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.
