Rainforest frogs: vanishing act? Frog populations around the world are dying off mysteriously. Can scientists save them--before it's too late? (Amphibians/Ecology).

RAINFORESTS SEEM to have everything any frog could crave--lots of water, an endless variety of bugs to prey oil, and thousands of hiding places. No wonder more than 2,100 frog species call rainforests their home. In a 3-square-kilometer (1.16-square-mile) site in Ecuador, "we found 81 different frog species," says William Duellman, leading tropical frog expert. "That's almost as many species as we have in all of North American!"

Biodiversity has long lured tropical field biologists (life scientists) like Duellman to rainforest. And while these explorers continue to stumble on never-before-seen frog species in fertile jungles, they're also startled to discover that hundreds of frog species are abruptly declining--and even extinct. "It used to be that in the Andes of Ecuador, toads were so abundant you had to be careful not to step on then," recalls Duellman, who is also curator emeritus of herpetology (reptile and amphibian science) at the University of Kansas Natural History Museum. "Now, they're gone." And not just in South American rainforests.

Amphibians like frogs, toads, and salamanders are dying off worldwide--especially in pristine cloud forests, rainforests so high in mountains that trees draw moisture directly from clouds. "We study frogs in national parks--the most protected areas," explains Karen Lips, a Southern Illinois University herpetologist. "There, no one's logging or spraying pesticides. But frogs are still dying. Why?"

There are multiple theories:

* Habitat destruction. "Dart-poison flogs deposit tadpoles in water-holding plants," says Jack Cover, general curator of fishes and rainforest exhibits at the National Aquarium in Baltimore, Maryland. "Clearcut the forest and the flogs lose the tools they need to reproduce."

* Pollution and climate change. A frog's skin is porous, riddled with tiny holes that easily absorb pollutants in air and water. For that reason they're called indicator species: Their health may mirror the well-being of the planet. Strong evidence suggests Earth's climate is warming, and at the Monteverde Cloud Forest Preserve in Costa Rica, biologist Alan Pounds has witnessed 20 of 40 frog species disappear from a 30-square-km (11.58-sq-mi) study area since 1989. Weather data suggests that global warming has caused clouds to shift upward, decreasing the amount of mist in the cloud forest--which impacts frogs' ability to thrive.

As ectotherms, frogs' body temperatures depend on surrounding temperatures. And many rainforest frogs are specialized to survive in specific habitat niches (places). Even slight changes in temperature or moisture disrupt their life cycle.

* Disease. Some biologists believe that a global disease-causing fungus (plant with no leaves, flowers, or roots) may rival habitat destruction as the largest single cause of amphibian population declines.

Frogs have existed on Earth for nearly 200 million years. "That's a pretty impressive amount of time to be around," says Coven "We can only hope they hang on through whatever this is."

MASS MURDER

Frogs are food to so many snakes and birds, a herpetologist could spend many seasons in some places without seeing a dead frog lying on the ground. So imagine Karen Lips's shock when she returned to her research site in western Panama and discovered dead and dying frogs everywhere: "I'd been going to Fortuna, Panama, since 1993. In 1997, I returned to find all these dead frogs. They looked fine, like they went to sleep and didn't wake up."

Lips collected bodies for autopsy. Cause of death? A fungal organism called chytrid had pervaded the frogs' skin--essentially the fungus suffocated them. Now researchers have found a plague of chytrid in moist, high-elevation forests throughout North, Central, and South America, as well as Australia.

Chytrid and frogs might have always shared a habitat. "But now something is off-balance, causing a disastrous interaction between flogs and fungus," Lips says. "Something's different." But what? That's where experiments come in, she explains. She's now comparing aspects of Fortuna with her new research site, El Cope in central Panama. Species that disappeared from Fortuna can still be found at El Cope, including four or five species of glass tree frogs.

Fearing another die-off like that at Fortuna, Lips wracks her brain to form strategies to save frogs. One option: an emergency captive-breeding program, "as a last-ditch measure only," she says. The idea is to preserve each species in captivity to prevent extinction, should something catastrophic happen to the wild populations. "We can put them back into the wild when"--or if--"the environment is okay again."

The National Aquarium has successfully bred 27 species of dart-poison frogs since 1996. Complex chemical toxins called alkaloids in these frogs' brilliantly colored skin excite medical researchers, who see the alkaloids as possible sources of potent new medicines to treat human diseases. "But there's a problem," says Jack Cover. "The ones we raise in captivity don't produce alkaloids at all."

The reason? New research suggests that dart-poison frogs don't manufacture their own toxins--they acquire them from insects they eat in the wild. The frogs concentrate alkaloids from the bugs in their skin, then use the poison for self-defense. "That makes me appreciate these frogs even more," says Cover. "And if you want to have both the frogs and their chemicals, you need to save the rainforests."

WHAT'S IN A FROG?

Take a look at a frog's anatomy. The large liver stores digested.

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FAR-OUR FROGS

Think you know about froggy metamorphosis--the physical transformation from pond eggs to aquatic tadpoles (fishlike larvae) to land-dwelling frogs? Well, surprise. Hundreds of rainforest species defy the norm: "When you get to the tropics, reproduction gets really wacky," says Jack Cover of the U.S. National Aquarium. "You've got frogs that incubate eggs in back pouches, frogs that skip the tadpole stage entirely, and frogs that carry tadpoles on their back." Here are three amazing rule-breakers:

PRIVATE POOL Female dart-poison frogs lay eggs in leaf-litter on the rainforest floor and urinate on the eggs to keep them moist! When the tadpoles hatch, they have the usual fishlike features--but no water to swim in. So mom helps a tadpole wiggle onto her sticky mucus-covered back for a ride to a rootless bromeliad plant that collects rainwater in its waxy leaves. She deposits the tadpole in the thimble-size pool and returns every few days to squeeze out unfertilized eggs for baby to eat. Such care is rarely seen in frogs.

Home: Lowland tropical rainforests of Central and South America

SIZE: Less than 50 millimeters (2 inches)

Distinguishing feature: Brightly colored toxic skin warns potential predators not to take a bite.

SEE-THROUGH The bare-hearted glass frog (Centrolenella colymbiphyllum) is a tiny arboreal (tree) frog that lives in remote Costa Rican cloud forests, where its high-pitched, musical call fills the air. Males seem to spend most of their time dangling from leaves by their toes, fighting to defend their eggs and turf. Their greenish bones and beating hearts are visible through transparent bellyside skin.

Home: Central America

Size: 19.05 to 63.5 mm (0.75 to 2.5 in.)

Distinguishing feature: Males sport a bony upper-arm hook for battle.

MUDDY PANCAKE A flick of the tongue is all it takes for most frogs to nab a meal. But beady-eyed Suriname toads (Pipa pipa) have no tongue. The toad lurks in the shallow muddy waters of the Amazon River--fingers outstretched. "If a fish swims by, the frog sucks in water and uses its fingers to tuck the fish into its mouth," Cover says. These frogs also play the most bizarre amplexus (mating) game. Males and females somersault underwater until the male has pressed 60 eggs into the female's spongy back. A few months later, fully formed froglets crawl out of her back!

Home: Amazon and Orinoco rivers of South America

Size: Up to 203.2 mm (8 in.)

Distinguishing feature: Looks like it was squashed by a car.

Lesson Plans

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Cross-Curricular Connection

Language Arts: Research the meaning of the prefix amphi- and explain why it's used to describe amphibians.

Did You Know?

* Members of Central and South American rainforest tribes hunt with darts dipped in batrachotoxins from dart-poison frog skin. To harvest the toxin, hunters catch and kill a frog, then roast it over fire until the skin sweats tiny drops of poison.

* The endangered Panamanian golden frog lacks an external eardrum. Although the frog can produce sound, most of its communication is visual. Golden flogs wave at each other with their front legs.

* All toads are frogs--but not all frogs are toads. Generally, the word toad is used to describe stocky flogs that walk rather than hop, live primarily on land, and have dry skin with poison-filled warts.

National Science Education Standards 

Grades 5-8: structure and function in living systems: natural hazards * diversity and adoptions of organism * populations and ecosystems

Grades 9-12: natural and human-induced hazards * interdependence of organisms * behavior of organisms

Resources

Dissect a frog online at www.froguts.com

In Search of the Golden Frog by Marty Crump, The University of Chicago Press, 2000

"Potions from Poisons" by Andre Dorfman, Time, January 15, 2001

CHECK FOR UNDERSTANDING

Directions: Circle the correct word(s) in the parentheses.

1. Herpetologists study (frogs, snakes, amphibians, birds).

2. As (ectotherms, endomorphs, exoskeletons), frogs' body (temperature, mass, flexibility) depends on external temperature.

3. Frogs' skin is (tough, porous, riddled with holes), causing them to absorb environmental pollutants. Thus, their health reflects that of the environment. That's why frogs are called (mirror, endangered, indicator) species.

4. Many frogs may be vanishing because of (a fungal disease, pollution, climate change, habitat destruction.)

ANSWERS

1. frogs, snakes, amphibians 2. ectotherms, temperature 3. porous, riddled with holes, indicator 4. fungal disease, pollution, climate change, habitat destruction