"Dear Prof. Meadows, please tell us about the frogs."That was the complete message that bounced in to me from cyberspace. I stared at it and sighed. I don't want to tell about the frogs. I don't even want to think about the frogs. The story is so complex and uncertain and sad.But, you know, there may be no more important story unfolding in the world today, important not only for the frogs, but also for us. So I guess I'd better try to tell about the frogs.Around 1990 herpetologists (people who study reptiles and amphibians) began noticing an alarming drop in frog and toad populations. Some of the decreases were sudden, some had been going on for decades. Some species seemed all right, others were decimated. There was no obvious geographic pattern; frogs were failing in wilderness and farmlands, in mountains and valleys, in just about every part of the world.In Yosemite National Park seven kinds of amphibians are declining and three are gone. In the cloud forest of Monteverde, Costa Rica, the beautiful golden toad has disappeared. According to "Froglog," published by the World Conservation Union, 25 percent of all known amphibians are endangered, vulnerable, or extinct. The Nature Conservancy says 38 percent of amphibians in the United States are endangered.The herpetologists are baffled.It's not that there is no plausible explanation, but that there are so many. Though they have thrived on earth for millions of years, frogs are especially vulnerable to environmental disruption. As tadpoles they live in water and eat plants; as adults they live on land and eat insects. Their eggs have no protective shells. Their skins are thin and wet and permeable. They are eaten by fish and birds and people. (The French eat 4000 tons of frog legs a year.)Each frog lives in a small, specialized habitat, but those habitats occur all over the planet. So frogs are incredible environmental sensors. Their decline should be a canary-in-coal-mine sort of wake-up call.Why are they declining? Habitat destruction is the most obvious cause. The U.S. has drained or filled half of its wetlands. Watery places that remain are likely to be polluted. Frog eggs, tadpoles, and adults are sensitive to many pollutants, especially acid rain and pesticides. Spraying for insects not only poisons frogs directly, it also wipes out their food supply. Some frogs are outcompeted by foreign species introduced by humans. Clearcutting opens shady habitat to the sun. Furthermore, the depleted ozone layer lets in ultraviolet light that damages the DNA in frog eggs and prevents them from hatching.Fungal or bacterial infections have been blamed for frog declines, but the infectious agents have always been around, so some scientists think recent epidemics may be a symptom of immune system weakening from other causes. Others think frog diseases could be spreading from aquaculture ponds.In short, they don't know. It could be that all the above causes are damaging frogs, which is why the pattern is so hard to figure out.It is also unclear how frog disappearances are related to frog deformities. In 1995 schoolchildren in Henderson, Minnesota, were catching frogs in a farm pond. They noticed that most of the frogs had extra legs, missing legs, misplaced eyes. The kids publicized their finding, along with pictures of gross-looking frogs. That caused other kids in Vermont and elsewhere to go out and find malformed frogs, lots of them.There seems to be dawning clarity about the malformations. A Canadian researcher has just reported that along the St. Lawrence valley fewer than two percent of frogs in ponds far from any pesticide use have deformities. Frogs from ponds near heavy pesticide use show at least 20 percent defects, and in one pond 100 percent.Researchers in Switzerland have exposed developing frog eggs to the pesticide triphenyltin and produced deformities similar to some seen in the field.The Scripps Research Institute has shown that S-methoprene, a popular ingredient in mosquito sprays and flea powders, breaks down into retinoids, which are known to cause birth defects not only in frogs but in humans. It is logical that this insecticide messes up the development of embryos; the way it kills insects is that it interrupts their pupation.Australia has banned use of the weedkiller Roundup (glyphosate) around ponds and streams because of its effect on tadpoles and frogs. The problem seems to be not the glyphosate itself, but an "inert ingredient" added to make the herbicide stick better to leaves.What does seem clear amidst the uncertainty is that when we humans spray poisons and send waste out through pipes and bulldoze land around, we push into the homes and food supplies of other creatures.I could point out how scary that is, because what poisons other creatures can also poison us.I could point out how stupid that is, because those creatures are economically valuable. Frogs eat insects, toads eat slugs in my garden, amphibians feed fish and birds and mammals, they are the source of some remarkable chemicals, including painkillers and antibiotics.But what I mainly feel is how sad that is, the thought of not having these humble, damp, jumpy animals as companions in the world. Not having bullfrogs for my dog to scare up and send plopping into the pond. Not looking forward through the long New England winter to the sound of the peepers on the first warm night of spring.That's why I'd rather not tell about, not think about the frogs. But thanks for asking.(Donella H. Meadows is an adjunct professor of environmental studies at Dartmouth College.)