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.
