Chasing Frogs and Phantoms: The Mystery
of Amphibian Declines
by Howard Youth
On August 8, 1995, teacher Cindy Reinitz and eight Minnesota New Country middle school students were padding down a farm road on their way to an outing in the nearby Ney Woods. "We were just going to natural areas around our communities, so the kids could understand and appreciate what they have in the backyards," recounts Reinitz. "But we never made it to the woods."
At a section of the road bordered by a corn field on one side, a bean field on the other, the students caught sight of scores of northern leopard frogs (Rana pipiens) hopping about a nearby pond. They interrupted their walk and began diving for frogs, playing a game of catch-and-release. But then a child stumbled upon a troubling find, the first clue in one of the greatest ecological mysteries of our time.
"One student found a frog with one very, very thin leg. Just bone covered with skin, no musclevery strange," says Reinitz. "I thought it had gotten hurt with the kids belly-diving for the frogs." Shortly thereafter, however, another student discovered a frog missing a hind leg.
Deciding that the situation merited better data, the students captured a total of 22 frogs. Half of them were deformed. "That just didn't seem like it was a normal thing," recalls Reinitz. Some students wanted to keep the frogs to show scientists. Others wanted to leave them unmolested. "We compromised and decided we'd take three of the most grotesque ones back in an ice-cream bucket."
Reinitz soon linked up with a Minnesota Pollution Control Agency scientist, and from there life got crazy. Reporters, satellite trucks, and scientists from all around descended upon sleepy Henderson, Minnesota, and its "mutant frog" pond. Deformed frogs were soon reported from two-thirds of the counties in Minnesota, and in other states. The inevitable question was raised by scientists and journalists alike: What could be disfiguring so many frogs? Studies began almost immediately to look for answersfrom pesticides and heavy metals to parasites, ultraviolet light, viruses, or just natural causes. But almost five years later, no clear explanations have emerged.
Frog deformities have been documented in different parts of the world for more than two centuries. But the sheer number of deformed frogs found in Minnesota alarmed scientists, and highlights heightened global concerns over mysterious amphibian declines and disappearances plaguing various regions of the world. Many scientists now think multiple factors come into play to create amphibian deformities and declines. And declines and deformities, they caution further, are not necessarily linked.
Something in the Air?
The planets hopping and crawling amphibians, biologists believe, can provide valuable insight into the health of their larger ecosystems. Most species of frogs, toads, and salamanders lead double lives, starting out as aquatic eggs and larvae and later switching to a terrestrial adulthood. Their development thus puts them in contact with two potentially polluted habitats. Also, many amphibians breed in small pools, then disperse after breeding to more far-flung haunts. Migrations to and from breeding pools are often cut short by the fragmentation or destruction of their habitat.
But researchers seeking answers to ecological whodunits especially prize amphibians' extremely sensitive skin. "Skin changes that would be considered relatively minor in a mammal, bird, or reptile can have highly significant effects in an amphibian," says National Zoo associate pathologist Don Nichols. "Amphibians rely on their skin to absorb water, and to control hydration and the concentration of some important minerals in the bloodstream. Also, to a certain degree they breathe through their skin, and it plays a role in temperature control," says Nichols.
According to the World Conservation Union's 1996 Red List of Threatened Animals, 25 percent of amphibian species for which ample data exist are classified either as endangered or vulnerable. The United States and Australia house the most species listed as suchabout two dozen eachbut that fact probably disguises the paucity of data from tropical countries, where the most species live and where habitat destruction is most rampant. Only 12 of the United States 17 amphibian species currently listed as endangered or threatened have recovery plans.
While habitat destruction undoubtedly constitutes one of the greatest threats to the world's amphibians and other wildlife, scientists are puzzled over inexplicable amphibian declines in more remote and seemingly pristine areas, including, for example, Monteverde Cloud Forest Preserve in Costa Rica. Established in 1973, Monteverde now protects 26,000 acres of cloud forest. The preserve, however, has not sheltered one of its rarest treasures: the world's only known population of the gaudy golden toad (Bufo periglenes). In 1987, golden toads suddenly vanished even from the heart of the refuge. Surveys in the early 1990s later revealed that the golden toads were not alone. Twenty of 50 frog and toad species in a 7,400-acre study area lying mostly within the preserve had disappeared without a trace.
Researchers believe that the world changed around these moisture-dependent, mountain-dwelling animals. Global warming likely played a role in their population crashes, which occurred abruptly in 1986 and 1987 during extremely warm, dry conditions. In a 1999 study published in the journal Nature, biologists J. Alan Pounds, Michael P.L. Fogden, and John H. Campbell wrote, "Our results indicate that these crashes probably belong to a constellation of demographic changes that have altered communities of birds, reptiles, and amphibians in the area and are linked to recent warming." Amphibians, they found, declined more dramatically than any other group. The scientists hypothesize that atmospheric warming raised the height at which condensation begins, so that higher clouds may now pass high over Monteverde, instead of dragging across the forest depositing mist. Meanwhile, there's been no sight of the golden toad, and no better hypotheses have emerged to explain its disappearance.
Acid precipitation is a more clear-cut threat to amphibians living in or near industrialized regions. For example, sulphur dioxide emissions from power plants and nitrogen oxides from car exhaustwhich can raise acidity levels in breeding poolshave been blamed for the virtual disappearance of natterjack toads (Bufo calamita) in southern England. In 1997, researchers examining northern leopard frogs concluded that acidified waters also lower frogs' resistance to a form of bacteria that can colonize their spleens, and kill them.
Finding New Enemies
Another malady linked with amphibian declines is a mysterious disease caused by the chytrid fungus Batrachochytrium dendrobatidis, the first such fungus known to harm vertebrates. Don Nichols began seeing the strange affliction back in 1991, when the Zoo received samples taken from a captive colony of dying arroyo toads (Bufo microscaphus californicus) in California. "It was a unique skin disease, unlike anything I'd seen before, associated with infection by these strange round, microscopic organisms," says Nichols. On and off over the next five years, he received samples, searched the literature, and consulted experts, but it wasn't clear exactly how to classify the organisms causing the disease: Were they algae, protozoa, or fungi?
When the disease hit the Zoo's White's treefrogs (Litoria caerulea), blue dart-poison frogs (Dendrobates azureus), and green-and-black dart-poison frogs (Dendrobates auratus) in 1996, Nichols and Allan Pessier, a resident in the Zoo's Department of Pathology, were able to collect fresh samples. After results of electron microscopic examinations suggested the organism might be a chytrid fungus, Pessier searched the Internet and found one of the world's few chytrid experts, Joyce Longcore at the University of Maine, who confirmed their suspicion. Meanwhile, having verified that the chytrid fungus indeed caused the disease, Nichols and Department of Pathology biologist Elaine Lamirande found ways to treat the Zoo's ailing frogs.
"This type of fungus had never been considered to be pathogenic," says Nichols, "so you couldn't find it in the veterinary literature. But here was a new chytrid that affects vertebratesthat was unheard of."
Troubling news soon came from the field: Frogs and toads were turning up with the disease in distant corners of the globe. In 1997, a veterinary pathologist named D. Earl Green told Nichols of samples of wild frogs and toads he received which had been found dead and dying in Panamanian streams. Soon after, word came that Australian scientists Rick Speare and Lee Burger were tracking a pathogen sweeping through the continents wild frog and toad populations. This killer is now considered among the possible causes in the decline or disappearance of at least 14 frog species in Queensland rainforests. While Nichols and his colleagues worked to identify the disease in Zoo amphibians, Green, Speare, and Burger studied the wild populations. "We all were coming to the same conclusion," says Nichols.
Although the culprit now has a name and classification, it remains very much an enigma. Nichols doubts the fungus is the sole killer. "It doesn't do a parasite any good to kill its host. Other factors may be tipping the balance," he says. "Something in the environment may have changed. These are all things that need to be sorted out."
Another question without an answer so far: Where did it come from? "This thing's popping up all over the placein zoos, wild leopard frogs in Arizona, boreal toads [Bufo boreas boreas] in Colorado," says Nichols. "How did it get all over the place, and apparently suddenly? There don't appear to have been cases before the late 1980s."
Evidence from Australia points to an introduced pathogen possibly borne by escaped pet frogs, or by giant toads (Bufo marinus) introduced in the 1930s (see sidebar). "For all we know, researchers could be carrying it in on their boots, going from swamp to swamp and carrying it from place to place," says Nichols. Increasingly, biologists disinfect their field equipment after visiting research sitesbut that was unheard of ten years ago.
While fungi present a clear problem, scientists also believe viruses may play a role in amphibian declines. For example, tiger salamander (Ambystoma tigrinum) die-offs in Utah and Maine in 1998 might have been the handiwork of a killer iridovirus, like the one thought to have caused earlier tiger salamander die-offs in Arizona and Saskatchewan. And in 1999, a study in California showed that both wild stickleback fish (Gasterosteus aculeatus) and the now threatened red-legged frogs (Rana aurora) carry the same iridovirus.
This finding disturbs conservationists trying to save the red-legged frog, the western United States largest native species, and the species Mark Twain likely had in mind in his novel, The Celebrated Jumping Frog of Calaveras County. Researchers already knew that introduced trout, bass, sunfish, large minnows, and bullfrogs (see sidebar) consumed and out-competed native frogs and California newts (Taricha torosa). But the new finding indicates that the introduced species may spread disease to them as well.
Searing Sun and Strange Brews
Andrew Blaustein, an Oregon State University biology professor, has long sought to pin down potential causes for the precipitous declines of western amphibian species. In a 1994 study, Blaustein and his colleagues offered one answer. Natural ultraviolet (UV) light, they found, was killing the eggs of declining Cascades frogs (Rana cascadae) and western toads (Bufo boreas), but not those of the stable Pacific treefrog (Hyla regilla). (The Pacific treefrog apparently produces more of an enzyme that enables it to heal from UV-caused damage.) Dozens of experiments and papers have since focused on the implications of ozone-layer depletionwhich leaks UV light into our atmospherefor amphibians elsewhere.
"We now know that a good couple of dozen species are affected by UV," says Blaustein. "From our studies, we know it's lethal in Cascades frogs, western toads, long-toed salamanders [Ambystoma macrodactylum], and northwestern salamanders [Ambystoma gracile]." Research has also uncovered non-lethal effects of UV exposure, says Blaustein, including the discovery that frogs that bask in sunlight are more likely to suffer damaged retinas.
Scientists are particularly concerned with how elevation, water depth, and water chemistry affect UV penetration. Steve Corn, a U.S. Geological Survey zoologist at the Montana-based Aldo Leopold Wilderness Research Institute, studies how UV exposure varies between amphibian habitats and over time. Ultraviolet radiation, Corn says, increases during the spring, remains high in summer, and then declines in the fall. "But that varies year to year due to weather," he says. "The UV reaching the surface is pretty variable, and amphibian breeding is also variable, sometimes ranging from 30 to 45 days from one year to the next."
Blaustein is also looking into another troubling linkthat between amphibian declines and fertilizers. In a 1999 study, he and his colleagues demonstrated that even at very low doseslower than E.P.A. standards allow for drinking waterfertilizer damaged larval stages of the Oregon spotted frog (Rana pretiosa), a species declining throughout its range. Blaustein and his colleagues also concluded that fertilizers building up in water also encourage algae growth that benefits parasitic flatworms called trematodes. Trematodes cause frog deformities by boring into larvae and adults, forming cysts that disrupt limb development. Blaustein and other scientists are further trying to understand if there is a link between UV exposure and lowered resistance to pathogens in amphibians.
Chemicals are also under scrutiny. USGS toxicologist Don Sparling and his colleagues have studied the effects of methoprene, an active ingredient in the insecticide Altosid, which is used to control mosquitoes in many areas, including some national wildlife refuges. Using a series of room-sized experimental wetlands at the Patuxent Wildlife Research Center in Laurel, Maryland, the researchers found that 15 percent of southern leopard frog tadpoles (Rana utricularia) hatching in sprayed pools had deformities, compared with 4.5 percent of tadpoles in unsprayed, control pools. Sparling says there is also strong evidence implicating methoprene in the delayed maturation and development of southern leopard frogs and northern cricket frogs (Acris crepitans).
Sparling and other scientists are collecting data that they believe could soon prove that the widely used insecticides malathion and chloropyrifos are harming Sierra Nevada amphibian populations just east of California's heavily agricultural Central Valley. Even more toxic than these compounds are chemicals called pyrethroidsinsecticides that can prove lethal to amphibian larvae at dilutions as low as one part per million. But in a literature search for an upcoming book, Sparling and his colleagues found only 221 studies on the effects of contaminants on amphibians, compared with some 7,000 such studies for fish. "There's a great need for additional research on amphibians," argues Sparling.
Human activities can affect amphibian populations more noticeably than the potentially insidious forces of disease, climate, and chemistry. About 10 to 12 million wild frogs are caught each year for dissection in U.S. classrooms, while the French alone annually consume six to eight million pounds of frog legsmany imported from Asia and other parts of Europe. Yet habitat destruction has a far greater impact. A 1993 study in western North Carolina concluded that it takes 50 to 70 years for forest salamander populations to recover after their habitats are clearcut, and that an estimated 14 million salamanders have died from clearcutting in the national forests of western North Carolina. Habitat fragmentation by roads also takes a toll. For example, 43 percent of young southern leopard frogs observed emerging from one Florida pond were mowed down by cars upon leaving their natal pool.
Hopping to It
Much remains to be learned about amphibians and the causes of their declines and deformities. Long-term data are lacking for most populations, partly because amphibians havent traditionally received nearly as much attention as is lavished upon fish, large mammals, or birds.
But that is quickly changing. The Declining Amphibian Populations Task Force (DAPTF), founded by the World Conservation Union's (IUCN's) Species Survival Commission in 1990, is bringing together amphibian researchers from around the world. "It's been amazingly successful in serving as the nervous system for the declining amphibian phenomenon and getting information into one central location, then distributing it back out," says DAPTF chair Ron Heyer, a research curator at the Smithsonian National Museum of Natural History's division of amphibians and reptiles.
DAPTF provides seed grants to researchers in foreign countries who focus on amphibian declines. Yet until a few years ago, even many North American amphibians escaped scrutiny. Now several programs are seeking to build much needed long-term data sets on frog, toad, salamander, and newt populations. In 1996, the U.S. Geological Survey launched a continent-wide monitoring effort called the North American Amphibian Monitoring Program (NAAMP). The program's first component, a frog call survey, is modeled after the USGS's long-standing Breeding Bird Survey. A federal, interagency amphibian monitoring program also begins this year in 11 national parks and in some national wildlife refuges.
"It's as close as you're going to get of an idea of what's happening all over the whole patchwork of U.S. and Canadian landscapes," says NAAMP coordinator Linda Weir, who works with states and provinces to set up monitoring programs to coordinate data collection and recruit volunteers. (No Mexico monitoring routes have yet been assigned.) So far, 29 states have come on board, using 1,000 to 2,000 volunteers to cover about 1,000 roadside routes. Volunteers go out three to four times a breeding season to ensure they hit their routes during the breeding peaks of different species. A similar program recently began in Australia, while regular monitoring programs have been ongoing in Great Britain and in parts of continental Europe.
"I think the general public is more concerned about amphibians than most people realize," says Weir. The U.S. Geological Survey is tapping this concern through its Frogwatch program, which encourages volunteers to monitor a selected backyard or neighborhood pond. Meanwhile, an amphibian monitoring program has just begun at the Zoo's Conservation and Research Center (CRC) in Front Royal, Virginia. "We hope to develop a network of science teachers, school science clubs, and young naturalists who will work together to establish a continuous, long-term monitoring of local amphibian populations in ponds and streams," says Chris Wemmer, the Zoo's associate director for conservation. Teacher workshops are just beginning, and the program is being launched in the seven counties nearest CRC.
"We're particularly interested in getting kids to learn to love nature," says Wemmer. "You've got to do more than surf the web and watch TV if you want to make deep connectionsyou have to have personal involvement in the real world of nature. That's what this is all about."
Five years ago, it was exactly such a connection that inspired eight students to dive after frogs in a Minnesota farm pond. "It's been a wonderful lesson for the kids to see that answers aren't always clear-cut and quick," says Reinitz.
Some of the students continue to investigate the frog puzzle. "It's also great that such an important ecological question was brought to the forefront by kids," says Reinitz, "and that they can know that not all great discoveries have been made. That's why I encourage them to always keep asking."
Howard Youth is a Contributing Editor to ZooGoer.
Dan Suzio is a California-based photographer who specializes
in reptiles and amphibians. His work may be seen on the web
at www.suziophoto.com.
ZooGoer 29(2) 2000. Copyright 2000 Friends of the National Zoo. All rights reserved.
