Smithsonian National Zoological Park l Friends of the National Zoo



Jumping into Trouble
by Howard Youth

In Mark Twain’s short story “The Celebrated Jumping Frog of Calaveras County,” gambler Jim Smiley puzzles over why his prize frog, Dan’l Webster, loses a frog-jumping match. After an unscrupulous challenger secretly stuffs Dan’l full of quail shot, Smiley says, “I do wonder what in the nation that frog throw’d off for—I wonder if there ain’t something the matter with him.” Today, this victimized, fabled frog provides a poignant metaphor for the fate of real-life amphibians—not just in the mid-1800s Gold Rush town of Angel’s Camp, California, where Twain set his tale, but well beyond.

golden mantella
Critically endangered by habitat loss and over-collection, the golden mantella (Mantella aurantiaca) is found in an area of Madagascar not much larger than that encompassed by Rock Creek Park. (Jessie Cohen/NZP)

The largest frogs around Angel’s Camp in 1850 would have been California red-leggeds (Rana aurora draytonii), also one of the U.S. West’s largest. Today, you won’t find any there. Since the Gold Rush, these dark-backed, rosy-legged amphibians have disappeared from 70 percent of their former range. In 1996, they hopped onto the U.S. Fish and Wildlife Service’s (USFWS) Endangered Species List.

With this foundering frog and thousands of others, scientists and conservationists find themselves more or less in Jim Smiley’s shoes, trying to figure out what’s wrong with their beloved amphibians. Over the last 20 years, herpetologists around the globe have joined forces to track frogs’ whereabouts, assess their well-being, and try to stem a rising tide of extinctions and declines that tugs at the complex natural fabric of ecosystems. But they have only gotten so far.

Frogs are sensitive creatures. Because they have permeable skin and eggs and typically depend upon moist habitats, they and other amphibians are particularly vulnerable to environmental changes. But most frog species remain poorly studied, so we can only guess at why many are in trouble. And as frogs vanish, new species are just being discovered.

The California red-legged frog’s tale illustrates the multiple threats frog species face. In the late 1800s, an estimated 80,000 wild frogs were carted off each year to California markets to fill the bellies of recently moneyed forty-niners and San Franciscans. Overharvesting was followed by widespread habitat loss, as farms and towns changed the landscape. By the early 1900s, wild frog populations dwindled but commercial frog-farming ventures attempted to meet the demand for frog legs. Instead of stocking native red-leggeds, though, businesses imported the largest frogs native to central and eastern North America, American bullfrogs (Rana catesbeiana). These robust amphibians, like the miners before them, came West and found great opportunity. Escaped farm bullfrogs, followed by those released by farmers, game-fish stockers, and frog-lovers, established wild western populations that competed with native species. They easily gained ground, often ate their competitors, and likely introduced new pathogens lethal to native frogs. In 1928, inspired by the Twain tale, Angel’s Camp townspeople started an annual frog-jumping event. This tradition has kept thousands of captured and released bullfrogs busy ever since.

An Ancient Line in Decline

About 290 million years ago, long before the dinosaurs, a panoply of amphibians prowled the primordial muck. Among them were some of the top predators of the Permian period, including large-headed, six-foot-long carnivores in the genus Eryops. Today, Earth is home to about 6,000 amphibian species—frogs (including toads), salamanders (including newts), and tropical burrowing caecilians.

The habitat of the Pine Barrens tree frog (Hyla andersonii), a threatened species, contains sandy, acidic soils and dense shrubs, heavy ground cover, and shallow ponds and bogs.

In sediments dating back 200 million years, paleontologists have unearthed fossils of the first known frog. Today, more than 5,200 frog and toad species, constituting more than 85 percent of amphibian diversity, inhabit all continents except Antarctica. Found not only in temperate wetlands but also above the Arctic Circle, in steamy equatorial rainforests, and even in some desert regions, they range in size from tiny Cuban and Brazilian species that fit comfortably atop a nickel to the largest, which can stretch three feet long from nose to hind toes.

In 1967, National Geographic Society scientist Paul A. Zahl wrote of his trip to equatorial Africa to encounter the world’s largest frog. In words and photos, he gave National Geographic readers a glimpse of the goliath frog (Conraua goliath), which had been first described six decades earlier. Zahl concluded his article by pondering the species’ future:

…the generous proportions of a goliath drumstick put it in a class by itself. Such appeal could quickly doom these rare animals, were it not for two circumstances: their preference for difficult terrain, and the sparseness of human population to prey upon them. Unless these factors change, their survival seems assured.

Today, the goliath is listed as endangered on the World Conservation Union’s (IUCN’s) Red List, and its numbers are estimated to have declined by more than half over the last 15 years. Restricted to fast-flowing jungle rivers and streams in Cameroon and Equatorial Guinea, the frogs are now trapped and carried off for sale in urban food markets. In addition, each year at least several hundred are exported by zoos and animal traders.

The red-eyed treefrog (Agalychnis callidryas) is a common rainforest inhabitant from southern Mexico to Panama.

Most scientists agree that Earth is undergoing its single largest mass extinction event since the catastrophic demise of dinosaurs 65 million years ago. Goliath and other frogs are among the fastest-falling victims. Since frog species began vanishing in the late 1980s, scientists have galvanized their efforts to track these diverse creatures, with sobering results. The Global Amphibian Assessment (GAA), the product of input from nearly 600 experts in more than 60 countries, first came out in 2004 and has yielded the first-ever worldwide snapshot of more than 5,900 amphibians species’ conservation status and distribution. This regularly updated inventory reveals that nearly one-third of existing amphibian species are threatened, compared with 12 percent for birds and 23 percent for mammals, the only other groups that have been subjects of global assessments. The GAA estimates that at least 43 percent of amphibian species are in decline, although the status of about 30 percent could not be determined due to lack of data. Just one-half percent were ranked as increasing. Experts also believe that as many as 165 frog species have gone extinct, with at least 122 of these vanishing since 1980.

A 2005 conservation summit convened by the IUCN Species Survival Commission helped further hone scientists’ and conservationists’ efforts to study and reverse declines. The summit yielded a 2007 Amphibian Conservation Action Plan, a comprehensive, global plan that outlines strategies for responding to amphibian losses. Individual country efforts have aided this mission. For example, U.S. Geological Survey (USGS) scientists have collaborated on an Amphibian Research and Monitoring Initiative since 2000 to address U.S. amphibian population slumps, and the agency recruits volunteers and other collaborators for a long-term project called the North American Amphibian Monitoring Program. And this year, the Amphibian Ark—a program that highlights zoo efforts to save rare frogs—was launched as part of the IUCN’s and the World Association of Zoos and Aquariums’ global Year of the Frog campaign.

The Smithsonian’s National Zoo is participating in the Year of the Frog. Educational activities and exhibits will take place throughout 2008, aimed at creating awareness in Zoo visitors and inspiring them to take action to save amphibians locally and globally. Also, the Zoo is breeding imperiled frogs, and scientists and keepers there hope to expand frog breeding programs in coming years.   

Wet, Waning, and Wasted

The greatest overall threat to frogs and other amphibians is habitat loss and degradation, which adversely affects about 4,000 species. A look at small countries with growing human populations and few remaining wild areas illustrates how amphibians and other wildlife are running out of space. In the Caribbean, for example, more than 80 percent of amphibian species in the Dominican Republic, Cuba, and Jamaica are threatened, while in impoverished, deforested Haiti, 92 percent are in trouble.

A natural match for its surroundings, the mossy frog (Theloderma corticale) leads a sheltered life on craggy forested mountain slopes in northern Vietnam. (Jessie Cohen/NZP)

Particularly in the tropics, frog diversity and distribution remain poorly studied. As trees fall and wetlands shrink, scientists find new frog species while others disappear. Up until the late 1990s, for example, fewer than two dozen frog species had been catalogued in the Asian island nation of Sri Lanka. Over the past nine years, however, following boosted efforts to census and classify frogs there, the count jumped to beyond 100 species. No one can know how many other frogs once occurred on the West-Virginia-size island. But following decades of deforestation, it is certain that at least some known species have recently been lost, along with more than 95 percent of the original forest cover. In 2005, for example, scientists from Sri Lanka’s Wildlife Heritage Trust documented 35 new frog species, but also found that 19 previously described species had gone extinct.

Plenty of habitat has also disappeared in temperate regions, including the United States. Since European settlement, widespread wetland losses undoubtedly slammed many U.S. frog populations, although data are not available for earlier years when the largest losses occurred. By 1980, California and five Midwest states had lost more than 95 percent of their original area in wetlands; 16 other states had lost between 50 and 95 percent. Today, the USFWS lists 21 amphibian species as threatened or endangered. The GAA, however, tallied 51 U.S. species as threatened, and 32 as nearly threatened. Three species are listed as extinct.

Chemical pollution poses another threat to frogs. For example, atrazine, one of the most commonly used herbicides in the world, is considered an endocrine disruptor—a substance that can alter the sexual reproduction and development of creatures exposed to it. In their article published in the Proceedings of the National Academy of Sciences in 2002, a team of biologists wrote that “hermaphroditic, demasculinized” African clawed frogs (Xenopus laevis) resulted after larvae were exposed to “realistic exposures” of atrazine. Frogs exposed at “low ecologically relevant” doses of the compound developed both male and female sexual organs. In males, exposure also drove down testosterone levels. The authors suggested that similar exposures in wild frogs could risk impairing the frogs’ sexual development. “This widespread compound and other environmental endocrine disruptors,” they wrote, “may be a factor in global amphibian declines.” Elsewhere, tests using northern leopard frogs (Rana pipiens) found that exposure to small amounts of atrazine yielded similar results, altering the frogs’ ability to breed.

Other agricultural and industrial chemicals kill or injure frogs. A study released in 2007 by scientists from the University of Colorado at Boulder fingered boosted levels of nitrogen and phosphorus, chemicals commonly used in fertilizers, as responsible for spikes in the rate of frog deformities. These chemicals spur algae growth, which helps snail populations increase and spread rapidly. The snails may indirectly spur debilitating frog deformities because they host microscopic parasites called trematodes, which form cysts on tadpoles that can cause disfigurement. Deformed frogs may not live as long as usual, and their breeding may be impaired. Other studies point to a link between an increased rate of trematode infections following pesticide exposures, hinting that animals’ immune responses may be altered by the chemicals. The potential dangers posed by agricultural and industrial chemicals to amphibians remain largely unexplored.

Invisible Killers

Scientists also suspect that recent environmental changes across the globe affect amphibians. Threats include increased acidity of water in areas affected by acid rain and rising levels of ozone, perhaps caused by the erosion of the ozone layer. Also, shifting patterns of precipitation, cloud cover, and temperature resulting from global climate change seem to be taking their toll. Although people tend to think of global warming as a phenomenon that primarily affects temperate and polar areas, it also seems to be having a strong impact in the tropics. A 35-year study of lowland forest habitats at the La Selva Biological Station in northern Costa Rica, for example, recently revealed that all terrestrial amphibians, along with lizards, declined by an average of 75 percent within the protected study area. The investigators believe that global warming is to blame—that with warmer temperatures, trees hold their leaves longer and/or decomposition rates are higher. The overall effect is a reduction in leaf litter, which denies ground-living amphibians and reptiles critically important food and shelter.

Global climate shifts may also be creating the “perfect storm” for the spread of pathogens that kill frogs. A recent survey of forest-dwelling Neotropical toads called harlequin frogs (Atelopus spp.) found that while only a few species have been affected by habitat loss, 81 percent of adequately studied species are in decline. Only ten species have healthy populations. Of the 113 harlequin frog species of Central and South America, at least 30 have vanished in the last 20 years.

“Our findings point to Atelopus as the most striking case of catastrophic species loss ever documented for a single amphibian, or perhaps vertebrate, genus in recent history,” wrote a scientist from the Smithsonian Tropical Research Institute on Barro Colorado Island, Panama, and his international colleagues in the journal Biotropica in 2005.

The reasons for these disturbing declines are still not entirely clear, but scientists think that two major suspects, a fast-spreading fungus called chytrid and quick-changing climate, might be working together. “Disease is the bullet that’s killing the frogs,” said biologist J. Alan Pounds of the Golden Toad Laboratory for Conservation in Puntarenas, Costa Rica. “But climate change is pulling the trigger.” In a 2006 study published in the journal Nature, Pounds and his colleagues noted a correlation between chytrid and rising temperatures, which leads to more cloud cover that cools days and warms nights in tropical mountain forests. This temperature moderation may spur the spread of the chytrid fungus. Alternatively, climate change may stress frogs, making them more susceptible to the fungus or other diseases.

Blue dart-poison frogs are vulnerable to chytridiomycosis. (Jessie Cohen/NZP)

This is but the latest twist in a disease-sleuthing saga that began about 20 years ago. In the late 1980s, Pounds and other herpetologists noted some of the first disturbing drops in frog numbers, in Monteverde Cloud Forest Reserve, a protected mountain area in Costa Rica. The standard culprits—habitat loss, pollution, introduced predators—didn’t figure there. An invisible killer was at work. In 1999, National Zoo pathologists were at the forefront in isolating this microscopic fungus and the associated fatal disease, chytridiomycosis, which they found while analyzing blue dart-poison frogs (Dendrobates azureus) that had died at the Zoo. “It was a unique skin disease unlike anything I’d seen before, with these round, microscopic organisms,” recalls one of the investigators, former Zoo pathologist Don Nichols.

Although named less than ten years ago, the chytrid fungus Batrachochytrium dendrobatidis, or Bd for short, has since been detected in frog species in the Neotropics, Australia, North America, and other parts of the world. In lab tests, the fungus and its resulting illness killed frogs of most of the tested species. In the tropics, Bd seems to flourish at higher elevations, and lab studies indicate that in general it may be most virulent in cool, moist areas. Seventy-five percent of the harlequin frog species restricted to high elevations have vanished, according to the 2005 Biotropica article. Among extinct species, chytrid was recorded in populations for an average of three years before all individuals vanished.

Where did the chytrid fungus come from? Some research suggests it originated in Africa; other recent evidence suggests a North American strain exists. It may have been spread by introduced game fish or frogs. The African clawed frog, for example, is widely sold in pet stores, and has been shipped all over the world for use in medical tests since the 1930s. Released individuals may have carried the fungus—and more—to wild areas. For example, in southern California, where this species now has self-sustaining wild populations, scientists found infestations of two parasitic worms previously known only from Africa. In addition to preying upon native amphibians, introduced bullfrogs and game fish such as trout may have helped spread the fungus and other parasites and illnesses to the western U.S., South America, and other parts of the world.

Some experts hypothesize that scientists or ecotourists might have transported the fungus to new areas on their equipment or boots. Biologists studying frog populations now disinfect their gear before moving from one study site to the next, but in the 1980s and earlier, such a practice was not commonplace.

If habitat protection alone cannot save amphibians, what can be done? In zoos and aquaria, certain treatments seem to help frogs survive chytrid infections. One of the more recent findings came from New Zealand, where scientists learned that chloramphenicol, a medicine used to treat human eye infections, cures frogs of the fungus.

Panamanian golden frogs, a critically endangered species, are bred at the National Zoo. (Jessie Cohen/NZP)

The National Zoo continues to play a part in helping save endangered frogs. Critically endangered Panamanian golden frogs (Atelopus zeteki) breed in the Zoo’s Reptile Discovery Center, contributing to a reservoir of zoo genes that can help ensure that these diminutive hoppers do not follow some of their brethren into extinction. They are now probably extinct in the wild due to habitat loss, overcollecting, and chytrid fungus.

While zoo breeding may not be able to save all Atelopus species, as the Biotropica authors wrote: “At present, two likely causes for population declines, Bd and climate change, cannot be counteracted in wild [Atelopus] populations. The only conservation tool available when either of these two factors is present is ex situ breeding programs.”

It’s a Frog’s Life

Despite all the attention now focused on amphibian declines, scientists have just started teasing apart the vexing mysteries of disappearing frogs. This work relates not just to frogs themselves, but to their ecosystems. As the Biotropica investigators wrote of the harlequin frogs, “These population changes are expected to cascade through aquatic and terrestrial food webs, with often unexpected consequences.”

Scientists’ detective work is stymied by the complexity of frog natural history. For example, frogs have varied breeding strategies. We are familiar with pond- or pool-breeding frogs, which lay eggs in water, and the young hatch as tadpoles before metamorphosing into adults. But almost half of all frogs follow different pathways to adulthood. Some carry their eggs on their back legs, and some lay eggs in trees and their tadpoles fall into water below when they hatch. Some tropical frogs, such as Solomon Island leaf frogs (Ceratobatrachus guentheri), which are exhibited in the Zoo’s Reptile Discovery Center, skip the tadpole stage and hatch directly into small froglets.  

Also, there’s no single, surefire way to survey these jumpy creatures. It’s not like counting crows for a Christmas bird count or looking for singing orioles for a spring breeding bird survey. While many frogs have loud voices that can be heard during their breeding season, they don’t breed on such predictable schedules as do most birds of temperate climates. Instead, they must wait until moisture levels and other environmental conditions are ideal. Some frogs have brief breeding periods, others have long ones. And frog populations frequently undergo dramatic natural ebbs and flows. Compounding the difficulty, species that live in rainforests and other densely vegetated habitats often avoid detection. The well-studied genus Atelopus is an exception: The majority of the species are brightly colored, diurnal, and found along stream edges.

Playing key roles as hunter and hunted in a variety of habitats, frogs are important cogs that keep nature’s gears smoothly turning. They also remain the important focus of human lore and interest, as in the former mining town of Angel’s Camp, California.

These days, in the protected habitats where they survive, California red-legged frogs face more than bullying bullfrogs. In some areas, chytrid fungus, trematode parasites, and introduced and hungry African clawed frogs are added dangers.

Meanwhile, the show must go on in Angel’s Camp. Each year, up to several thousand bullfrogs compete, spurred on by their handlers. The capture, trade, and rerelease of bullfrogs is carefully monitored by fair organizers and state wildlife agencies so as not to affect any of the remaining red-legged frog populations outside of town. Not that the thousands of transported bullfrogs would follow rules given the chance: USGS scientists studying introduced bullfrogs in the Southwest recently found that these pioneering jumpers can disperse on their own more than six miles between suitable wetland habitats.

Do people in “Frogtown, U.S.A.” miss the native red-legged frog? Some say yes, others no. The naysayers resist any suggestion that native frogs be reintroduced into nearby wetlands, lest there be more restrictions on where new homes or businesses can be built, or where people can seek and release bullfrogs.

So, even where frogs are celebrated, it’s a brave new world for declining amphibians. And in this shaky, human-dominated world, it seems that only the most adaptable can keep their heads above water.

—Contributing editor Howard Youth last wrote about declining amphibians in ZooGoer in the March/April 2000 issue.