The species diversity and ultimate survival of amphibians, one of the major groupings of vertebrate animals, face tremendous challenges today for reasons that are still not entirely understood. Since the 1980s, the pace of amphibian extinction has been alarming, and almost 200 identified species have become extinct. About one-third of all amphibian species have seen dramatic declines in populations, and are now considered to be endangered or threatened with extinction. See also: Biodiversity; Endangered species; Extinction (biology); Extinction of species
Amphibians (members of the class Amphibia) comprise one of the four major groups composing the superclass Tetrapoda of the subphylum Vertebrata (the other three classes are Reptilia, Aves, and Mammalia). Currently, there are approximately 7370 known species of extant amphibians, which are classified in three orders: Anura, consisting of the frogs and toads (≈6500 species); Urodela (Caudata), consisting of the salamanders and newts (≈670 species); and Apoda (Gymnophiona), consisting of the caecilians (≈200 species). See also: Amphibia; Anura; Apoda; Lissamphibia; Origins of modern amphibians; Urodela
The crisis now facing amphibians is occurring across the globe, with Central America, the Caribbean, Southeast Asia, and Australia showing the greatest overall reductions in numbers. Habitat destruction, especially the clearing of forests in the tropics, is one of the most important factors leading to the vast declines in amphibian populations. Still, even in locations where habitats have been preserved and protected, amphibians are vanishing. Investigations have revealed that chytridiomycosis, an emerging disease caused by the waterborne fungal pathogen Batrachochytrium dendrobatidis, is directly responsible for the spectacular loss of amphibian biodiversity in many locales. The pathogen that causes chytridiomycosis has a particularly adverse effect on the skin of amphibians. In infected animals, the resultant thickening (hyperkeratosis) of the skin inhibits its capability for respiration, water hydration, and salt adsorption, leading to death. So far, there are no adequate measures to prevent this devastating disease in the wild, so chytridiomycosis continues to affect numerous populations around the world. See also: Fungi; Skin
Other factors contributing to the loss of amphibians seem to include global climate change, contamination by chemical toxins and pollutants, higher levels of ultraviolet radiation, and the loss of native species resulting from predation by invasive species. Although not necessarily the chief reasons for the declines observed in amphibians, these factors and others are collectively increasing the already heavy casualties directly associated with habitat degradation and chytridiomycosis. See also: Global climate change; Hellbender salamanders; Invasion ecology; Invasive species and their effects on native species; Species and global climate change