Pity the poor frogs: they're one of the most endangered group of vertebrates on the planet, and new research shows that two of the factors in their plight are common, everyday farm chemicals. The study shows that atrazine, a weedkiller that's widely used in agricultural areas, not only boosts the levels of parasitic flatworms in frog ponds, it also decreases tadpoles' ability to fight off infections. If that wasn't bad enough, previous research has found that runoff from phosphate fertilizers also boosts parasite levels. Taken together, researchers say, the weedkiller and the fertilizers are hitting frogs with a double whammy.

Amphibian populations around the world have been declining in recent decades, with many species on the brink of extinction. Infection with any of several species of tiny flatworms, known as trematodes, can trigger debilitating limb deformities in frogs. Severe infections can kill the amphibians. The question was why high rates of those deformities — and, presumably, trematode infections — began showing up across the nation in the mid-1990s [Science News].

The new findings suggest that the growing prevalence of the weedkiller atrazine in corn-growing regions since that time may be partly to blame for the woeful state of American amphibians. In the study, published in Nature [subscription required], researchers found that the weedkiller and the phosphate fertilizers both played a role in causing algae blooms in frog ponds, which feed the snails that serve as temporary hosts for the flatworms--so when snail populations go up, so do flatworm populations. Ecologist Pieter Johnson comments that

it's surprising that atrazine would stimulate algal growth. "It's an example of complex ecological interactions that might not be anticipated," he says. Why would an herbicide lead to more algae? Apparently, the kind of algae favored by the snails recovers faster than competing algae [ScienceNOW Daily News].

The new study also showed that frogs that are exposed to atrazine have suppressed immune systems, and therefore can't fight off the flatworms effectively. Atrazine's use is somewhat controversial:

The chemical was banned on fears of groundwater contamination by the European Union in 2004. But it is still commonly used in the US; in 2006, the country's Environmental Protection Agency concluded there was not enough evidence to suggest the chemical was harmful to humans."I doubt that further documentation of atrazine's harmful effects on frogs will prompt a ban in the US anytime soon," says Pieter Johnson [Chemistry World].

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