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The Trouble with Frogs
When deformed frogs started hopping out of this country’s wetlands during the last decade, scientists and the public alike took notice. biologist David Skelly thinks he knows why the deformities occurred—and what they have to say about human health.
October 2002
by Bruce Fellman
At first glance, the Ward Marsh wildlife management area on the western border of Vermont is the epitome of nature at its most pristine. Surrounding the wetland is a grassy meadow through which deer and wild turkeys roam, and guarding it is a rugged cliff that shelters the Green Mountain State’s last remaining population of rattlesnakes. The marsh’s sparkling waters are rich with cattails, dragonflies, catfish, and birds, and this calm backwater of the Poultney River just might be amphibian heaven.
But in 1997, there was serious trouble in paradise. For when the resident leopard frogs began making the age-old transition from tadpole to adult, nearly half of the creatures emerged missing limbs, eyes, or other body parts.
The scene was unnerving, and it was repeated throughout the state. Since these animals spend so much of their early lives in close contact with the water, they’re often seen as the equivalent of the canaries that miners once carried to warn them of impending doom. Amphibians unable to hop in a straight line were surely a warning sign—but of what?
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“The environment is under a well-documented assault.”
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Last summer, ecologist David Skelly, an associate professor at the School of Forestry and Environmental Studies (FES), waded into Ward Marsh and other sites in Vermont as part of an ambitious effort to solve what remains a scientific mystery. Although there are many theories, no one knows precisely why the frogs suddenly failed to develop normally—or why the “hot spots” where deformities were prevalent tended, like fireflies in a meadow, to wink on and off.
“The key is to figure out why it happened then, and why it isn’t happening now,” says Skelly, a scientific detective on the trail of an old crime. “If we can understand what caused the abnormalities, maybe we can determine how to prevent them in the future.”
Armed with a $2.1 million, five-year grant from the National Institutes of Health (NIH) and the National Science Foundation (NSF), Skelly and Joseph Kiesecker, a former postdoctoral researcher at Yale and now an assistant professor at Penn State, are midway through a massive study aimed at discovering what can turn batrachian heaven into frog hell in Vermont and, it turns out, elsewhere. In the mid-1990s, observers in Minnesota spotted frogs that had developed too many legs and since then, deformed amphibians have been discovered in 44 states and 4 Canadian provinces.
For scientists hoping to find a single cause for the problem, nothing has seemed to add up. “The more I looked, the more I realized that there wasn’t going to be a magic bullet,” says Skelly. “The patterns we’re seeing don’t fit any one explanation. More likely, the cause will be a complex interaction among several factors.”
Rick Levey, a Vermont department of environmental conservation aquatic biologist who has monitored the deformities since the first reports started arriving, agrees. “When you examine everything that might be involved, it begins to read like a Chinese restaurant menu: one from column A, one from column B, and so forth,” says Levey. “I’m still hoping that the bulk of the abnormalities will be chalked up to fluctuations in natural conditions, but we all know that the environment is under a well-documented assault.”
Skelly and Kiesecker proposed looking at how the perfectly normal changes in nature and a multitude of human-caused insults to the natural world might come together to wreak havoc among frogs. This approach appealed to the NIH and the NSF. Two years ago, the federal agencies inaugurated a joint research program called “The Ecology of Infectious Diseases,” and the Skelly-Kiesecker proposal was among the first funded by the initiative. Deformed amphibians, the NIH and NSF felt, might have something profound to say about our own health.
“We’re trying to understand how human modifications of the natural world can influence the patterns of disease we see in frogs as well as in our own species,” says Skelly.
Amphibians seem to be an ideal model system for testing theories that could explain the emergence of new ailments, such as West Nile fever and Lyme disease, and the reappearance of old scourges, tuberculosis among them. “We can do experiments in the lab and in the field and modify the frogs' natural environment in ways we could never do with humans,” the ecologist continues. “Then, we can see if these changes alter an animal’s ability to cope with infection and if they have longer-term consequences for the population. We can also determine if there are ways in which we can minimize our impact.”
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“The tadpoles in the mesocosm grew way beyond anything in the natural world. I called them Frankentadpoles.”
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It is a tall order, and to ferret out answers, Skelly and his research crew—Susan Bolden, Nicole Freidenfelds, and Nancy Cothran, an FES master’s degree student—have spent much of the past year slogging through a wide variety of Vermont wetlands. Some of the ponds, marshes, and swamps under study are located in the relatively urban area of Burlington, while others are in the neighborhood of farm fields or hidden in undeveloped mountain hollows. The research followed a similar tack in Connecticut in 2001, surveying wetlands along an urban-to-rural gradient from city parks in Manchester to the relatively untouched woodlands of the Yale-Myers Forest in the northeastern part of the state. (Kiesecker and his colleagues are conducting comparable investigations in Pennsylania and upstate New York.)
In Vermont, the Skelly team routinely monitored some 40 sites, each of which is within the Lake Champlain watershed in the western part of the state. The study area began at Ward Marsh in the south and continued almost 200 miles north to the Canadian border, and the researchers gave special attention to four spots. These were among the sites that had been monitored since 1996 by Rick Levey, so there was already plenty of data that Skelly could use to determine what effect, if any, his experiments might have on the deformity situation.
Since the first reports surfaced almost ten years ago, scientists have attributed the abnormalities to a variety of factors. Among the favorites are parasites, pollutants, changes in ultraviolet light, shifts in the weather due to the El Nino phenomenon, amphibian overcrowding, even an increase in the number of amateur naturalists. (In response to the news, many conservation organizations enlisted corps of citizen observers; perhaps they were seeing a phenomenon that had always been present but that, for lack of human eyes, had been overlooked.) But while a “one size fits all” explanation might be desirable—if for no other reason than a single cause could suggest a single solution—Skelly’s lengthy experience in wetlands suggested that nature isn’t likely to be so accommodating.
For most of his career, the scientist has concentrated on the ecology of “temporary” ponds—“mudholes” that teem with life for part of the year and then dry to a dusty memory. “These are small, complete worlds,” says Skelly. “I work in places in which I can walk around the entire ecosystem and characterize it, and then sample the universe of nearby ponds, compare them, and experience first hand how dynamic these habitats are.”
Skelly began this pursuit early. “I had the great fortune to grow up in a swamp,” he explains. Raised in Wilton, Connecticut, then a largely undeveloped town in northern Fairfield County, the scientist recalls rearing frog eggs, culled from nearby wetlands, in styrofoam-lined milk coolers when he was 6 years old. His first love, however, was fishing, and after completing a bachelor’s degree at Middlebury College in 1987, he went to the University of Michigan with the intention of studying fish. But ecologist Earl Werner, his mentor at graduate school (and, since that time, a frequent collaborator), had begun to study amphibians. Skelly was quickly hooked on frogs, toads, salamanders, and the like.
As Skelly’s career progressed, ecology was making a transition from a descriptive science, in which practitioners spent years outdoors observing aspects of the natural world, to an experimental endeavor where nature could be mimicked in the laboratory. Among freshwater ecologists, the cattle watering tank, a fixture on dairy farms and ranching operations, became the set-up of choice, and in these tubs—researchers also used kiddie wading pools and plastic sweater boxes—scientists would put a standard “recipe” of water, nutrients, amphibians, and other aquatic life. By carefully measuring what happened to each component of these ersatz ponds, ecologists attempted to zero in on the fundamental principles, particularly the roles of predation and competition, that drove the ecosystem and determined winners and losers.
The results might have been statistically rigorous, but the more Skelly worked with cattle tanks and other “mesocosms,” the more he felt they weren’t providing an adequate proxy for nature. “The tadpoles in the tanks were growing superfast and getting enormous—way beyond anything I’d ever seen in the natural world,” he says. “I called them ‘Frankentadpoles.’”
Something was missing in the mesocosm, the underappreciated but critically important factors of disease and parasites. “We were raising tadpoles in what amounted to little quarantine wards,” says Skelly. “But we’ve learned that a wild tadpole, however healthy it looks, is usually infected with something that makes it grow slower. This is entirely normal, and we believe that infection and disease play an important, though overlooked, role in population regulation and in how communities are structured.”
And in why frogs developed abnormal limbs.
Skelly has developed an experimental approach which, he believes, provides a more realistic look at the basic workings of the natural world than can be seen in mesocosms. While he still uses tanks and wading pools, the ecologist does much of his work in the wetlands themselves. In Vermont, for example, the intensive research at the four sites involves raising tadpoles in special cages designed to help the scientists understand the importance of parasites in causing deformities.
In laboratory studies, biologists have shown that infection by trematodes, a group of parasites known to cause such human diseases as schistosomiasis, can result in the abnormal limbs found in frogs. And in a landmark paper published earlier this year, investigators demonstrated a convincing correlation in several western states between the presence of trematodes and instances of malformed and missing legs in amphibian populations.
But researchers note that correlation is not causation, so in the four Vermont study sites, Skelly and his crew put together an experiment that would enable scientists to see how—or if—parasites were working in the real world. A parasite is a plant or animal that makes its living at another organism’s expense; the trematodes of interest to deformity investigators actually need to exploit three “hosts” in order to prosper.
This complicated life cycle begins when a trematode egg, carried in the feces of an infected raccoon, snake, heron, or the like, is deposited in a pond. The egg hatches, and from it emerges a microscopic creature that swims off in search of snails, which are very common in wetlands. Once the parasite has located and entered a snail host, it bides its time. At a certain point, when it is ready for another round of travels, it undergoes a transformation, emerging from the snail as a cercaria, a sperm-like swimmer looking for tadpoles. If the mission goes as planned, the cercariae form persistent cysts in the infected animal, which, the parasite hopes, will soon be eaten by a predator. Once ingested, the cysts open and give rise to sexually mature trematodes, and inside the third host, these meet, mate, and lay eggs. The cycle starts anew.
Scientists suspect that the cysts, which are often found attached to the places from which limbs develop, are interfering with the process. From the parasite’s viewpoint, this is an ideal tactic. “It needs to get eaten by a predator to continue the life cycle, so what better thing to do than mess up limb development and make the host easier to capture?” says Skelly.
To show that this is actually happening in nature, both Skelly and Kiesecker set up a series of similar experiments. From the ponds they would study, the scientists harvested frog eggs, which were hatched and reared in parasite-free conditions in laboratories in New Haven and State College. The uninfected tadpoles were then brought back “home” and placed in cages built from plastic mesh.
In three of the cages, the mesh was too small to allow cerceriae to enter; in the other three, the parasites could cruise in at will. If all went as planned, the tadpoles in the smaller mesh cages would develop a normal complement of legs, while their vulnerable colleagues would show clear signs of abnormal limbs.
If this prediction came to pass, it would establish beyond a reasonable doubt that parasites played a major role in causing the deformities. But the two scientists also believed that it wasn’t going to be the entire answer.
As long as tadpoles have patrolled the wetlands, these animals have had to contend with parasites, and, except for a handful of rare reports, development has proceeded without a hitch. Something else had to be going on, too—something that made tadpoles unusually vulnerable.
Kiesecker believes he’s found at least one critical factor: a common herbicide called atrazine. Earlier this year, researchers showed that when tadpoles in a laboratory setting were exposed to levels of this chemical that were well within EPA limits in drinking water, some of the animals developed into hermaphroditic adults. And in a paper published in July in the Proceedings of the National Academy of Sciences, Kiesecker demonstrated that atrazine could greatly enhance the ability of trematodes to cause limb abnormalities in frogs.
“Our working hypothesis is that this chemical somehow surpresses the amphibian immune system and leaves the animals more susceptible to parasite infection,” says Kiesecker. “It’s not killing them—it’s a subtle effect.”
Together, trematodes and atrazine can, the theory goes, act in synergy. However, the past few years in Vermont and elsewhere have shown that often enough, they don’t act at all. The parasites are going through their complicated life cycle, low levels of the herbicide are washing off corn fields and entering wetlands, but in places where deformities had been common, the frogs are doing just fine.
This was just what occurred in Skelly’s cages in Vermont, a finding that mirrored the relative paucity of limbless frogs in the state in 2002. The explanation, the ecologist surmises, lies in a third suite of factors: the year-to-year changes that take place in the natural environment.
Water levels fluctuate annually. If they’re too low, tadpoles may be crowded too closely together, become overly stressed, experience higher levels of atrazine or other pollutants, and wind up as relatively easy targets for parasites. On the other hand, if precipitation is unusually abundant, as it was this spring in Vermont, fewer of the stress-producing situations are present and, Skelly would predict, fewer of the frogs should be stricken with abnormalities.
The scientist will have a better handle on the complex interaction of factors that are required to create trouble among frogs—and people—in a year or so when he gets the results of an experiment just underway at Yale-Myers. There, Skelly is working with a dozen woodland ponds, all currently free of amphibian deformities. He’s thinning the forest around half of the ponds and leaving the trees around the other half. In half of the altered ponds, Skelly is adding snails; he’s doing the same thing in half of the untouched ponds.
“We’re modifying the vector of infection directly, and we’re also modifying the context in which infection might occur,” he says.
Skelly calls this his “field of dreams” experiment. “We’re building various scenarios and seeing who comes,” the ecologist continues, noting that what happens may have important implications for frog and human health. “People tend to consider only the vector of a disease, that is, animals get sick because there’s more of the infection source present. But there’s another way to think about it—the reason you get sick is because the environmental context has changed. The way we change the environment may come back to change us.”  |
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