Why? For one thing, amphibians, more than any other group of terrestrial vertebrates, rely intimately on water. Their eggs dry out on land, and their larvae (such as tadpoles) are often aquatic. Across the planet, aquatic ecosystems are being destroyed and degraded -- by pollution, dams, river channelization, introduced predators and competitors, and other maladies -- at a horrific pace.
Beyond this, amphibians seem unusually vulnerable to exotic diseases. The water that they rely on so intimately is an excellent medium for transmitting pathogens, many of which will quickly die if they dry out.
Combine that trait with our hyper-mobile modern society -- where exotic organisms are suddenly being moved across the planet at lightening speed, often by accident -- and you have the makings of environmental Armageddon.
For example, a few decades ago a mysterious chytrid fungus suddenly appeared and swept like a tsunami across Australia, Latin America, and parts of Asia and Europe with deadly effect.
In total, some 200 species of frogs and salamanders have declined catastrophically or been driven to global extinction by the fungal disease (notably, ALERT director Bill Laurance and colleagues did some of the earliest work on the pathogen's impacts).
Now, there is a new disease threat. Exotic viruses have appeared that seem to be targeting a range of amphibian species. This is frightening, because viruses are usually specific to a particular species.
A virus that can attack several species at once could be catastrophic for amphibians. Normally, a virus dies out when it kills most individuals of its host species, because the host becomes so rare that the virus is no longer transmitted successfully.
But a multi-species virus is different. One of its host species might be largely wiped out, but the virus can still thrive in a different species. Because it persists in the environment, even when one of its hosts becomes perilously rare, such a virus can drive a species completely to extinction.
That is exactly what seems to be happening now in Europe. At least two new types of Ranaviruses are plaguing frogs and salamanders there, causing massive die-offs in several different species of hosts at once.
The viruses induce hemorrhaging in the frogs, creating open sores and killing their limb tissues.
Alarmingly, even reptiles might be affected; a snake that ate an infected salamander died soon afterward, apparently from the virus.
Such events suggest that one of the most damaging features of modern humanity might be our proclivity for moving exotic organisms all around the planet. Species have adapted to ecosystems in which a major new pathogen might come along once every few centuries or millennia. Now they're arriving at a pace that's blindingly faster than that.
Little wonder that the amphibians -- among the most ancient denizens of the Earth -- are having a hard time surviving the onslaught.