At the end of the twentieth century, biologists uncovered growing evidence of a global decline in amphibian populations. AmphibiaWeb (http://amphibiaweb.org), a conservation organization that monitors amphibian species worldwide, reported in 2006 that more than 160 species have become extinct in recent decades and at least 2,400 other species are declining in population. Amphibian declines have been documented worldwide, though the degree of decline varies across regions. Areas that have been hardest hit include Central America and Australia. In the United States, amphibian declines have been concentrated in California, the Rocky Mountains, the Southwest, and Puerto Rico. Particularly disturbing is the loss of numerous populations within protected and relatively pristine wildlife refuges.
Scientists are concerned because a large number of amphibian species—particularly frogs—has become extinct over a very short period of time. Other species are either declining or showing high levels of gross deformities, such as extra limbs.
The golden toad, named for its unusual and striking orange color, is a prime example of the global amphibian decline. Over a three-year period, golden toads disappeared inexplicably from their only known habitat in the Monteverde Cloud Forest Reserve in Costa Rica. In 1987 herpetologists observed an apparently healthy golden toad population estimated at 1,500 adults along with a new generation of tadpoles. The following year, in 1988, there were only eleven toads. In 1989 only a single surviving toad was found. It was the last individual on record for the species (Britton Windeler, "The Extinction of the Golden Toad [Bufo periglenes]—Symptom of a Worldwide Crisis," 2005, http://jrscience.wcp.muohio.edu/fieldcourses05/PapersCostaRicaArticles/TheExtinctionoftheGoldenT.html).
Recent amphibian declines appear to result from a combination of causes. Loss of habitat is a major factor in the decline of numerous amphibian species, as it is for many endangered species. The destruction of tropical forests and wetlands—ecosystems that are rich with amphibians—has done particular damage to populations. In the United States, deforestation is blamed for the loss or decline of salamander species in the Pacific Northwest and Appalachian hardwood forests. In addition, some amphibians have lost appropriate aquatic breeding habitats, particularly small bodies of water such as ponds. These aquatic habitats are often developed or filled in by humans, because they appear to be less biologically valuable than larger aquatic habitats.
Finally, habitat fragmentation may be particularly harmful to amphibian species that migrate during the breeding season. These species require not only that both breeding and nonbreeding habitats remain undisturbed, but also that there be intact habitat along migration routes.
Pollution is a second major factor in global amphibian declines. Because amphibians absorb water directly through skin and into their bodies, they are particularly vulnerable to water pollution from pesticides or fertilizer runoff.
Furthermore, air pollution by substances such as chlorofluorocarbons (CFCs) has reduced the amount of protective ozone in the Earth's atmosphere. This has resulted in increased levels of ultraviolet (UV) radiation striking the Earth's surface. UV radiation has wavelengths of 290 to 400 nanometers (nm). Wavelengths between 290 and 315 nm are called UV-B radiation and are the most dangerous, because they can damage deoxyribonucleic acid (DNA) by producing chemicals called cyclobutane pyrimidine dimers. (See Figure 7.2.) Exposure to UV-B radiation causes genetic mutations that can prevent normal development or kill eggs. Increased UV-B levels particularly affect the many frog species whose eggs lack shells and float on the exposed surfaces of ponds. Tadpoles and adults are also at risk, because of their thin delicate skins.
Many amphibian species have also been affected by the introduction of nonnative species that either compete with them or prey on them. These include fish, crayfish, and other amphibians. The bullfrog, the cane toad (a very large frog species), and the African clawed frog (a species often used in biological research) are some of the invasive species believed to have affected amphibian populations. In addition, introduced trout are blamed for the extinction of several species of harlequin frogs in Costa Rica. It is hypothesized that trout consume tadpoles. Similarly, introduced salmon have affected native frog populations in California.
Amphibian diseases caused variously by bacteria, viruses, and fungi have devastated certain populations. Of particular importance in recent years is the chytrid fungus. This fungus attacks skin, and was first identified in 1998 in diseased amphibians. There are often no symptoms initially, but eventually affected individuals begin to shed skin and die. The precise cause of death is not known, though damage to the skin can interfere with respiration. The chytrid fungus is believed to be responsible for the demise of numerous species in Australia and Panama. In 2000 it was also documented in populations of the Chiricahua leopard frog in Arizona
FIGURE 7.2 The effects of ultraviolet-B radiation on a living cell Erin Muths and Steve Corn, "Why Is UV-B 'BAD'" in Amphibian Decline: Still an Unexplained Phenomenon—From the Rain Forest to Rocky Mountain National Park, U.S. Department of the Interior, U.S. Geological Survey, Fort Collins Science Center, Undated, http://www.fort.usgs.gov/products/presentations/amphibian/sld035.html (accessed March 6, 2006)and the boreal toad in the Rocky Mountains. Officials at the Klondike Gold Rush National Historic Park in Skag-way, Alaska, reported in May 2006 that five of nine western toads evaluated there during the summer of 2005 had tested positive for the fungus. The disease was being aggressively investigated as a cause of a decline in toad population in southern Alaska during recent years (http://www.nps.gov/applications/digest/printheadline.cfm?type=Announcements&id1/44457).
Global warming is blamed for destroying unique habitats such as cloud forests (forests containing large amounts of water mists) in tropical regions, resulting in the loss of some amphibian species.
Quirin Schiermeier in "The Costs of Global Warming" (Nature, January 26, 2006) reported that global warming was also aggravating infectious diseases in the frog populations of Central and South America. The scientists found that warmer temperatures were associated with increased cloud cover over tropical mountain areas. These conditions were conducive to increased growth of the deadly chytrid fungus.
Many amphibian species are vigorously hunted for food, the pet trade, or as medical research specimens.
Amphibian deformities (see Figure 7.3) first hit the spotlight in 1995, when middle-school students discovered large numbers of deformed frogs in a pond in
FIGURE 7.3 A frog showing deformed and extra limbs. The high incidence of amphibian deformities in the United States is cause for concern. (JIM Visuals.)Minnesota. By 2000 scientists had documented malformed frogs in forty-four states and fifty-seven species. According to FrogWeb, an online service of the National Biological Information Infrastructure (http://frogweb. nbii.gov/index.html), rates of deformity as high as 60% have been documented in some local populations.
The high incidence of amphibian deformities in U.S. species appears to have multiple causes, as no single hypothesis accounts for all the different types of deformities seen. The most common deformities include missing hind limbs and toes, missing feet, misshapen feet, missing eyes, deformed front legs, and extra legs. Some of these malformations are believed to be related to a parasitic trematode, or flatworm, which in experiments causes the development of additional limbs. Aquatic trematodes have increased in number due to human activity, via a complicated chain of events. First, fertilizer runoff increases nutrient levels in ponds, allowing more algae to grow. An increase in algae results in a larger population of algae-eating snails, and snails host juvenile parasitic trematodes. Trematodes move on to frogs when they mature, forming cysts in the vicinity of developing frog legs. Chemical pollution and UV radiation may account for some of the other observed deformities.
The U.S. Geological Survey (USGS) set up a system whereby members of the public can report observations of deformed amphibians. The North American Reporting Center for Amphibian Malformations (NARCAM) is managed by the USGS National Biological Information Infrastructure and the University of Georgia's Savannah River Ecology Lab. Reports can be submitted online at the Web site http://frogweb.nbii.gov/narcam/.