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Scientist Suspects Pesticides Play A Role In Declining Frog Population
(Beyond Pesticides, August 9, 2004) The Pittsburgh Tribune-Review reported on July 30, 2004 that University of Pittsburgh biology professor Rick Relyea, PhD, believes that early findings suggest EPA’s acceptable levels of pesticides might be far more deadly to amphibians -- and might pose a greater threat to human health -- than once thought. His research may add to the work of other scientists who are helping to unravel a mystery that has baffled scientists for years: Why are frogs around the world disappearing in record numbers?

According to the Tribune-Review, five years ago, on a scientific whim, Dr. Relyea and a colleague decided to find out what would happen if they exposed tadpoles to low doses of a common pesticide called carbaryl in the presence of a predator.

You can buy carbaryl in your local hardware store as the brand-name product Sevin. Ten to 15 million pounds of this nerve agent are used every year on farms and in homes across America to keep crops bug-free and fleas off the family dog. Carbaryl represents one of about 21,000 chemical pesticides in a multibillion-dollar industry.

Dr. Relyea explains that while carbayl is registered by EPA, most toxicology data are collected under artificial conditions: Researchers slowly add a chemical to a tank of critters and find out how much is needed to kill half of them over a period of several days. This is the standard way to test the potency of a substance, but the procedure might grossly underestimate the dangers of pesticides by failing to account for the real-world effects of predators and other natural stressors, Dr. Relyea tols the Tribune-Review.

At the time of Dr. Relyea's initial carbaryl experiment, ecologists knew that tadpoles use chemical cues to detect when predators are nearby and often reduce their activity or growth rate when they sense danger. But no one had ever studied what happens when tadpoles, already scared by the "scent" of a predator, are then treated with supposedly nonlethal amounts of pesticide.

The results were shocking. So shocking, in fact, that Dr. Relyea repeated the experiment before he believed what he saw: Lots and lots of dead tadpoles.

"I said to myself, 'Holy mackerel! These guys are all dead. That wasn't supposed to happen,'" said Dr. Relyea, who had expected his ill-fated tadpoles to stop growing and become sluggish, at worst. With further experimentation, Dr. Relyea found that low, so-called safe concentrations of carbaryl are up to 46 times more deadly to tadpoles when combined with another stressor, the presence of a hungry predator.

Then there's the enigma of the vanishing frogs. About a decade ago, scientists realized that amphibian populations around the world were in decline. The most important reason for this trend is habitat loss resulting from draining wetlands and harvesting timber. But frogs are disappearing in seemingly pristine areas, too, and no one is quite sure why.

Dr. Relyea's experiments suggest that low concentrations of pesticides at work in natural conditions could be a prime suspect, which in turn would be an indicator of more widespread water quality problems.

This hypothesis is supported by recent findings of declining amphibian populations upwind from agricultural lands in central California, Dr. Relyea said. Amphibians collected from this region have decreased amounts of an enzyme found in the nervous system, evidence that neurotoxins such as carbaryl are at work, he said.

Back on Pennsylvania, every trough scoured and tadpole hatched in this remote corner of northwest the state could bring Dr. Relyea one step closer to finding answers to these global questions. Along the way, the scientific pioneer is having the time of his life.

TAKE ACTION: The Bush Administration has recently weakened the regulations that protect threatened and endangered species from pesticides. See Daily News from August 3. Write to the Bush Administration at president@whitehouse.gov and Mike Leavitt, EPA administrator and tell then that you oppose the change made and support the checks and balances that the Endangered Species Act provides against the destruction of endangered plants and animals by harmful pesticides.