Why the Frogs Are Dying

Draped like a verdant shawl over Costa Rica's Tilarán Mountains, the Monteverde cloud forest has long been a nature lover's idyll. Hidden birds flirt to the whisper of rushing streams and epiphytes tumble from the mist, while delicate flowers bloom impossibly from the jungle's maw. With luck you might even catch the iridescent flash of the resplendent quetzal, the elegant symbol of the Central American rain forest.

There's one member of this pageant that won't be turning up, however: the Monteverde harlequin frog. Named after its palette of yellow, red and black, this miniature amphibian--a member of the genus Atelopus --had thrived in these Costa Rican mountains for perhaps a million years. Yet the last time J. Alan Pounds, an ecologist who has studied the cloud forest's wildlife for 25 years, spotted one in Monteverde was in 1988. Its cousin, the golden toad, went missing about the same time. Indeed, the more scientists search, the grimmer the situation looks. A study by 75 scientists published earlier this year in the journal Nature estimated that two thirds of the 110 known species of harlequins throughout Central and South America have vanished. And that may be just the beginning.

The loss of a species is sad enough, not least a jewel like the harlequin, which one researcher described as a tropical Easter egg. What has puzzled scientists is why. For millennia, this denizen of tropical America survived by adapting to whatever changes nature threw its way. Suckers lining the underbelly of tadpoles allow them to cling to rocks without being flushed downstream. The adult's carnival-like costume warns potential predators to stand clear or risk a deadly dose of tetrodotoxin. But apparently there's one peril the harlequin couldn't trump: climate change.

Monteverde gets its lifeblood from the trade winds, which blow moisture uphill where the air cools and condenses into clouds. An ark of plants, insects and animals flourishes in the cool misty mountains. Gradually, though, a warming trend has raised nighttime temperatures and increased cloud cover, which makes for cooler days by blocking solar radiation. The subtle change, which might go unnoticed by us bipeds, is thought to have been ideal for chytridomycosis, a disease caused by a waterborne fungus that has flared up throughout tropical Central and South America. Scientists believe the chytrid disease kills the frogs by blocking their natural ability to absorb water through their porous skin (and perhaps also by releasing a toxin), essentially causing them to die of dehydration. What really frightens researchers, however, is the potential implications of the die-off. "There's basically a mass extinction in the making," says Pounds. "I think amphibians are just the first wave."

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For years now, eminent researchers have been warning of a gathering climate disaster. The findings at Monteverde, and scores of other research stations around the globe, have shaken people's complacency. This was not just another computer model spitting out mathematical warnings but a whole living genus on the brink. Alarmed at the portents, a network of conservationists is trying to evacuate the remaining harlequin frogs to fungus-free zones and frog farms. But such heroics may be futile. Scientists monitoring wildlife around the world are echoing Pounds's research. Their conclusion: many more species will perish.

A global temperature rise of a mere 0.6 degrees Celsius over the last century has sent shock waves through the animal kingdom. From the desiccating rain forests of Australia to the thawing Arctic, the warmer weather is expelling animals from age-old homelands, scrambling mating and nesting habits, and putting competitors on a prickly collision course. As habitable spaces get smaller, competition for food grows fierce. Meanwhile, insects and pests, which flourish in the heat, abound. So may the diseases they carry, like dengue fever, avian pox or cholera. Scholars are asking whether the loss of individual species could have a knock-on effect all through the food chain. "We are seeing problems from pole to pole; we see them in the oceans and we see them on land," says Lara Hansen, chief climate-change scientist at the World Wildlife Fund. "There are very few systems that I can think of that are untouched by climate change."

Not all the science points to disaster. Some species can adapt to the changing climate. But to what extent? "Climate change is happening a lot faster than the process of evolution can," says biologist Camille Parmesan, at the University of Texas. "The fact that species are going extinct is telling you that they didn't adapt."

Still others parry that the havoc credited to climate change owes more to deforestation or diseases spread by humans. Yet to many experts, that misses the point. "We already know that all kinds of diseases respond to climate conditions. We also know that the interaction of species, especially predators and parasites, can also complicate the equation--which is something the computer climate models don't take into account," says Pounds. "That makes the impact of climate change difficult to predict, but probably even more severe than you'd imagine."

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The trouble at Monteverde only heightened a mystery that had scientists stumped for years: why do whole species of wildlife disappear in apparently pristine parks and nature preserves? There had been no shortage of theories to explain the demise of the harlequins, from acid rain to an overdose of ultraviolet rays. By the late nineties, attention shifted to the chytrid fungus outbreaks, which many amphibian experts concluded were the smoking gun. But Pounds wasn't satisfied. After all, it wasn't just harlequins, but all kinds of amphibians that were dying. And if the chytrid disease was killing the frogs, what was behind the deadly outbreak?

In time, Pounds learned that the fungus flourished in the wet season and turned lethal in warm (17 to 25 degrees Celsius) weather--exactly the conditions that climate change was bringing to the cloud forest. More important, he found that 80 percent of the extinctions followed unusually warm years. "The disease was the bullet killing the frogs, but climate was pulling the trigger," says Pounds. "Alter the climate and you alter the disease dynamic."

In a broad survey of scientific literature, Parmesan and Wesleyan University economist Gary Yohe recently concluded that hundreds of animals and plants had responded to climate change by jumping their biological clocks. Yellow-bellied marmots stir from hibernation 23 days later than they did in the mid-1970s, when temperatures in the Rocky Mountains were 1.4 degrees cooler. Some 65 bird species in the U.K. are laying eggs nearly nine days earlier than they did in 1971. Others have literally fled, pushing north to cooler climes or to higher altitudes. Nearly two dozen species of dragonflies and damselflies are now wandering nearly 90 kilometers north of their habitual range in the U.K. of four decades ago, while in Spain a steady warming trend has reduced the habitat of 16 species of highland butterflies by a third in just 30 years.

On a boundless planet such artful dodging would not be a problem. But climate change is beginning to crowd animals together. Canada's red fox has moved 900 kilometers north into Baffin Island, where it is trespassing on the grounds of the Arctic fox. Scientists are reporting a complex ripple effect at Monteverde. The same warming trend that makes for hotter nights in the wet season also provokes prolonged dry spells in summer, attracting all sorts of fair-weather strangers. One is the aggressive keel-billed toucan, which has climbed from the foothills to the cloud forests, competing for food and nesting spots with the quetzal.

On the ground, Pounds's team has noticed a dramatic decline in the population of lizards, and some snakes like the cloud-forest racer and the firebellied snake, which once fed on the harlequin frogs. The loser, again, looks to be the quetzal, which is already capturing fewer frogs and lizards--a key protein and calcium source for its nestlings. "When interactions between species are disrupted, the outcome can sometimes be devastating," says Pounds.

Pests are the big winners in a warming world. A parasite called the nemotode, which dies off in the heat, has compensated by breeding faster, which causes fertility to plunge, or even death, among infected wild musk oxen. A kidney disease has flourished in the warming streams of Switzerland, ravaging trout stocks. Meanwhile, the oyster parasite, a scourge to shell fishermen in Chesapeake Bay, has crept all the way to Maine because of milder winters. Though there's little hard science linking climate change to farm pests, most agricultural experts say it's a matter of connecting the dots. "There is good evidence that warmer conditions favor more invasive species," says David Pimentel, who studies invasive plants and pests at Cornell University. "Invasive plants can compete with native varieties and cause extinctions."

Global warming is taking an especially heavy toll on specialists, species whose biology tailors them to specific geographic areas and narrow climate and temperature ranges. A recent casualty is the honeycreeper, a tiny songbird found only in the mountains of Hawaii. It has been decimated by a plague of avian pox carried by mosquitoes that have moved steadily farther into the highlands. An even bleaker example is the pika, a small, mountain-dwelling lagomorph related to the rabbit, with a low threshold for heat; it starts to die as soon as the mercury tops 24 degrees, which is exactly what is happening in its native habitat. Nine of 25 pika communities known in the western United States in the 1930s have now vanished, while fully half of those that once roamed the Tian Shan Mountains of northwest China are gone.

One of the most besieged of all the specialists is the polar bear, which hunts seal from floating chunks of sea ice. Warmer currents in the Arctic Ocean have hastened the breakup of ice floes and forced the bears to swim greater distances for their meals, putting them at risk of drowning or starving. Already bear watchers say the average weight of polars in Hudson Bay has dropped from 295kg to 230kg--near the threshold below which they stop reproducing. Polar bears now top most green groups' endangered lists.

More than polar bears will be in trouble if atmospheric temperatures rise two more degrees--far from the worst-case climate forecasts. The Greenland ice shelf would melt, posing a threat to a whole web of life that depends on ice, including plankton, which feed fish, which are eaten by seals, which are meals for both polar bears and Inuk hunters. In the Southern Hemisphere, many researchers have already linked sharp declines in penguins like the rock hopper, Galápagos, blackfoot, Adélie and the regal emperor to warmer ocean currents, which have flushed away staple food supplies like krill, a coldwater crustacean.

The loss of creatures is alarming enough. What about losing an entire ecosystem? For most of the last two decades, Stephen Williams, a tropical ecologist at James Cook University in Australia, has been studying the evolutionary biology of the Australian rain forests. The sprawling experiment was meant to plot how wildlife evolved in the mountainous cloud forests along the coast of northeast Queensland, where thousands of unique animal and plant species have thrived for 5 million years. But when Williams ran his data through a computer model, testing for a modest rise in world temperatures (3.5 degrees Celsius over a century), he was floored. By 2100, his team concluded, up to 50 percent of all species would be gone. "I expected to see an impact, but this was shocking," says Williams.

Perhaps what is most alarming about Williams's study is that even if not another tree ever falls to the chainsaw or bulldozer, one of the planet's most heralded World Heritage sites will still be under silent siege. "We're looking at losing most of the things that the protected areas were put in place to preserve," he warns. Already the populations of the gray-headed robin and a small frog belonging to the species Cophixalusneglectus are beginning to thin, while marsupials, reptiles and a host of forest birds are fleeing the heat ever higher up the mountainside, to where the life-giving clouds have retreated. "Soon," says Williams, "there will be nowhere to go." Nowhere, perhaps, but heaven.

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