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Imperiled Amphibians and Reptiles - Amphibians

species frogs frog salamander

Amphibians represent the most ancient group of terrestrial vertebrates. The earliest amphibians are known from fossils and date from the early Devonian era, some 400 million years ago. The three groups of amphibians that have survived to the present day are salamanders, frogs and toads, and caecilians. Table 6.1 describes some of the major amphibian groups in North America.

"Amphi-" means "both," and amphibians get their name from the fact that many species occupy both aquatic and terrestrial habitats. In particular, a large number of amphibian species undergo a dramatic change called

TABLE 6.1
Major amphibian groups

A. Completely aquatic
1. Salamanders (for example, hellbender, mudpuppy, siren, amphiuma, neotenic1 ambystomatid salamanders).
2. Frogs (for example, African clawed frog; introduced)
B. Lentic (still water) breeding/semi-terrestrial adults
1. Salamanders (for example, ambystomatid salamanders, newts2)
2. Frogs (for example, spotted frogs, wood frogs, treefrogs, toads)
C. Lotic (running water) breeding/semi-terrestrial adults
1. Salamanders (for example, red and spring salamanders)
2. Frogs (for example, foothill yellow-legged frog, tailed frog)
D. Completely terrestrial
1. Salamanders (for example, red-backed salamander, slender salamanders)
1Animals reach sexual maturity but retain the larval form.
2The eastern red-spotted newt (Notophthalmus viridescens) has an aquatic larval form that metamorphoses into a terrestrial subadult form (red eft). When the newt reaches sexual maturity (3-7 years) it makes a few more changes (morphological and physiological) and returns to the water for the rest of its life.
SOURCE: J. K. Reaser, "Major Amphibian Groups of North America," in Amphibian Declines: An Issue Overview, Federal Task Force on Amphibian Declines and Deformities, Washington, DC, 2000

metamorphosis, in which individuals move from an aquatic larval stage to a terrestrial adult stage. In many frog species, for example, aquatic, swimming tadpoles meta-morphose into terrestrial jumping frogs. In the process, they lose their muscular swimming tails and acquire fore-limbs and hind limbs. Many amphibian species occupy terrestrial habitats through most of the year, but migrate to ponds to breed. However, there are also species that are either entirely aquatic or entirely terrestrial. Whatever their habitat, amphibians generally require some moisture to survive. This is because amphibians pass some oxygen and other chemicals in and out of their body directly through their living skin, using processes that require water to function.

A large number of amphibian species are in serious decline due to factors such as habitat loss, pollution, and FIGURE 6.1
This Asian box turtle, native to China, is seriously threatened due to demand for trade. (Photograph by David Northcott. Reproduced by permission of the Corbis Corporation.)
climate change. Amphibians are particularly vulnerable to pollution because their skin readily absorbs water and other substances from the environment. For this reason, amphibians are frequently considered biological indicator species. There are currently 30 amphibian species listed with the Fish and Wildlife Service as either threatened or endangered. The 21 U.S. species listed—11 salamanders and 10 frogs and toads—appear in Table 6.2.

Sudden Disappearances

At the end of the twentieth century, biologists uncovered growing evidence of an unexplained global decline in amphibian populations. AmphibiaWeb, a conservation organization that monitors amphibian species worldwide, reported in 2004 that at least 200 amphibian species had experienced serious population declines in the last few decades. In addition, no fewer than 32 amphibian species have gone extinct. 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 U.S., 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.

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 11 toads. In 1989 only a single surviving toad was found. It was the last individual on record for the species.

Concern regarding declining amphibian populations led then-U.S. Secretary of the Interior Bruce Babbitt to meet with amphibian biologists in 1998. These scientists reported that a large number of amphibian species—particularly frogs—had become extinct over a very short period of time. They also noted that numerous other species were either declining or showing high levels of gross deformities, such as extra limbs, and that amphibians were dying out in unexpected places, such as protected national parks in the western United States. Secretary Babbitt commented:

Many of these frogs and amphibian species have been in an evolutionary relationship with our landscape for millions of years, and when all of a sudden they start to

TABLE 6.2
Endangered or threatened amphibians, February 2004

Status Species name
T Coqui, golden (Eleutherodactylus jasperi)
T Frog, California red-legged (Rana aurora draytonii)
T Frog, Chiricahua leopard (Rana chiricahuensis)
E Frog, Mississippi gopher (Rana capito sevosa)
E Frog, mountain yellow-legged (Rana muscosa)
T Guajon (Eleutherodactylus cooki)
E Salamander, Barton Springs (Eurycea sosorum)
E Salamander, California tiger (Ambystoma californiense)
T Salamander, Cheat Mountain (Plethodon nettingi)
E Salamander, desert slender (Batrachoseps aridus)
T Salamander, flatwoods (Ambystoma cingulatum)
T Salamander, Red Hills (Phaeognathus hubrichti)
T Salamander, San Marcos (Eurycea nana)
E Salamander, Santa Cruz long-toed (Ambystoma macrodactylum croceum)
E Salamander, Shenandoah (Plethodon shenandoah)
E Salamander, Sonora tiger (Ambystoma tigrinum stebbinsi)
E Salamander, Texas blind (Typhlomolge rathbuni)
E Toad, arroyo (=arroyo southwestern) (Bufo californicus [=microscaphus])
E Toad, Houston (Bufo houstonensis)
T Toad, Puerto Rican crested (Peltophryne lemur)
E Toad, Wyoming (Bufo baxteri [=hemiophrys])
E = Endangered
T = Threatened
SOURCE: Adapted from "U.S. Listed Vertebrate Animal Species Report by Taxonomic Group as of 02/17/2004," Threatened and Endangered Species System (TESS), U.S. Fish and Wildlife Service, Washington, DC, 2004 [Online] http://ecos.fws.gov/tess_public/TESSWebpageVipListed?code=V&listings=0#E [accessed February 17, 2004]

just, in the blink of an eye, disappear, there's clearly some external cause that's probably related to something that we are doing across the broader landscape. The deformities are particularly ominous because of the potential human implications as well.

Recent amphibian declines and deformities appear to result from a combination of causes.

HABITAT DESTRUCTION.

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 U.S., deforestation has caused the loss or decline of at least seven salamander species in the Pacific Northwest and 16 salamander species in Appalachian hardwood forests. Global climate change has also destroyed unique habitats such as cloud forests (forests containing large amounts of water mists), resulting in the loss of cloud forest amphibian species. 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 non-breeding habitats remain undisturbed, but also that there be intact habitat along migration routes.

POLLUTION.

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.

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 UV radiation striking the Earth's surface. Exposure to UV radiation causes genetic mutations that can prevent normal development or kill eggs. Increased UV levels particularly affect the many frog species whose eggs float on the exposed surfaces of ponds.

INVASIVE SPECIES.

Many amphibian species have also been affected by the introduction of non-native 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 used in much 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.

EPIDEMICS.

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 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 and the boreal toad in the Rocky Mountains.

HUMAN COLLECTION.

Many amphibian species are vigorously hunted for food, the pet trade, or as medical research specimens.

Amphibian Deformities

Amphibian deformities (see Figure 6.2) first hit the spotlight in 1995, when middle-school students discovered large numbers of deformed frogs in a pond in Minnesota. Deformed frogs have since been found in 44 states in the United States. These include representatives of 38 different frog species and 19 different toad species. In some populations, over 60 percent of individuals are deformed.

FIGURE 6.2
A frog showing deformed and extra limbs. The high incidence of amphibian deformities in the United States is cause for concern. (JLM Visuals)

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. More algae results in more 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.

Salamanders

Salamanders are tailed amphibians. The group contains over 500 described species, including the newts. The majority of salamanders are fairly small in size, most often six inches long or less. The Chinese and Japanese giant salamanders, which grow to as large as five feet long, are the largest of all amphibians. In 2004, salamanders comprised 11 of the 21 U.S. amphibians on the Endangered Species List. Some endangered salamanders, including many cave species (see Figure 6.3), have highly restricted habitats. The Barton Springs salamander, for example, is only found in a single locale in and around the Barton Springs pool in Austin, Texas. The Barton Springs salamander has been the subject of contentious debate between conservationists and those who wish to guarantee free recreational use of the pool.

GIANT SALAMANDERS.

There are two species of giant salamanders, the Chinese giant salamander and the Japanese giant salamander. These are by far the largest currently living amphibian species, reaching lengths of up to five feet. Both are listed with the Fish and Wildlife Service and FIGURE 6.3
The Texas blind salamander has been listed by the Fish and Wildlife Service as endangered since 1967. Because it lives in underground caves, the salamander has only vestigial eyes, found below the skin. (U.S. Fish and Wildlife Service)
are highly endangered. Giant salamanders are aquatic, and have folded and wrinkled skin that allows them to absorb oxygen from their watery habitats. The Chinese giant salamander is found in fast mountain streams in western China. Despite official protection, the species is endangered partly because of hunting for food or medicine. The Chinese giant salamander is also harmed by loss of habitat and aquatic pollution. Its close relative, the Japanese giant salamander, is also endangered and protected. This species inhabits cold, fast mountain streams in northern Kyushu Island and western Honshu in Japan. Japanese giant salamanders have been successfully bred in captivity.

Caecilians

Very little is known about most species of legless, worm-like amphibians called caecilians. Some caecilians are aquatic, but most of these elusive animals are underground burrowers that are difficult to locate and to study. Caecilians generally have very poor eyesight because of their underground habitat—some have no eyes at all or are nearly blind. There are 160 described species of caecilians. All live in tropical climates. Because so little is known about this group, it is difficult for environmentalists to assess the level of endangerment of these animals. Although there are no currently listed species, the loss of tropical habitats worldwide suggests that many caecilians are likely imperiled.

Frogs and Toads

Over 5,000 species of frogs and toads have been described worldwide, making this by far the most diverse group of living amphibians. Most occupy tropical habitats, though two species are found within the Arctic Circle. Many frog species go through a swimming tadpole stage before metamorphosing into a tailless, jumping, adult frog. However, in some species, eggs hatch directly as juvenile froglets, which are miniature versions of the adults. Tadpoles are most often herbivorous, although there are some carnivorous tadpoles, including cannibalistic species. Adult frogs are carnivorous and catch prey with their sticky tongues. Altogether, ten frogs and toads were listed by the Fish and Wildlife Service as threatened or endangered in 2004.

GASTRIC-BROODING FROGS.

There are two species of gastric-brooding frogs, both found in Australia. Gastric-brooding frogs are described as timid and are often found hiding under rocks in water. These species were only discovered in the 1970s, and, unfortunately, went extinct only a decade after their discovery. One species was last seen in the wild in September 1982; the other was last seen in March 1985. Gastric-brooding frogs get their name from their unusual reproductive strategy—females brood their young in their stomachs! During brooding, the mother does not eat and does not produce stomach acids. The gestation period lasts about eight weeks, and as many as 30 tadpoles may be in the brood. Juveniles eventually emerge as miniature froglets from the mother's mouth. Although it is not certain what led to the extinction of gastric-brooding frogs, one hypothesis is that populations were killed off by the chytrid fungus, which is also responsible for the decline of other frog species.

CALIFORNIA RED-LEGGED FROG.

The California red-legged frog, made famous by Mark Twain's short story "The Celebrated Jumping Frog of Calaveras County," experienced a significant decline during the mid-twentieth century. By 1960 California red-legged frogs had disappeared altogether from California's Central Valley, probably due to the loss of 70 percent of their habitat. The California red-legged frog, the largest native frog in the western United States, was officially listed as a threatened species by the Fish and Wildlife Service in 1996.

California red-legged frogs require riverside habitats covered by vegetation and close to deep water pools. They are extremely sensitive to habitat disturbance and water pollution—tadpoles are particularly sensitive to varying oxygen levels and siltation (mud and other natural impurities) during metamorphosis. California red-legged frogs require three to four years to reach maturity and have a normal life span of eight to ten years.

Water reservoir construction and agricultural or residential development are the primary factors in this species' decline. Biologists have shown that California red-legged frogs generally disappear from habitats within five years of a reservoir or water diversion project. The removal of vegetation associated with flood control, combined with the use of herbicides and restructuring of landscapes, further degrade remaining habitat. Finally, non-native species have also attacked red-legged frog populations. These include alien fish predators as well as competing species such as bullfrogs.

In 2004 California red-legged frogs were known to occupy approximately 238 streams or drainages, primarily in central and southern California. Only four localities are known to support substantial populations (over 350 individuals) of adult frogs. The recovery plan for the California red-legged frog includes eliminating threats in current habitats, restoring damaged habitats, and re-introducing populations into the historic range of the species. The U.S. National Park Service helped to preserve one current frog habitat by altering water flow in the Piru Creek connection between Lake Piru and Pyramid Lake, located in the Los Angeles and Los Padres National Forests about 60 miles northwest of Los Angeles. This also benefited another threatened species, the arroyo southwestern toad.

The Fish and Wildlife Service has taken measures to preserve habitat in the foothills of the Sierra Nevada, in the central coastal mountains near San Francisco, along the Pacific coast near Los Angeles, and in the Tehachapi Mountains. Protected frog habitats have also been established in Marin and Sonoma Counties. The Contra Costa Water District east of San Francisco Bay has established protected habitat areas in an attempt to compensate for habitat destruction caused at the Los Vaqueros watershed and reservoir. Finally, captive breeding of California red-legged frogs is being considered for feasibility at the Los Angeles Zoo, in a coordinated effort with The Nature Conservancy.

GUAJÓN.

The threatened web-footed guajón is a Puerto Rican cave-dwelling frog species. Its decline has resulted largely from introductions of alien species such as mongooses, rats, and cats, all of which eat unhatched guajón eggs. In addition the species has experienced habitat loss from garbage dumping in caves and deforestation for agriculture, roads, and dams. Deforestation also creates the potential for future environmental disasters such as flash floods, which drown adult frogs and destroy nests. Encroaching agriculture causes pollution from fertilizer runoff. Finally, the guajón, with its large white-rimmed eyes and phantom-like appearance, is frequently killed by superstitious local residents who believe the mere sight of the animal can bring disaster.

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