Wednesday, September 26, 2007

frogs, frogs and fewer frogs

Oh, the beleaguered frogs. You probably already know that amphibian species are declining around the world. You have probably seen the depressing photos of deformed frogs trying to get through life with too many (or too few) legs. You may even have seen Dr. Tyrone Hayes' breathtaking presentation on how the herbicide atrazine turns boy frogs in to hermaphrodite frogs.

This week the N & O ran a story about a new study that reinforces the theory that farm runoff is causing the deformed limbs. Excess nutrients in the water lead to lots more parasites in the water that turn normal tadpoles into sickly, deformed adult frogs.

One of the questions about this research is, how come the trematodes make frogs so sick? They're not a new pathogen - they've always been in the frogs' environments. It's just that lately the frogs can't seem to fight them off. Another stumper: if it's one disease deforming the frogs, why does it affect so many species? Leopard frogs, bullfrogs, wood frogs, and many others have shown up with the deformed limbs, in many different parts of the U.S. and Canada.

The answer may actually lie in the frogs' immune systems: one of Tyrone Hayes' experiments found that wild frogs who live in pristine waters are easily able to fight off common infections, while wild frogs who live in waters containing agricultural runoff die at astonishing rates from the same exposure to disease. Distinguished researchers around the world have pointed at all sorts of explanations for the frog decline, deformities and hermaphrodism: climate change, habitat destruction, parasites, pesticides, and more. The sad answer may be that there is no smoking gun, but that an alphabet soup of environmental changes have over-burdened the frogs' immune systems to the point of destruction. Parasites and infections that formerly posed little or no threat to amphibian populations become deadly.

Biologists like to call frogs a "sentinel species," because they are so sensitive to their environments and serve as indicators for problems that can grow to affect other species as well. I hope we're paying attention.

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