Introduction to The Declining Amphibian Population
The golden toads of the Monteverde Cloud Forest Preserve, a rain forest in the mountains of Costa Rica, were numerous and easy to find as recently as 1987. The yellow-orange males and the brownish females would be seen during breeding season after they emerged from their burrows. In 1988, however, scientists reported a huge drop in the number of these toads--they saw only 10 of them. The next year, they could find only one. In December 1997, after no golden toads had been seen for several years, biologists said that the species was almost certainly extinct. Moreover, they said, the toad's disappearance could not be attributed to natural causes. Although animal numbers fluctuate naturally from year to year, the golden toad's precipitous decline led biologists to suspect that human activities were involved. Whatever the cause of its disappearance, the golden toad was not the only amphibian species in the Monteverde preserve to be affected. Six other species of frogs or toads in the forest also declined in numbers during the same period.
The frogs and toads of the Monteverde rain forest, like other amphibians, are characterized by a life cycle that is partially terrestrial (on land) and partially aquatic (in water). A typical amphibian lives on the land until it is time to breed and lay eggs, when it returns to the water. Eggs hatch into aquatic larvae (immature organisms), which live in the water until undergoing metamorphosis (changing into adults). Not all amphibians, however, go through this cycle. Many species live entirely on land or in water. Amphibians are also characterized by a lack of protective body covering, such as scales, feathers, or fur. The skin of amphibians is exposed and easily penetrated by substances in the water and air, allowing them to breathe through their skin as well as with their lungs.
Scientists have been reporting since the late 1980's that certain populations (regional groups within a species) of amphibians are in decline. And in the mid-1990's, there was another worrisome development. Many people began seeing amphibians with various kinds of malformations, including extra limbs and abnormal sex organs. Scientists are concerned about these problems because amphibians are thought to be good indicators of environmental health. The ability of amphibians to readily absorb substances from the water and air makes them more sensitive than most other animals to environmental conditions. Amphibians may thus be providing an early warning of a deteriorating environment. If so, researchers caution, whatever is harming amphibians might also pose a threat to human health.
Some biologists, however, see little cause for concern. They contend that many amphibian population declines are just natural fluctuations. Some scientists also question whether malformations are truly on the rise among amphibians. They note that deformities in frogs had been reported as far back as the 1800's.
Researchers were engaged in many studies in the 1990's to learn what--if anything--was happening to amphibians. By 1998, these studies had uncovered a number of important clues that helped investigators better understand this amphibian mystery.
Frogs, Toads, and Their Kin
There are more than 4,000 species of amphibians in three main groups: frogs and toads (3,500 species), salamanders and newts (350 species), and caecilians (see SIL ee uns) (160 species). Frogs and toads have short tailless bodies with long hind legs. They range in length from 1 centimeter (0.4 inch) to 35 centimeters (14 inches). Toads differ from frogs in having rougher skin and spending most of their life on land. Most frogs and toads lay their eggs in water. Tadpoles (the aquatic stage of frogs and toads) develop from these eggs and live in the water until growing legs and becoming land-based adults. Many frogs, however, continue to spend much of their time in or around water.
Salamanders and newts have long bodies with tails. They range in length from 3 centimeters (1.2 inches) to 160 centimeters (63 inches). Although most salamanders and newts have two pairs of limbs, some aquatic forms have no hind limbs and only small forelimbs. Caecilians are limbless, wormlike amphibians found throughout the tropics. They range in length from 7 centimeters (2.8 inches) to 150 centimeters (59 inches). Some species lay eggs, while others give birth to their offspring.
The scientific community first began to learn about amphibian population declines at the First World Congress of Herpetology [the study of reptiles and amphibians], held in Canterbury, England, in 1989. David Wake, director of the Museum of Vertebrate Zoology at the University of California at Berkeley, presented evidence of dwindling frog populations in central and northern California. In 1990, an international group of about 40 biologists gathered in Irvine, California, to discuss research findings. As the scientists exchanged information, it became apparent to them that amphibian disappearances were a global phenomenon. The most disturbing development was that amphibians seemed to be disappearing not only from areas known to be disturbed by human activity but also from areas thought to be pristine wilderness.
In 1991, the Species Survival Commission, a division of the World Conservation Union (an international organization made up of more than 500 environmental groups), established the Declining Amphibian Populations Task Force (DAPTF). The DAPTF, based at the Open University in Milton Keynes, England, recruited more than 1,200 scientists, including me. Our research goals were to determine: (1) if amphibian numbers are, in fact, declining; (2) if so, why; and (3) what can be done to halt these losses?
Evidence For Population Declines and Malformations
Proving that amphibian populations are truly dwindling has been challenging for scientists. A major problem with documenting amphibian declines is that, in most regions and for most species, there are no reliable data on past population sizes to serve as a basis for comparison with recent observations. Because populations fluctuate naturally--due to such factors as drought, variations in the availability of food, and changing numbers of predators--an apparently worrisome decline could be just a short-term phenomenon. In addition, it is difficult to confirm the suspected extinction of a species. Just because a species is not found by a group of researchers at a certain time and in a certain place does not necessarily mean that the species no longer exists there.
Because of these difficulties, scientists participating in the DAPTF focus their studies on areas where historical data on amphibians are available. And they make observations over several years under a variety of conditions. A number of such studies have found unmistakable declines in amphibian populations. For example, in the mid-1990's, my research team (affiliated with Iowa Lakeside Laboratory near Milford) found a hundredfold to a thousandfold decline in amphibian numbers in the Prairie Pothole Region of northwest Iowa, compared with the early 1900's. Of the seven native species of amphibians in the region, we believe that one, Blanchard's cricket frog, has become extinct. A second species, a type of salamander called the mudpuppy, has not been seen in the region in about 30 years. It's hard to say if this species is extinct however, because its silent, nocturnal (active at night) nature makes it difficult to track down.
Other research teams have made similar findings. A group at the University of Alberta in Canada discovered that the northern leopard frog, once common in Alberta, had virtually disappeared since 1979. In the Canadian province of Quebec, researchers documented a dramatic decline in the number of chorus frogs and chorus-frog habitats throughout the St. Lawrence River Valley since 1988. And Australian scientists found that 14 species of frogs in the rain forests of eastern Australia had either disappeared or greatly declined in number since the late 1970's. These are just a few examples among many.
Some Populations Are Stable
In contrast to these reports of declines, other studies have found that some amphibian populations are stable. For example, researchers studying amphibians in the Canadian province of New Brunswick over several years found no evidence that any species there have declined in number. And studies since the early 1980's at the Savannah River Ecology Laboratory, affiliated with the University of Georgia, have discovered that amphibian populations on the Georgia-South Carolina border undergo considerable fluctuations. But the researchers have found no indications that any of the populations in the area are in permanent decline.
Some studies have obtained contrasting results by looking at different populations of the same species. For example, in North America, studies indicate that northern populations of Blanchard's cricket frogs have declined, while southern populations have remained robust. Such studies underscore the fact that amphibians in different regions confront different environmental conditions. In addition, within particular regions, some species of amphibians may be in decline while others are stable. Although researchers have documented cricket frog declines in the upper Midwest, many other amphibian species that share the cricket frog's wetland habitat in the region appear to be doing well. These sorts of findings indicate that different amphibian species respond in their own individual ways to the same kinds of environmental influences.
While some researchers are studying trends in amphibian populations, other investigators are focusing on amphibian malformations. The phenomenon of amphibians with various kinds of physical abnormalities came to national attention in 1995, when a group of Minnesota students discovered hundreds of deformed frogs during a field trip to a wetland. Soon, other reports of malformed amphibians were being reported from many parts of the United States and Canada. Malformations have also been reported from Japan. Observed deformities have included misshapen, extra, or missing limbs; missing or shrunken eyes; and abnormally small sex organs. The relationship between amphibian population declines and amphibian malformations--if any--was still unclear in 1998. Observers have seen malformations in amphibian populations that appeared to be stable, and they have noted a decline in numbers among many amphibian populations that have no malformations.
An analysis of the various studies done in the 1980's and 1990's has led most biologists affiliated with the DAPTF to conclude that amphibians in many parts of the world are suffering population declines and malformations. Researchers in 1998 were trying to determine the causes of these problems. They were investigating causes both originating in nature and stemming from human activities.
Some Possible Causes of Amphibian Problems
One natural factor that scientists think is probably involved in some amphibian population declines is drought. Because most amphibians lay their eggs in water, droughts can have a devastating effect on them. Food shortages and greater numbers of predators can also decimate amphibian populations. However, biologists believe that when a population experiences a steady decline over many years, other factors must be involved.
One additional possibility of natural origin is disease. Since the late 1980's, many amphibians in Oregon have been sickened by a fungus known as Saprolegnia. Researchers in 1997 reported that an unknown type of protozoan or fungus was killing frogs in Panama's Fortuna Forest Reserve, and frogs in Australia were found to be dying from a similar cause. Also in 1997, investigators in the United States implicated a bacterium called Aeromonas hydrophila in the disappearance of several populations of western toads in Colorado. Parasitic organisms have been implicated in certain amphibian malformations. Flukes--a type of parasitic worm--burrow into tadpoles and form small cysts, which some investigators believe can cause deformities as the tadpole develops.
Although natural causes may contribute to amphibian problems, human disruptions of the environment undoubtably play a more important role. In fact, most biologists believe that the destruction and alteration of wetlands is the single biggest cause of amphibian population declines. In September 1997, the U.S. Fish and Wildlife Service released a report estimating that more than 405,000 hectares (1 million acres) of wetlands--which include various kinds of watery terrain, such as marshes and swamps--vanished in the United States between 1985 and 1995. This loss of wetlands has harmed many animals besides amphibians, including numerous species of birds and mammals. Construction has been a major cause of the problem. The building of roads, housing developments, and commercial strips fragments the landscape, reducing wetlands and cutting them off from one another.
The use of surviving wetlands for aquaculture (fish farming) has also imperiled wildlife. Since the 1980's, an increasing number of natural wetlands in North America have been converted into aquacultural basins for the raising of both game fish and bait fish. In Wisconsin alone in 1997, some 7,000 wetlands were made available to commercial interests for the creation of fish farms. During the conversion of a wetland to an aquacultural basin, toxic chemicals are sometimes added to the water to kill existing fish in order to make room for fish of commercial value. These chemicals may also kill amphibians. In addition, when wetlands are too shallow for aquaculture, they may be dredged to make them deeper. Amphibians, which generally prefer shallow water, may not be able to survive in such an altered habitat.
Nonnative animals released in aquacultural basins, streams, and other habitats can have devastating effects on the ecology of an area by preying on amphibian eggs and larvae. Salmon and trout added to Sierra Nevada mountain streams in Yosemite National Park, for example, have been implicated in the decline of the park's mountain yellow-legged frogs. Even bullfrogs, which were introduced to western North America from eastern North America around 1900, have caused the decline of many native western amphibians by competing with them for food and preying on them. Nonnative animals may also spread diseases to native amphibians.
The Role of Pesticides
Another factor that scientists believe is responsible for amphibian problems is the use of agricultural pesticides, which may be transported by water or wind into amphibian habitats. Pesticides can produce fatal genetic mutations and developmental malformations in animals and can weaken their immune systems. Biologists suspect that agricultural pesticides have played a role in the collapse of the amphibian populations in the Monteverde rain forest and in Alberta and Yosemite National Park.
Even pesticides that have been banned from use may be killing amphibians. Studies in the 1990's found that toxic residues from the breakdown of the insecticide dichloro-diphenyl-trichloroethane (DDT) were present in the bodies of frogs in Point Pelee National Park, in southern Ontario, Canada. Although DDT, which was once widely used to control mosquitoes in the park, was banned in the United States and Canada in the 1970's, its toxic breakdown products are still present in the environment. In addition, DDT itself is carried to North America by winds and migratory birds from countries where it is still used.
Scientists have found that certain pesticides, fertilizers, and other synthetic chemicals mimic female hormones. These synthetic chemicals, sometimes called endocrine disrupters, have been linked to defects of sex organs in amphibians and other animals in various locations in the United States and Canada, including the Minnesota region where amphibian deformities were first observed in 1995. Such animals may be unable to breed.
Another environmental threat that may be harming amphibians is acid rain (precipitation containing sulfuric or nitric acid), formed when certain kinds of pollutants in the atmosphere combine with other chemicals. The pollutants come mainly from the burning of fossil fuels. Excessively acidic water and soil can kill amphibians, affect their behavior, or cause them to suffer developmental disorders. In England, population declines among natterjack toads have been attributed, in part, to acid rain.
Other Factors That May Be Harming Amphibians
Many studies indicate that amphibian declines are most prevalent in high-altitude areas, a finding that points to increases in ultraviolet radiation from the sun as a possible cause of some amphibian deaths and malformations. This intense form of light energy, which can cause genetic changes and other physical damage, is strongest at high altitudes, where there is less atmosphere to filter damaging solar rays. Ultraviolet radiation may have grown stronger in these areas since the 1970's due to the thinning of the ozone layer, a protective blanket of oxygen molecules that absorbs ultraviolet rays. Scientists believe that the ozone layer has been eroded by certain reactive chemicals--principally chloro-fluorocarbons--that have drifted into the upper atmosphere.
In December 1997, Oregon State University researchers reported that ultraviolet radiation was killing embryos (an early developmental stage) of the long-toed salamander in lakes of the Cascade Mountains. The scientists said that more than 90 percent of the embryos they studied that had been exposed to direct sunlight died before hatching or hatched with deformities. In contrast, almost all of the embryos that the researchers had shielded from sunlight with ultraviolet filters developed normally.
One last factor that should not be overlooked is the collection of amphibians--mostly frogs--for biological-supply companies and the food industry. The biological-supply trade provides dissection specimens for school classrooms. Such collecting is not well regulated and has reduced the number of frogs in many areas. Hunting frogs for their legs, a gourmet delicacy, has also depleted frog populations.
The various factors affecting amphibians do not occur in isolation. Scientists believe that several negative influences probably work in combination to push an amphibian population into decline. For example, pesticides and ultraviolet radiation could weaken amphibian immune systems, making them more vulnerable to bacterial infections and parasitic diseases.
Effects On Both Nature and People
A die-off of amphibians can have a profound impact on a wetland area. Amphibians are often the most abundant vertebrates (animals with a backbone) in wetlands. Therefore, their disappearance can lead to a proliferation of insects, their main prey. In addition, reptiles, birds, and mammals that feed on adult amphibians or their larvae are liable to face a serious food shortage.
The problems threatening amphibians may have implications for human life. Some researchers believe that humans are being affected by the same hormone-mimicking chemicals linked to amphibian sexual abnormalities. A few disputed studies in the 1990's found that men were producing fewer sperm than in previous decades, and some scientists speculated that the decline might be due to endocrine disrupters. Other studies in the 1990's linked lower IQ scores in children to these chemicals, but that research, too, was controversial. A more accepted scientific conclusion was that the higher levels of ultraviolet radiation that seemed to be affecting amphibians may be causing a greater incidence of human skin cancer and eye disease.
The loss of amphibian species may in itself have human implications, particularly in the development of new drugs. In 1973, for example, a frog called the gastric brooding frog was discovered in Australia. This frog was of interest to scientists because it was somehow able to switch off the production of digestive chemicals in order to brood its eggs in its stomach. Researchers hoped that learning how the frog did this might lead to new treatments for stomach ulcers. Unfortunately, the gastric brooding frog has not been seen in the wild since 1979 and is thought to be extinct.
Steps Toward Protection
The loss of such a potentially valuable amphibian species--as well as those species whose primary value was in preserving life in wetland communities--emphasizes the importance of protecting these vulnerable creatures. The most important step we can take is to preserve amphibian habitats. In the upper Midwest, my colleagues and I have found that the ideal amphibian habitat is a series of wetlands containing no fish and connected by stretches of undisturbed landscape. In other regions of the United States and the world, ideal amphibian habitats include old-growth forests, diverse prairies, open savannas, flowing streams, and vegetation-fringed lakes.
Another important step that could protect amphibians, and people as well, is a further reduction of chemical pollutants in the environment. By the mid-1990's, a number of nations had taken action to reduce the use of chemicals responsible for acid rain and ozone depletion. However, some other chemicals suspected of harming amphibians--including endocrine disrupters--were still being widely used in the United States and other countries.
I and other scientists affiliated with the DAPTF acknowledge that more research is needed before the extent and causes of amphibian disappearances and deformities are fully understood. Nonetheless, we believe that the findings to date clearly show that there is indeed a problem and that humans are partly to blame for it. Habitat destruction, pesticide use, and ozone depletion seem to be magnifying such natural factors as disease and drought. Perhaps the possibility that amphibians are sounding an early warning of environmental damage with potential human consequences may prompt people to practice better stewardship of the Earth as we enter a new millennium.