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From NPR News, this is All Things Considered. I'm Michele Norris.


And I'm Robert Siegel. All over the world, frog populations are declining because of diseases and the destruction of wetlands. Well, a new study suggests another reason, a cascade of environmental changes. As NPR's Dan Charles explains, these changes were set off by farmers who spray crops with a weed killer called atrazine.

DAN CHARLES: Farmers have been using atrazine for 50 years. It's cheap and not very toxic to humans, but it's controversial because it persists in the environment for a long time. Several years ago, scientists at the University of California, Berkeley reported that incredibly small amounts of atrazine kept frogs from reproducing. The Environmental Protection Agency commissioned two big studies. But David Skelly, an ecologist at Yale University, said those studies didn't show the same damage.

Dr. DAVID SKELLY (School of Forestry & Environmental Studies, Yale University): And so far, the lack of someone else being able to reproduce these results has left this in limbo.

CHARLES: So, farmers continue to spray tens of millions of pounds of atrazine on fields, mostly corn fields, every year. But now, another group of scientists is saying atrazine may hurt frogs in a different way. There are little flatworms called larval trematodes that infect frogs. The scientists studied 18 ponds and wetlands in Minnesota trying to find out why some ponds had lots of these worms and others didn't. Jason Rohr from the University of South Florida says the one thing that seemed to explain the difference best was the level of atrazine.

Dr. JASON ROHR (Biology, University or South Florida): The more atrazine that we had in ponds, the greater the abundance of the larval trematodes.

CHARLES: But why would atrazine residues lead to more flatworms? Rohr designed another experiment.

Dr. ROHR: We ran a study in 300-gallon tanks - outdoor tanks that had members of the aquatic community, typical aquatic community, including snails and amphibians.

CHARLES: And some of the tanks were dosed with a bit of atrazine. It turned out the atrazine set off an ecological chain reaction. It killed off algae floating in the water. But algae attached to the bottom of the tank survived. In fact, those algae prospered because they were getting more sunlight.

Dr. ROHR: So, there was more food for the snails, and, as a result, there were many more snails in those tanks.

CHARLES: Four times more snails, in fact. And more snails mean sicker frogs because snails carry those flatworm parasites. Rohr and his colleagues also noticed something else. In the tanks with atrazine, the number of frogs infected with parasites jumped even more drastically than the scientists expected based on the increase in snail population. It's a hint, Rohr says, that atrazine may also damage the frogs' immune system so they can't fight off the parasites. Experts on amphibians like Andrew Blaustein at Oregon State University say atrazine and flatworms probably are not the biggest reasons for the decline of frogs.

Dr. ANDREW BLAUSTEIN (Zoology, Oregon State University): I don't think it's as big a deal as habitat destruction or some of the main diseases or even climate change, but it can do some damage.

CHARLES: And David Skelly, the Yale ecologist, says government regulators don't usually look for this sort of damage resulting from indirect ecological effects.

Mr. SKELLY: They just haven't been part of the paradigm for how we decide what the effects of pesticides are going to be on the environment.

CHARLES: The EPA got chemical companies to spend millions testing for direct toxic effects of atrazine. But it never demanded studies of the herbicide's effects on the food chain that links frogs with algae, snails, and flatworm parasites. Dan Charles, NPR News, Washington.

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