questions occupy Fellers' thoughts at pond "Y2016," an unnamed
body of water at the foot of Mount Dana where he is counting amphibians. He
holds an elegant, energetic Pacific treefrog as he considers.
Could it just be a population fluctuation that he's seeing in the Sierra? Isn't
it possible that these amphibians have been coming and going like this for the
past million years or so?
"I haven't sat down and gone through the exact numbers," Fellers says.
"Some people ask if they've declined by 40% or 80%. We haven't put those
numbers together yet, but it is a major decline. This is no fluctuation we're
He is just as quick to say there is no "smoking gun." He has doubts
about the popular wisdom that trout, which eat a lot of tadpoles, are among
the big causes of the amphibian declines.
No matter what the ultimate causes are, Fellers buys the argument that human
intervention is having an impact on amphibians. He says people should be interested
if only because a widespread amphibian die-off may be a prelude to serious environmental
problems that will prey on humans.
But Fellers has an emotional attachment as well.
"Sometimes, people ask why we should care whether or not a bunch of frogs
die off," he says, releasing the brilliant green treefrog and watching
it swim away. "All you need to do is spend a day with frogs in a mountain