Amphibian species are distributed worldwide and include frogs, toads, salamanders, and newts. They are cold-blooded animals, or ectotherms, and their physiology is affected by their external environment. Amphibian populations face an extinction crisis, as they have experienced dramatic population decreases worldwide since the 1980's. An estimated one-third of amphibian species are currently in decline, with many amphibian species now either threatened or extinct.

In the late 1980's, scientists began to report mass mortalities of amphibians at an alarming rate, with causes not well understood. Although amphibian extinctions have occurred globally, declines have been particularly significant in the western United States, Central and South America, and Australia. Among the amphibian species that experienced dramatic population declines are the golden toad, Bufo periglenes, of Costa Rica and many harlequin frog species (Atelopus) that were once common in South America. These frog species are listed as critically endangered. These species lived in pristine areas, so their extinctions raised particular concern, because they could not be linked to human activities.

A number of potential explanations for amphibian declines have been proposed, with many of the causes also affecting other organisms. The causes of amphibian declines are likely to be complex; many probably act synergistically. Some causes may include destruction of both terrestrial and aquatic habitats, introduced species, overexploitation, pollution, and pesticides. Many amphibian declines, however, have occurred in pristine habitats where such effects are unlikely. Therefore, although habitat loss is known to have affected amphibians for decades, recent research has focused on the effects of environmental contaminants, increased ultraviolet radiation, emerging diseases, and climate change.

Since the health of amphibian populations is thought to be an indicator of overall environmental health, reports of global amphibian declines have led to considerable public concern. The causes of amphibian declines might also threaten other species of animals and plants. One reason amphibians are thought to be indicator species for environmental health is their sensitivity to the environment. Amphibians' skin is extraordinarily thin, which makes them very sensitive to even small changes in temperature, humidity, and air or water quality. Their skin is also very permeable, which makes them very sensitive to toxins in both land and water environments.

Climate change has probably contributed to the observed decline of many amphibian species worldwide. Although there is no simple answer to what is causing amphibian declines, many factors related to global warming are believed to play a role. Global warming is thought to initiate amphibian declines by triggering epidemics such as fungal diseases. Ozone layer depletion as a result of increased pollution leads to greater ultraviolet (UV) radiation exposure, which damages the delicate skin of amphibians, as well as their eyes, eggs, and immune systems.

Amphibian reproduction is affected by climate change: Changes in breeding behavior have been linked to increased temperatures due to global warming. UV radiation may also reduce hatching success and the rate of survival to metamorphosis. Global warming increases the metabolic rate of toads during hibernation and thus affects their body condition. Environmental temperature has a dramatic effect on amphibians' immune systems, so climate change may affect their defenses against invading pathogens. Tropical amphibian species, especially those in higher altitudes, are particularly susceptible to adverse effects related to global warming.

Although other pathogens play a role, chytrid fungi are believed to be responsible for many amphibian declines. About two-thirds of the 110 known harlequin frog species in Central and South America disappeared in the 1980's and 1990's; the cause was thought to be an infectious disease triggered by changes in environmental temperatures. These abundant frog deaths led to the discovery of a pathogenic fungus, Batrachochytrium dendrobatidis, which belongs to a family known as chytrids. By moderating temperature extremes that used to keep the growth of this fungus in check, global warming has arguably created ideal conditions to support its growth and reproduction. However, although many scientists support a connection between outbreaks of chytrid fungus, amphibian population declines, and global warming, this link is now controversial, with some questioning the role of temperature changes in fungal outbreaks.

The disease caused by chytrid fungi, chytridiomycosis, is fatal for otherwise healthy animals. Frogs with chytridiomycosis generally exhibit skin lesions that affect respiration across the skin, resulting in mortality. Since its discovery, the fungus has been linked to many amphibian extinctions. Amphibian declines due to chytrid fungus are most common at higher elevations in the tropics, and chytrid fungus is believed to be the cause of decline in threequarters of frog species in Costa Rica and Panama and in species in the United States as well.

Bibliography:

1)         Cherry, Lynne, and Gary Braasch. How We Know What We Know About Our Changing Climate: Scientists and Kids Explore Global Warming. Nevada City, Nev.: Dawn, 2008.

2)         Hofricher, Robert. Amphibians: The World of Frogs, Toads, Salamanders, and Newts. Toronto: Key Porter Books, 2000.

3)         Lannoo, Michael, ed. Amphibian Declines: The Conservation Status of United States Species. Berkeley: University of California Press, 2005.

4)         Linder, Greg, Sherry K. Krest, and D. W. Sparling. Amphibian Decline: An Integrated Analysis of Multiple Stressor Effects. Pensacola, Fla.: Society of Environmental Toxicology and Chemistry, 2003.

5)         Pounds, J. A., et al. "Widespread Amphibian Extinctions from Epidemic Disease Driven by Global Warming." Nature 439 (2006): 161-167.

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