Endangered Species Handbook

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Aquatic Ecosystems

Threatened Species

    Approximately 41.4 percent of the 5,435 animals listed by the 2000 IUCN Red List, 2,251 species, are native to aquatic environments. Many more are in lower categories of threat such as near threatened or possibly threatened. Of these, 88 percent are native to inland or freshwater. At least 790 species of fish are threatened with extinction, 627 of these native to freshwater. This is an indication of the effects of dams, pollution and introductions of non-native fish that out-compete native species. Reptiles, totaling 111 species, are threatened by habitat destruction through wetland drainage and pollution, as well as demands of the pet, meat and skin trades that are pushing a growing number of turtles, crocodiles, chameleons and other lizards toward extinction. The threats to amphibians, especially frogs and toads, have grown dramatically in recent years, with environmental factors, such as UV radiation from ozone thinning, pesticide use, wetland destruction and pollution, playing important roles.

(Higher Categories of Threat)

Marine Inland Water Total
Mammals 25 31 56
Birds 105 78 183
Reptiles 9 111 120
Amphibians 0 131 131
Fish 163 627 790
Crustaceans 0 408 408
Insects 0 125 125
Mollusks 13 420 433
Corals & Anemones 2 0 2
Worms 3 0 3
Total Animals 320 1,931 2,251
Plants 0 14 14
Grand Totals 320 1,945 2,265


    Based on 2000 IUCN Red List of Threatened Species Compiled by Craig Hilton-Taylor. International Union for the Conservation of Nature and Natural Resources, Gland, Switzerland, Cambridge, U.K. 

    The number of threatened whales, dolphins, porpoises, manatees and seals has greatly increased in recent years. Moreover, many species have been upgraded in status to more endangered categories. Pollution, killing for meat and drowning in fishing nets play roles, but clearly more research needs to be carried out as to whether there is sufficient food for the large whales, such as the Blue, Fin, Sei, Gray and Northern Right Whales, none of which has recovered from past whaling. In the early 1980s, the IUCN, in its various Red Data Books, listed only three species of dolphins and six whales as threatened. In the 2000 IUCN Red List, 25 dolphins, six porpoises and 35 whales are listed in various categories, a dramatic increase. Marine ecological systems are becoming imbalanced as a result of overfishing. Gray whales have been washing ashore dead in unprecedented numbers on the North American Pacific coast. Many are thin and appear to have died of starvation, either from overfishing of their food supply or effects of changes in ocean temperatures brought about by El Nino or La Nina, which can greatly alter the types of fish remaining in an area. Destruction of sea beds is caused by fishing trawlers who rake sea beds for scallops and groundfish, obliterating entire communities of anemones, starfish, sea grass and habitats for a variety of fish and invertebrates. 

    A major increase in the number of threatened sea birds has also occurred. The number of threatened albatross has risen from two in the early 1980s to 20 in recent years. Similarly, only two species of penguins were threatened in the early 1980s, while 12 were listed in 2000. Petrels, small nocturnal sea birds that were once abundant, had 12 threatened species in the 1980s, mainly from disturbance of their nesting islands. At present, more than 40 species of petrels and storm-petrels are threatened with extinction. A major cause is long-line fishing for tuna, swordfish and other billfish, with baited hooks that attract albatross and petrels. Sea birds and birds that live along coastlines, lakes and rivers have been endangered by marsh drainage and pollution, human encroachment on their nesting habitats and invasions by exotic species of plants and animals. 

    The vast majority of threatened fish listed by the IUCN are freshwater species, but this may be an indication of our ignorance about the status of marine fish. The 1990 IUCN Red List included 751 species of fish in various categories (WCMC 1990). That number rose to 979 in 1994 (WCMC 1993), and in the 1996 IUCN Red List, 1,091 species were listed in all categories. The compilers of the 1996 edition of the IUCN Red List noted that only a small portion of the world's fish have been evaluated in terms of their status, and almost no marine species have been researched (Baillie and Groombridge 1996). The 2000 IUCN Red List listed 752 species in its three highest categories, an increase of 18 species from 1996. Worldwide, the status of the majority of fish has not been appraised. Ignorance of marine fish populations has contributed to their depletion in commercial fisheries, which are pushing many species toward extinction (see Trade chapter). 

    North American freshwater fish are perhaps the best known in terms of the status of the world's fish because of long-term biological studies and the state Natural Heritage Programs, which have a data bank of rare, endemic, declining and imperiled species. Two reports compiled by The Nature Conservancy, 1997 Species Report Card: The State of US Plants and Animals (Stein and Flack 1997) and the book Precious Heritage: The Status of Biodiversity in the United States (Stein et al. 2000), were based on data from the Natural Heritage Programs in all 50 states. The organization found that 16 species of United States fish have become extinct in recent times, and one species is possibly extinct; 93 species are critically imperiled, 88 species are imperiled and 116 are vulnerable. This total of 314 species comprises 36.1 percent of all American fish, which makes for an extremely high rate of endangerment (Stein and Flack 1997). 

    A major threat to native fish in the United States is the introduction of non-native species, usually game fish such as exotic trout, by state fish departments. Rainbow trout, for example, have been introduced in areas where they are not native, and have out-competed and endangered native cutthroat trout in the West, as well as introduced disease caught in fish hatcheries. Few other countries have undertaken such detailed studies of endangered native wildlife, but in countries where freshwater ecosystems have been greatly altered by channeling, diversion, dams and pollution, research would likely find similar rates of endangerment. In China, 80 percent of major rivers are so degraded they no longer support fish (Barlow 1999). 

    Frogs and toads represent the most dramatic rise in threatened species of any vertebrate. Their plight may be indicative of the serious environmental threats facing the Earth. Because of their thin, porous skin, frogs are extremely sensitive to environmental contaminants. Within the past few decades, a large number of frog and toad species have disappeared all over the world. Many have been found with grotesque deformities and diseases. What is happening to the frogs appears to be an early warning of serious threats to the Earth's aquatic ecosystems and to its atmosphere. The scale of the recent disappearance of frogs around the world has not been seen since mass extinctions that occurred millions of years ago. The ecological effects will be significant because of their important role in consuming insects and as food items for a large number of animals, from birds to reptiles. 

    In the 1979 IUCN Red Data Book of Threatened Animals, a world review found 18 species of frogs and toads to be in various categories of peril; in the 1990, the number of threatened frogs and toads had almost doubled to 35, and in 1994, 130 species were listed ‒ more than triple the previous total (WCMC 1993). The 141 species in various categories in the 1996 IUCN Red List represented a fourfold increase since 1990 and an almost eightfold increase since 1979. This total grew to 173 frogs and toads in all categories in the 2000 IUCN Red List, with 112 in the three highest categories of threat. Thus, threatened frogs and toads have increased 10 times in number from 1979, a trend not seen in any other large group of vertebrates since such studies began. 

    The threats to these amphibians are not regional. Frogs are threatened wherever they exist, declining in 140 countries, with status research continuing (Stolzenburg 1997). A worldwide phenomenon is occurring, endangering amphibians and causing extinctions at a spiraling rate. Among the first to disappear were two newly discovered and highly unusual frog species that had been acclaimed as remarkable by scientists around the world. These frogs came from the rainforests of Australia and had astounded the scientific world with their breeding biology. Their life history was so extraordinary that the man who first described it had his paper rejected by a prestigious journal for being implausible. Michael J. Tyler, associate professor of Zoology at the University of Adelaide, discovered these species, known as gastric brooding frogs (Rheobatrachus silus and Rheobatrachus vitellinus), and was beginning research on potential applications of their amazing ability to turn off gastric acids to the treatment of human stomach ailments. These frogs were filmed before their disappearance, which documented that they swallow fertilized eggs that grow in the female's stomach and emerge six weeks later as tiny froglets when she opens her mouth (Yoffe 1992). In 1980, only a few years after their discovery, this species and others in the area vanished; they have not been seen since (Yoffe 1992). 

    The dazzling golden toad (Bufo periglenes) also disappeared suddenly. Jay M. Savage of the University of Miami came upon this species, native to the cloud forests of Costa Rica, in 1964 in Monteverde Cloud Forest Reserve, witnessing large gatherings of males during the breeding season (Phillips 1994). National Geographic Society filmmakers, in making the film, “Rainforest,” recorded the assemblages of hundreds of these colorful toads perched on rocks among misty streams, and also filmed their far less flamboyant mates. Less than 2 inches long, male golden toads were bright, shiny orange, in contrast to the slightly larger females who were dark olive to black with spots of red encircled by yellow (Phillips 1994). Depicted on tourist posters and protected in its reserve, the Golden Toad's future seemed assured. No studies were carried out to discover its haunts during the rest of the year, however (Phillips 1994). Tourists flocked to Monteverde, many drawn to see the beautiful toads. In 1987, an American biologist, Martha Crump, witnessed the last breeding group of golden toads. "They've been described as little jewels on the forest floor, and that's really the impression you get," she recalled years later (Phillips 1994). With a grant from the National Geographic Society, she returned in 1988 for two seasons of research (Phillips 1994). In 1988, only 10 toads were seen, and only one male was in the breeding area; in 1989, only one toad was seen (Phillips 1994). None have been seen since (Yoffe 1992); the protected area was enlarged to 26,000 acres in the early 1990s to preserve as much forest as possible. 

    Two other inhabitants of this cloud forest, a species of translucent glass frog (Yoffe 1992) and an endemic harlequin frog (Stolzenburg 1997) have become very rare. A total of 20 other frog species are also missing from this region, based on a survey of Monteverde by a University of Miami biologist in 1996 (Blakeslee 1997). In Ecuador's high Andes, an estimated 50 species of frogs have disappeared since 1990. In Puerto Rico's rainforests, 12 of 18 endemic frogs are extinct or nearly so (Stolzenburg 1997). 

    The first World Congress of Herpetology, for the study of reptiles and amphibians, convened in 1989, and many of the 1,300 participants began comparing notes on the shocking fact that the subjects of their research were disappearing from under their very eyes. A comprehensive study was undertaken under the guidance of the IUCN's Declining Amphibian Populations Task Force (DAPTF) (www.open.ac.uk/daptf) to standardize methods of study of amphibian population changes and the environmental factors that may be affecting amphibian reproduction and survival (Stebbins and Cohen 1995). The 1989 conference inspired a symposium the following year that addressed this problem, recruiting 1,000 researchers in 40 countries. The reports from these researchers are published in a bulletin known as the Froglog, published by DAPTF on its website. 

    What is happening to the world's frogs? Many of the traditional causes still contribute to declines in amphibian populations. Marshlands and boggy areas continue to be filled in and polluted; pesticides, herbicides and other toxic chemicals are entering aquatic ecosystems, causing mortality and mutations. Introduced predatory fish and even exotic frogs have eliminated some frog populations (Yoffe 1992). Trade in frogs for the gourmet food market and for high school biology dissections has also played a role in the declines of some species. Some tropical frogs are known to have succumbed to a protozoan, which kills many and weakens the immune systems of others, making them vulnerable to a host of other pathogens (Blakeslee 1997). The disappearing frogs are not all from polluted areas. Some are native to remote wilderness areas, national parks and other pristine areas. The most surprising possible cause for their disappearance is the increased ultraviolet (UV) radiation resulting from the thinning of the ozone layer. This has been the subject of much research. The thinning of the Earth's ozone layer is caused by Chlorofluorocarbons (CFCs), chemicals used as coolants, aerosol propellants and for other industrial purposes. When released into the atmosphere, they turn into chlorine and destroy the thin-but-vital layer of ozone that shields the Earth from harmful UV radiation. Chlorine-based solvents have also had this effect. First noticed in the 1970s and early 1980s when holes appeared in the ozone layer at the Earth's poles, many years passed until the cause was proven. Those who discovered this phenomenon were awarded the 1995 Nobel Prize in Chemistry, marking the first time this prize was ever awarded for achievement in environmental science. The recipients were Dr. F. Sherwood Rowland of the University of California, Dr. Mario Molina of the Massachusetts Institute of Technology and Dr. Paul Crutzen of the Max Planck Institute for Chemistry in Germany. 

    Holes and thinning of the protective ozone layer have allowed an increase in the amount of injurious ultraviolet rays reaching the Earth, which has brought about a dramatic rise in the number of human skin cancer cases, as well as declines in amphibian populations. Although the use of CFCs has been banned in new appliances and air conditioners in the United States, the chemical is still used in older cars and refrigerators, escaping to the air from malfunctioning units and improper disposal of these appliances. Illegal CFC imports into the United States have been documented, and some countries still allow their use. CFCs will take at least a century to disappear from the upper atmosphere, according to scientists. 

    In 1996, Yale University researchers made a discovery that may help stem the breakdown in the ozone layer. A chemical extract obtained from rhubarb and related plants, sodium oxalate, can chemically break down CFCs when heated to 520 degrees F. (Sharkey 1997). The practical use of this discovery is years away, but it may prove to be effective in controlling this lethal phenomenon. 

    As ancient bridges between land and sea, amphibians were named from the Greek words amphi and bio, meaning living a double life, indicating they were both aquatic and terrestrial. Frogs and salamanders are the only two major groups of amphibians, and they form an evolutionary link between fish and reptiles. First emerging in the Triassic period 200 million years ago, frogs have outlasted dinosaurs, and until recently, most biologists considered them hardy and adaptable species (Yoffe 1992). The vast majority of the approximately 4,100 species live in tropical areas. They must hibernate to survive cold, temperate winters, and relatively few species have evolved this adaptation. 

    The breeding biology of frogs and toads is a fascinating study in variety. There are frogs that develop marsupial pouches like tiny kangaroos to carry their fertilized eggs, and species in which the males incubate tadpoles in their vocal sacs until they emerge (Stebbins and Cohen 1995). The parent of one tropical frog hops across the forest floor carrying a single tadpole sticking with a mucus to its back; it climbs a tree and steps into a bromeliad plant, placing the tadpole into a tiny amount of water (Roberts 1996). The female returns to lay unfertilized eggs for the tadpoles to eat; she must remember which bromeliads, of the thousands in the area, contain her tadpoles. When the female enters the bromeliad containing her tadpole, it instinctively wriggles, causing vibrations in the water which stimulate the mother to lay eggs for it to eat (Roberts 1996). Wendy Roberts, a biologist who has studied the many reproductive methods used by tropical frogs, believes that as long as frog populations do not continue to disappear, more discoveries of the extraordinary variety of their reproductive biology, parental care and behavior will be made (Roberts 1996). Some toads have even adapted to desert mud holes by burying themselves and entering a torpor-like state until it rains again. 

    The decline of North America's frogs and toads afflicts fully one-third of the 86 species (Yoffe 1992). Their disappearance in wilderness areas was documented as early as the 1970s. David Bradford, a biologist now with the Environmental Protection Agency, studied the Yosemite toad (Bufo canorus) and Mountain yellow-legged frog (Rana muscosa) in Yosemite National Park in the 1970s at 26 sites. When he returned in 1989 and visited each site, he found frogs had disappeared from all but one site (Yoffe 1992). A 1992 survey concluded that three of the eight toads and frogs native to the Yosemite region have nearly disappeared: the red-legged frog (Rana aurora), which has since been listed on the US Endangered Species Act; the Foothill yellow-legged frog (Rana boylei); and the Great Basin spadefoot (Scaphiopus intermontanus) (Yoon 1996). The Yosemite toad declined or disappeared at nine of the 13 original sites where a 1915 survey had found them abundant (Yoon 1996). Because of the longevity of toads, some living to be 35 years old, declines may go unnoticed as long as adults are alive, even though all their offspring may be dying year after year. Stebbins and Cohen (1995) documented declines in a number of once-common species in their text, A Natural History of Amphibians. Northern leopard frogs (Rana pipiens) and nine other species have become rare or absent in former haunts (Tyning 1995). The latter species has been dissected by the millions in high school biology classes, many of which require the students to kill the frogs, often by cruel means. 

    Dr. Andrew Blaustein, a herpetologist at Oregon State University, studied the Western toad in lakes in the Cascades Range at 4,000 feet for many years. By the mid-1980s, he found that these toads were becoming rare, yet in the same habitat, Pacific tree frogs (Hyla regilla) seemed to be thriving (Blaustein 1994). Female Western toads produce 12,000 eggs in long strips that are laid in unshaded ponds, and then they disappear into the forest, leaving the eggs to hatch in the shallow water (Blaustein 1994). Dr. Blaustein began to find eggs that did not develop properly, turning white, and then dying and leaving a fetid, viscous mass (Blaustein 1994). Other amphibians of these mountains, the Cascades frog (Rana cascadae) and the red-legged frog, had also disappeared completely from most of their range (Blaustein 1994). 

    Dr. Blaustein and a colleague decided to conduct outdoor and indoor experiments to test the reactions of frog eggs to ultraviolet radiation. Trays of frog and toad eggs in water were placed outdoors, and some were exposed to UV rays; others screened from UV-B rays, the most harmful type of radiation; still more eggs were brought into the lab and kept in darkened conditions (Blaustein 1994). Ninety percent of the eggs raised indoors, as well as those screened from UV rays outdoors, developed normally, while 40 percent of the unscreened eggs exposed to UV rays died (Blaustein 1994). Here was proof of the effect of present levels of UV rays. Dr. Blaustein and other biologists have concluded acid rain is combining with UV radiation in some areas to kill frogs and toads (Yoon 1996). 

    Some frogs and toads are dying of unknown causes. One biologist studying the Western toad (Bufo canorus) in a wilderness area in Colorado in the 1970s came upon dozens of dying toads, barely able to move because of their puffy red legs (Yoffe 1992). She took several of them to a veterinarian and was told their immune systems had collapsed, making them vulnerable to infection (Yoffe 1992). Biologists in Massachusetts have found wood frog (Rana sylvatica) tadpoles dying of disease outbreaks in three of four recent years (Knight 1995). 

    Grotesquely deformed frogs have been discovered recently in many parts of the United States. In 1995, a biology class found frogs with shocking malformations near Henderson, Minn. Some were missing both hind legs; others had as many as six legs, and at least one had an eye growing in its mouth. Another had a leg growing from its stomach. Many had only one leg, one eye, four tangled hind legs, misshapen limbs and tails, missing or shrunken eyes and small sex organs (Associated Press 1996). These deformed frogs were shown on national television news programs throughout the country. Researchers found similar frogs in 1996 and 1997 in California, South Dakota, Wisconsin, Vermont and Massachusetts, as well as in Quebec, Canada. Up to 35 percent of frogs in these areas may be deformed. Within a year, deformed frogs were seen in 54 of 87 counties in Minnesota. A herpetologist from the University of Minnesota at Morris examined 10,000 frogs in the summer of 1996, finding the greatest number of abnormalities in frogs that spent the majority of their lives in water, rather than coming to water only to lay eggs. Vermont's leopard frogs had a 45 percent deformity rate, and for the first time, deformed salamanders were found. Three five-legged salamanders were found in Massachusetts, an ominous sign this phenomenon is spreading to another type of amphibian. 

    At the Scripps Institute in California, laboratory tests on frogs produced similar deformities when they were exposed to the commonly used mosquito pesticide, methaprine. This chemical affects growth hormones, causing them to malfunction. In September 1997, officials of the Minnesota Pollution Control Agency announced they reproduced the deformities when frog eggs were raised in tap water from areas with deformed frogs. They issued a public health alert based on this research, providing bottled water to people living in these areas. One Minnesota biologist found internal and external abnormalities in these frogs. EPA studies found deformities when frogs were exposed to ultraviolet rays as well. These findings indicate certain pesticides and UV radiation present major threats to amphibians. Other studies have shown parasites can also cause deformities (Yoon 1999).* 

    North American researchers have organized the Great Lakes Declining Amphibians Working Group, as well as the North American Amphibian Monitoring Program, maintained by the US Geological Survey of the Department of the Interior. The FROGWATCH USA website gives advice to concerned citizens to learn more about aiding this situation. The US Fish and Wildlife Center provides information about the population status of various amphibians, as does AmphibiaWeb, run by noted herpetologist, Dr. George B. Rabb. 

    The disappearance of frogs and toads may seem insignificant to many people. From an ecological point of view, however, these animals play an important role in controlling insect populations. After frogs were nearly extinguished in Bangladesh to supply gourmet tables in France with frog legs, mosquito numbers and the incidence of malaria skyrocketed (Yoffe 1992). Frogs themselves are a major food item for a wide variety of animals, from storks to raccoons. They are an important link in the food chain, and their extinctions may have a major effect on a wide variety of other species and even evolution itself (Yoffe 1992). 

    From a human health point of view, frogs and toads have proven to be the source of many valuable medical compounds. Dart-poison frogs of Central and South America are more terrestrial than species that spend their lives in water, and to protect themselves, they have developed some extremely potent poisons that they excrete through their skin. Natives have used these chemicals on the tips of their arrows for hundreds of years to kill prey as large as monkeys. These tiny, brightly colored frogs are being studied by the National Institutes of Health for the alkaloid compounds found in their skin, which are used in research on nerve and muscle function, and as anti-convulsants and anti-arthritics (Sharp 1995). Another compound found in the skin of an Ecuadorian dart-poison frog has 200 times the pain-killing power of morphine (Sharp 1995).


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