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 cientists have confirmed that agricultural contaminants
may be an important factor in amphibian declines in California.
According to an article recently accepted by the journal Environmental
Toxicology and Chemistry, a study by scientists of the US
Geological Survey and US Department of Agriculture indicates
that organophosphorus pesticides from agricultural areas, which
are transported to the Sierra Nevada on prevailing summer winds,
may be affecting populations of amphibians that breed in mountain
ponds and streams.
 Dramatic
population declines in red-legged frogs, foothills yellow-legged
frogs, mountain yellow-legged frogs and Yosemite toads have occurred
in California over the last 10-15 years, but no single cause
for these declines has been positively identified. Scientists
and managers have been especially concerned because many of these
declines occurred in some of the state's most seemingly pristine
areas. Declines have been particularly drastic in the Sierra
Nevada, which lie east of the intensely agricultural San Joaquin
Valley. The red-legged frog is listed as threatened under the
US Endangered Species Act, and the mountain yellow-legged frog
and Yosemite toad have been proposed for listing.
"While
crucial to the agriculture industry, pesticides by their very
nature can result in serious harm to wildlife both by directly
killing animals and through more subtle effects on reproduction,
development and behavior," said Dr. Donald Sparling, a research
biologist and contaminants specialist at the USGS Patuxent Wildlife
Research Center in Maryland. "Unfortunately, now there appears
to be a close correlation between declining populations of amphibians
in the Sierra Nevada and exposure to agricultural pesticides."
The scientists
found proof that pesticides are being absorbed by frogs in both
aquatic and terrestrial systems and are suppressing an enzyme
called cholinesterase, which is essential for the proper functioning
of the nervous system. Modern-day pesticides function by binding
with this enzyme in animals and disrupting nervous system activity,
usually causing death by respiratory failure. Decreased cholinesterase
activity can indicate exposure to certain commonly used pesticides
and can be harmful to animals.
The scientists
collected 170 tadpole and 117 adult Pacific tree frogs, a species
that still is fairly abundant in the Sierra Nevada, from a total
of 23 sites in six locations including coastal, foothill, Lake
Tahoe Basin, Yosemite and Sequoia national parks; adult frogs
were also collected from Lassen National Park. They found that
cholinesterase activity levels in tadpoles were significantly
lower in the mountains east of the San Joaquin Valley compared
with similar sites farther north and east of the Sacramento Valley
where agricultural activity is less intense. Moreover, cholinesterase
activity became decreasingly lower in tadpoles from both northern
and southern sample sites as the sites moved in a gradient from
the coast to the higher elevations. Thus, tadpole populations
in the mountains had lower cholinesterase values than those along
the coast. Similar but less significant trends were seen in adult
frogs.
The researchers
also measured concentrations of particular pesticides in the
bodies of tadpoles and adults. More than 50 percent of the adult
frogs and tadpoles at Yosemite National Park had measurable levels
of chlorpyrifos or diazinon, compared to only 9 percent at the
coastal reference sites. Frogs at Yosemite National Park also
had a higher frequency of detection for chlorpyrifos than those
on the coast. Both diazinon and chlorpyrifos degrade very rapidly
in organisms, and the detection of either compound indicates
recent exposure to the chemicals.
Of the pesticides
tested in related lab studies, chlorpyrifos and diazinon, commonly
used organophosphates, suppressed nervous system activity and,
along with endosulfan, a frequently used organochlorine pesticide,
proved the most highly toxic to frogs. Diazinon has recently
been targeted for a three-year phaseout by the US Environmental
Protection Agency [see story on page 13].
"The
presence of pesticides and the decrease in cholinesterase activity
in Pacific tree frogs suggest that other species, which are more
closely associated with water, could be even more affected, said
Dr. Gary Fellers, a research biologist and amphibian specialist
at the USGS Western Ecological Research Center in California.
"Mountain yellow-legged frogs, for example, spend two or
three years as tadpoles before they metamorphose and then spend
considerable time in the water as adults. Melting of pesticide-contaminated
snow could provide a pulse of toxic chemicals at a critical time
in the life history of these frogs."
As the nation's
largest water, earth and biological science and civilian mapping
agency, the USGS works in cooperation with more than 2,000 organizations
across the country to provide reliable, impartial, scientific
information to resource managers, planners, and other customers.
This information is gathered in every state by USGS scientists
to minimize the loss of life and property from natural disasters,
contribute to the sound conservation, economic and physical development
of the nation's natural resources, and enhance the quality of
life by monitoring water, biological, energy and mineral resources.
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