The case of the croaking frogs: a deadly fungus is spreading among frogs. Could warming temperatures be to blame?

When scientist J. Alan Pounds first visited the Monteverde Cloud Forest Preserve in Costa Rica 25 years ago, a deafening chorus of frog calls filled the air. At night, frogs sat along stream banks, sounding off low, noisy croaks. Others belted out high-pitched "ping-pingpings" like giant dripping faucets. "Now, the sound is a pale version of what it used to be," says Pounds.

Why the silence? Monteverde's frogs are rapidly disappearing. The area's harlequin frogs have been hit especially hard. At least 110 species of these bright orange, green, or yellow striped frogs used to live near streams in Monteverde, Costa Rica, and in 10 other Central and South American countries. But during the 1980s and 1990s, these frogs started dying out in droves. Today, approximately two thirds of harlequin frog species are extinct in the wild.

Researchers have puzzled for decades over why harlequin frogs and other amphibian species have been vanishing (see pie chart, p. 15). But now, Pounds and other scientists think they've found an answer: Warming temperatures around the globe may be making frogs more prone to disease. "Disease is the bullet killing frogs, but climate change is pulling the trigger," Pounds says.

ALARM CALL

Frogs are one type of amphibian, a group of animals that also includes toads, salamanders, and wormlike caecilians (si-SIL-yuns). The features that distinguish amphibians from other creatures are also what make them so vulnerable, says Andrew Blaustein, a zoologist at Oregon State University. For instance, amphibians have very permeable skin. It's riddled with tiny holes that enable substances to pass easily into their bodies. Moisture from the air or ground passes through the porous skin, and helps the amphibians stay moist. They also "breathe" oxygen through their skin. The downside to such permeable skin? Amphibians also absorb environmental pollutants.

Amphibians, whose name means "double life," live a part of their life on land and a part in water "It's a double whammy--they can get exposed to bad things in both places," says Blanstein. Because amphibians have such sensitive bodies, they are more likely than other animals to be affected by pollution and other environmental hazards. That's why scientists call amphibians indicator species. Their health informs researchers about Earth's well-being.

LOST IN THE MIST

That's one reason why scientists are so worried about amphibians becoming extinct in Monteverde and around the world (see map, p. 17). It's a sign that the environment is in trouble. But for researchers studying Monteverde's frogs, figuring out the exact cause of death has been like trying to solve a murder mystery.

Years ago, researchers noticed that many of Monteverde's frogs were infected with a disease caused by the chytrid (KIT-rid) fungus. This waterborne organism--which reproduces by spores--creeps along a frog's skin and interferes with the animal's ability to take in water. That can rob an amphibian of the oxygen that's present in water. But infected frogs, although weakened by the disease, don't die right away. They spend their remaining days interacting with other frogs, spreading the killer fungus (see Nuts & Bolts, below).

Scientists first noted frogs infected with the chytrid fungus in Costa Rica in 1986. What mystified scientists was that if the fungus existed in the 1980s, why didn't it destroy the frog populations back then? Why are so many frogs dying from the fungus now?

To solve the mystery, scientists studied air-temperature records in the tropics over the past 30 years. They found that more frogs seemed to die during the warmest years. But they don't think heat alone is killing frogs. Pounds says that the rising temperatures seem to be making it easier for the deadly chytrid fungus to grow.

Monteverde sits in the middle of a cloud forest, a rain forest so high on a mountain that its trees draw moisture directly from mist in the air. When temperatures cool, mist-filled clouds hover lower on the mountain--below where the frogs live. But today's warmer temperatures mean that the clouds now form higher on the mountain and float above the frogs' habitats. These clouds insulate the ground where the frogs live, preventing heat from moving in or out. This keeps ground temperatures more constant throughout a 24-hour period.

Pounds believes the smaller temperature fluctuations may be creating ideal conditions for the chytrid fungus to grow. "[The] fungus is like an evil Goldilocks. It doesn't like extremes," says Pounds. So with more-constant temperatures, the thriving fungus can spread from frog to frog--killing the amphibians in droves.

SIMMERING SITUATION

Blaustein says that a hotter Earth could hurt more than just frogs. Average global temperatures have climbed about 0.6[degrees]C (1[degrees]F) over the past century. Scientists expect temperatures to rise even more over the next century. These changing conditions enable disease-causing species like the chytrid fungus to thrive in places where they didn't before. That could put other species--including people--in danger of contracting new types of illnesses.

Many scientists think that people have caused much of this global warming by burning oil, coal, and other fossil fuels for energy. Burning these fuels releases greenhouse gases that surround Earth and trap in heat. "Frogs are sending out an alarm call about climate change," Blaustein says. How can we slow global warming--and protect frogs and other species? "Saving energy, or becoming energy efficient, is one of the best things we can do," he says.

Nuts & Bolts

The chytrid fungus only infects areas of the frog's skin that contain keratin, a tough protein also found in your hair and nails. Within

the keratin-filled skin cells, the fungus develops and reproduces, sickening the frogs.

1 FUNGUS MEETS TADPOLE 

Skin near a tadpole's mouth contains keratin, A zoospore (swimming spore, or reproductive body) of the fungus attaches to the developing frog's mouth.

2. The zoospores absorb its own tail, forming a resting spore. This cyst enters the tadpole's skin cells. Within days, a sac-like structure sorrounds the spore. The spore undergoes cell division to form new zoospores.

3. Once the fungus's life cycle is complete; each sac-like structure produces a pinhead-size discharge tube, This tube pokes through the tadpole's skin, releasing tens to hundreds of zoospores into the water.

4 NEXT ATTACK

New zoospores infect other tadpoles and grown frogs. Mature frogs have many keratin zones--entry spots for the zoospores. The cycle begins again.

AMPIBIAN RISKS

The graph below shows the status of the world's 5,743 known amphibian species. The species range from "extinct"--the most serious threat--to "least concern" meaning a species is widespread and abundant.

1 0.6% Extinct / Extinct in the Wild (35 species)

2 7.4% Critically Endangered (427 species)

3 13.3% Endangered (761 species)

4 11.6% Vulnerable (668 species)

5 6.3% Near Threatened (359 species)

6 38.4% Least Concern (2,203 species)

7 22.5% Data Deficient (1,290 species)

Worldwide losses

Of the 5,743 known species of amphibians (frogs, toads, salamanders, and wormlike caecilians) in the world, nearly half are declining in number. Roughly one third are threatened with extinction. Habitat loss, pollution, and climate change are partly to blame for the decline.

EARTH DAY ACTION

In addition to the fallout from global warming, the frogs of Monteverde face other survival challenges--like loss of habitat due to logging and development. Protecting important habitats like Monteverde might seem like a daunting task. But in 1987, a 9-year-old Swedish boy came up with an idea for how kids could make a difference. He and his classmates raised money to buy up chunks of rain forest in Costa Rica. To make cash fast, "We baked cakes, sang our own songs ... everything you can imagine," says Eha Kern, the children's teacher.

The kids eventually bought approximately 15 acres of land in Monteverde. Since then, an additional 54,000 acres have been purchased with contributions from around the world. The area is now known as the Children's Eternal Rainforest and is owned and protected by a group called the Monteverde Conservation League (MCL). "Buying land gives you an enduring possibility of controlling its fate," says Frank Joyce, the treasurer for MCL. If young kids could have such a positive impact on the environment, just think what you could do!