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In the summer of 1995, middle school students on a field trip to a farm pond in southern Minnesota discovered large numbers of frogs with misshapen, extra, or missing limbs. About 50% of the northern leopard frogs they caught that day were malformed. Since then, reports of amphibian malformations from other parts of North America have drawn public attention. Species that have been reported with malformations include northern leopard frogs, wood frogs, bullfrogs, green frogs, mink frogs, gray treefrogs, Pacific treefrogs, spring peepers, American toads, long-toed salamanders, tiger salamanders, and spotted salamanders. Malformed amphibians are not a new phenomenon, but reports were only infrequent until recently. Since 1995, reports have become increasingly common, and a number of scientists -- herpetologists, developmental biologists, aquatic toxicologists, and parasitologists -- are looking for the cause(s). Possible Causes Hypotheses abound as to the causes of abnormal amphibian development in the wild. Occasional occurrances of malformations appear to be normal, since reports of malformed frogs exist from as long ago as 1740. In addition, there is evidence that a naturally occurring parasite -- a trematode -- can alter limb development in amphibians. The trematodes form cysts in amphibian limb buds that can disrupt limb development, potentially causing a range of deformity types (Sessions and Ruth 1990; see Bibliography). Some scientists are questioning whether the parasite hypothesis can explain the recent increase in malformation reports. Malformations may also occur as a result of xenobiotic chemicals -- chemicals humans add to the environment -- such as herbicides, insecticides, fertilizers, or byproducts of manufacturing. For example, some insecticides mimic a growth hormone that may cause amphibian embryos and larvae to develop abnormally (Maden 1993, Harmon et al. 1995; see Bibliography). Several biologists are conducting laboratory experiments to investigate the developmental mechanisms that may be disrupted or altered by such chemicals. In addition, biologists in Quebec found a higher rate of malformed amphibians in areas with a history of pesticide use than in sites not known to be exposed to pesticides (Ouellet et al. 1997; see Bibliography). However, other scientists argue against the xenobiotic chemical hypothesis, maintaining that no new pesticides, herbicides, or fertilizers have been introduced extensively in the past two years, when malformation reports suddenly increased. Not only do several potential causes exist, but factors may interact. For example, animals stressed by xenobiotic chemicals in their breeding habitat may be more susceptible to diseases or parasites. Or, some scientists have hypothesized that increased ultraviolet radiation (a result of the planet^s thinning ozone layer) is responsible for breaking down non-toxic chemicals into toxic ones. Separating out the potential causes is difficult. Without understanding all possible causes of malformed amphibians in the wild, we do not know for certain whether human health is also at risk. However, the possibility that malformed amphibians indicate a greater environmental problem is currently a real concern. According to the Minneapolis/St. Paul Star Tribune, Hillary Carpenter, a toxicologist with the Minnesota Department of Health, stated: "What^s driving this whole issue is not deformed frogs. It^s the potential for effects on human health." On the other hand, biology professor Stanley Sessions is not as concerned: "This whole deformed frog incident has been blown way out of proportion...these deformed frogs appear to have more to do with basic natural history than with human-caused environmental problems." Currently, there are few solid data on the causes of amphibian malformations, and even the extent of the problem (e.g., the rates of malformations and the full geographic range) is not well understood. Although several biologists in North America are focusing their research on the amphibian malformation phenomenon, the lack of long-term data hinders their ability to detect trends and draw sound conclusions. Furthermore, common themes among sites where malformations occur may not be apparent without samples that cover a wide area of the continent. This Reporting Center is designed to help overcome these problems. Amphibian Declines Many amphibian populations throughout the world are in decline. Although habitat loss is the most pervasive threat, populations of frogs, toads, and salamanders have disappeared from a variety of remaining habitats as well, including those in relatively undisturbed wilderness areas. Two species of gastric brooding frogs, native to Australia, have not been seen since the early 1980s. The golden toad of Costa Rica was last seen in 1989. The red-legged frog is no longer present in entire counties and valleys on the North American Pacific coast, where it was once abundant and common. These are just a few examples. People all around the world have told accounts of a "silent spring": where they used to hear the chorus of frogs or toads, they no longer do. Although amphibian populations fluctuate naturally -- they depend on widely varying climatic conditions such as rainfall -- several human-induced causes are also likely culprits and exacerbate the natural fluctuations. These causes operate at the local, regional, and global levels. Without a clear understanding of causes of amphibian malformations, the link between malformations and widespread amphibian declines remains uncertain. However, the fact that most malformations have been found in recently metamorphosed individuals suggests that malformed amphibians often do not survive into adulthood. Depending on the percent of malformed individuals in a population, this can potentially cause a specific population to decline.
Uploaded: 2/21/2004
Uploaded: 2/21/2004