Frogs, toads and other amphibians are among the world's most widely distributed species, thriving on every continent except Antarctica.

Or, at least, they were thriving: Something is happening to salamanders, newts, frogs and toads, and it has scientists and environmental advocates very concerned: Amphibians are disappearing all over the world. Why?

Amphibians: Out of the Primordial Seas

Amphibians are one of the most ancient classes of animals alive today, evolving almost 400 million years ago.

Unlike fish, the earliest amphibians had limbs that allowed them to leave water and move across dry land, and lungs that were well-adapted to inhaling air.

Most amphibians require fresh water at some point in their lives, usually for reproduction and egg-laying. (The term "amphibious" generally refers to anything that's able to function on both water and dry land.)

There are 3 orders of amphibians: Anura, the frogs and toads; Caudata, the salamanders and newts; and Gymnophiona, the caecilians (worm-like animals common in the tropics).

Amphibians have been a wildly successful and highly diversified group, with roughly 7,000 different species. They range from the 6-foot Chinese giant salamander to a frog from Papua New Guinea so tiny you could fit 10 of them on a U.S. quarter.

Without teeth or claws, amphibians might be defenseless but for one thing: their skin. An amazing adaptation on many levels, the skin of many amphibians is distasteful or downright poisonous, and thus offensive to predators.

Their permeable skin can also function as a breathing apparatus, allowing some amphibians to hibernate underwater through the winter months.

Endangered Frogs and the Disappearing Amphibians

During the 1980s, people around the world started reporting something unusual: Frogs were disappearing. Croaking toads, bellowing bullfrogs and spring peepers -- the tiny North American frogs whose chorus of peeping heralds the end of winter -- were in decline or completely gone from places where they once were common.

About one-third of all amphibian species are currently on the International Union for Conservation of Nature's (IUCN)"Red List" of declining or endangered species. Many of those amphibians are now feared extinct.

The latest news does nothing to reassure conservationists: A 2013 study from the journal PLoS One found that U.S. populations of salamanders, toads and frogs are declining at an average rate of 3.7 percent each year.

At that rate, amphibians will disappear in 20 years from about half of the places they now live.

And among the one-third of amphibians that are on the IUCN Red List, the rate of amphibian die-off is even faster, 11.6 percent each year. In just 6 years, those amphibians will be completely gone from half their habitats.

"They just disappear," said ecologist Michael J. Adams, as quoted by ABC News. "Populations are going away."

Adding to the alarm is the fact that many of these declines in amphibian populations are occurring in national parks, places that are supposed to function as animal sanctuaries.

"Even in what we consider pristine areas, we are seeing amphibian decline," biologist Erin Muths, who helped conduct the study, told the Denver Post. "If anything is doing poorly in an area we think is protected, that says something about our level of protection and about what may be happening outside those areas."

Why Are Amphibians Disappearing?

Nobody knows for certain why frogs, salamanders, newts and other amphibians are disappearing, but there are a few obvious candidates.

Habitat destruction, including the development of wetlands and waterfronts on which amphibians depend, is perhaps the single biggest threat to amphibian populations.

Climate change and a disease-causing fungus that's spreading among amphibians are also partly blamed for the decline. But, especially in protected parklands, these can't explain all disappearing frogs and toads.

Some experts are pointing to the unique role that amphibian skin might play in the destruction of the animals. Since their skin is very permeable, they are especially sensitive to water pollution, including endocrine disruptors.

Why Amphibian Disappearance is Important

Biologists are concerned about the disappearance of amphibians for several important reasons.

Amphibians play a critical role in the control of populations of insects, including mosquitoes and other water-borne insects. They're also an essential source of food for birds, snakes, fish and other animals.

"Amphibians do a lot of the important jobs," according to a statement on the PLoS One study from the U.S Geological Survey. "They control pests, provide medicines, feed other animals, and help make ecosystems work. They are extraordinary, diverse, beautiful animals. They have social value that inspires art and culture, and they are an ancient source of biodiversity. We have fossil records of amphibians going back some 350 million years."

Because of their important place in the ecosystem, and their land-water lifestyle, amphibians are often referred to as "indicator species," i.e., species whose health is indicative of the health of the ecosystem as a whole.

"Amphibians are going, but a lot of other species are going, too. Snakes are declining. Mammals are declining. We're seeing bird declines. Amphibians are probably declining at a faster rate than other groups, and they may be a little more sensitive," USGS zoologist Stephen Corn, one of the authors of the study, told the Denver Post.

Amphibians "are a good example of the collapse of the world's ecosystems that we seem to be seeing right now," he said. "We're seeing a lot of species in a lot of places declining at the same time."