Along with other zoo favorites, such as the orangutan, the gorilla, and various lemurs, tigers and giant pandas are threatened with extinction—the total disappearance of a living species—unless they are defended with strict protective measures.

Throughout the world, in the ever-shrinking wilderness areas, wild animals and plants are facing the ever-increasing encroachment of humans. From the vicuñas of the Andes and the whales of the open seas to the delicate orchids of Hawaii—everywhere there are animals and plants whose survival hangs in the balance. Action to save these species is of utmost importance for the future of all living things.

Extinction is nothing new. Since life first appeared on Earth billions of years ago, animals and plants have lived and died, and species have evolved and become extinct. Why and how this occurs is not always clear. Did the dinosaurs, which dominated Earth for millions of years, disappear because of massive changes in global climate? Did the last of their kind become extinct because enormous meteorites filled the sky with dust, blocking sunlight and causing plants to die? There are an endless number of possible scenarios.

What scientists do know, however, is that animals and plants either adapt or fail to adapt to changes in their environment. If they fail to adapt, or if the changes are so rapid that it is impossible to adapt, they become extinct.

Interactions Among Species

Until recently, most environmental changes took place relatively slowly, over thousands or millions of years. It was rare for one species to be directly responsible for the extinction of another. Predators might reduce the population of prey animals, but before the prey disappeared, the population of predators would be reduced because of the lack of food, thereby allowing the prey species to rebound. Such a balance can be seen in the 10-year population cycle of lynx and snowshoe hares in Canada. When the number of hares is high, lynx thrive, producing more young. In the following years, there are more lynx to eat more hares, thus reducing the hare population. Soon, because of the decrease in the number of hares, the lynx have less food available. During such times of stress, the lynx reproduce less, and fewer of their young survive. As their number declines, the population of hares increases, and the cycle begins once again.

A similar equilibrium is generally maintained between other predators and prey across the world. For this reason, scientists think that the direct effects of one species on another were probably not a major cause of extinctions in the far-distant past.

However, the presence of one species can indirectly cause the extinction of another. When several species with similar habits live in the same region, for instance, they compete for food, nesting sites, and the other necessities of life. Perhaps one species may be able to feed higher in the trees because it is slightly taller or climbs better. Thus, during years when there is a shortage of food, the better-adapted species has a greater chance of survival than the one that cannot feed so high or climb so well. It is also able to reproduce more successfully than the less-adapted one, producing more young and raising more of them to maturity. The offspring consume more of the food supply and put further pressure on other species, contributing to their extinction. Similar competition can occur over water supplies during drought years or over protected nesting sites when there are numerous predators. Through the gradual effects of such indirect competition, many species have become extinct.

Humans and Other Species

In recent centuries, the rate of extinction has quickened because of human activities. Unlike other animals, which slowly evolve the adaptations they need to survive, human beings can quickly change the environment on a large scale. Although some animals, such as beavers, can make changes in their territories, the alterations are relatively limited in extent. People, on the other hand, can drastically transform huge areas in a very short time. They dam rivers to form reservoirs, and clear forests and irrigate deserts to raise crops and animals for food. These activities change the environment in such a way that the native plants and animals must adapt, relocate, or die. In this sense, humans compete with wildlife. Furthermore, in order to protect crops and domestic animals, humans kill species that are perceived as competition or a threat.

Scientists believe that prehistoric people were at least partially responsible for the extinction of some of the larger North American mammals. The great mammoths, the huge ground sloths, and perhaps even the wild horses of the plains are animals that became extinct soon after humans arrived in the New World from Asia more than 10,000 years ago. These animals could coexist with their natural predators, but they could not survive the shock of human encroachment.

With the development of agriculture and the domestication of animals about 12,000 years ago, the threat to wildlife increased greatly. In 2005, a major scientific study found that explosive growth of the human population is straining Earth’s ecosystems. Now, 10 to 30 percent of all mammal, bird, and amphibian species are threatened with extinction.

Mammals Affected

Certain kinds of animals, and animals living in certain areas, are particularly vulnerable to the threat of extinction. Many of the more than 120 species and subspecies of mammals that have become extinct since 1600 lived on islands. These extinctions often resulted from such indirect human activities as the introduction of alien species to the islands. Beginning with the first European explorers in the 15th century, rats and domestic dogs and cats were often accidentally released on islands. With no way to defend themselves against the invaders, many native animals were exterminated.

Of the continental areas, Australia, the island continent, leads the list with 22 extinct mammals in the past 400 years. In North America, the sea mink of the northeastern coasts was hunted to extinction by 1880, and several subspecies of wolves, bison, caribou, and elks have disappeared.

There are more than 5,400 species of mammals in the world. According to a recent report issued by the United Nations Environment Program (UNEP), nearly a quarter of the world’s mammal species (about 1,100 species altogether) face extinction within 30 years, due to habitat loss and competition from nonnative species.

Other Species Lost

The pattern of extinction is the same for birds as for mammals. Of the approximately 10,000 species of birds living in the year 1600, more than 100 have become extinct. Among today’s living birds, 1,200 species are endangered or threatened. As with mammals, most of these losses have occurred in isolated island regions, including New Zealand, Madagascar, Mauritius, the West Indies, Hawaii, and Guam.

In North America, two of the most tragic extinctions were those of the Carolina parakeet (Conuropsis carolinensis) and the passenger pigeon (Ectopistes migratorius). Both species originally existed in such vast numbers that they darkened the skies during their migrations. Coincidentally, the last member of each species died in the Cincinnati Zoo in 1914.

Each group of the cold-blooded vertebrates—the reptiles, the amphibians, and the fish—has also lost several dozen representatives since the year 1600. Among the extinct reptiles are several species of giant tortoises once common to small Indian Ocean islands near Madagascar. Currently there are 304 reptile species, 1,770 amphibian species, and 800 fish species listed as endangered or threatened.

Many Plants Threatened

Although the public focus has been on endangered animals, there are also many species of plants that are threatened with extinction. Recently, two U.S. botanists who conducted a global study reported that between 22 percent and 47 percent of the world’s plants—or about 94,000 to 144,000 species—were at risk of dying out. Again, it is human activity that constitutes the most serious threat. One of the primary causes of the loss of plants (and animals) is the deforestation of tropical regions, where each day thousands of acres of rain forest are being cleared for farming. Another threat is the introduction of new animals and plants that overwhelm the native species in a particular area. Fortunately, botanical gardens are organizing banks to preserve the seeds of endangered plants. Such seed banks have already saved many plants that would otherwise have disappeared forever.

If threatened plant species are allowed to vanish from Earth, the consequences could be serious and far-reaching. For example, many beneficial drugs are obtained from plants, and it is likely that some of the threatened plants—particularly those that occur only in endangered rain forests—contain still-undiscovered compounds that are medically useful.

As with animals, island-dwelling plant species are the most vulnerable to extinction. In many cases, the plants found on an isolated island are not found anywhere else in the world. The introduction by humans of plant-eating animals, particularly goats, has almost destroyed the native vegetation of several islands, causing the extinction of plant species. Many others have become extremely rare. In Hawaii, nearly 300 plant species are threatened or endangered, representing fully one-third of the plant species so classified in the United States.

Forest plants are also threatened, and some are considered endangered or extinct. Among these plants are several species of orchids found in Brazil and India, as well as a crocus native to Chile. Many species of cacti in the southwestern United States are also in danger; about one-quarter of them are listed as threatened or endangered.

Classification of Threatened Species

Animals and plants are typically deemed threatened when their survival is considered unlikely because of reduced population levels or drastically restricted habitats. Determining if an animal or plant is in danger of extinction is frequently a complicated task, one rarely achieved by simply taking a population count. For example, a species with a total population of 5,000 individuals living in one small area is in much more critical condition than a species with 5,000 individuals living in several areas. A species represented only by a small population on a single island could, for example, be entirely wiped out by a local catastrophe, such as a flood or an earthquake.

A giant boost to the efforts of conservationists was the enactment of the Endangered Species Act of 1973, which was established to protect species that were endangered (at risk of extinction) and threatened (at risk of becoming endangered). More-precise categories are used by the Survival Service Commission of the International Union for the Conservation of Nature and Natural Resources (IUCN).

IUCN Status Categories

The IUCN system categorizes the status of species and also describes the degree to which an individual species is threatened with extinction. Some of the labels used by the IUCN include extinct, endangered, vulnerable, extinct in the wild, near threatened, and data deficient.

Extinct (EX)

According to the IUCN, a species is considered to have become extinct if it has not been located in the wild during the past 50 years.

Endangered (EN)

A species is considered endangered when its numbers are so few or its homeland so small that it will probably disappear if not given immediate special protection.

The Hawaiian monk seal (Monachus schauinslandi) can be used to illustrate such an endangered species. The total population of less than 1,500 seals breeds only on five small islands in the Leeward Islands, northwest of the Hawaiian Islands. By 1825, sealers who hunted them for their skins announced that the species was extinct. Fortunately, a remnant population survived. They have been protected since 1909, and have slowly increased in number to their present level.

Unfortunately, nearly a century of protection may not be enough to save the Hawaiian monk seals. If they are disturbed on the beaches where they give birth, the mothers rush away into the water, leaving the pups to fend for themselves. With all the Hawaiian monk seals located on just a few islands, it is easy to see how a local catastrophe, such as an oil spill, could wipe them all out. There are a few Hawaiian monk specimens in captivity, but they have never bred.

The Caribbean monk seal (Monachus tropicalis) and the Mediterranean monk seal (Monachus monachus) are also endangered species. The former has not been seen alive since 1952, and is probably extinct. Less than 1,000 of the Mediterranean species remain.

Vulnerable (VU)

A vulnerable species is one that will become endangered in the near future if causal factors, such as overexploitation and habitat destruction, continue to exist.

Extinct in the Wild (EW)

This category indicates any species that exists only in cultivation—that is, in captivity (a zoo or a wildlife park) or, in the case of a plant, in a nursery. A species is also considered extinct in the wild if a limited population survives outside of its traditional range, perhaps because it had been unnaturally introduced into another area. Scientists undertake exhaustive surveys to locate the life-form in its known or expected habitat—during times of the year in which the species would likely be spotted. If no individuals are found during a pre-established time frame (usually coinciding with the species’ life cycle), it is likely that the plant or animal in question no longer exists within its historic range.

Near Threatened (NT)

The “near threatened” designation replaces the “rare” category, which had been used to describe species that share some of the same traits as those labeled endangered and vulnerable. Such species are likely to qualify for one of the “threatened” categories in the near future.

Data Deficient (DD)

This category (formerly “indeterminate”) refers to a species not officially classified as threatened. In this case, there is inadequate population data available, making it difficult for scientists to assess the risk of extinction for that species. Typically, when a life-form is labeled data deficient, further research ultimately shows that one of the threatened designations would be appropriate. Such was the case with the Amazon manatee (Trichecus inunguis), a freshwater sea cow, which went from indeterminate to endangered in just two years. The now-endangered snow leopard (Uncia uncia), hunted for its beautiful fur, followed a similar pattern.

According to two separate scientific studies in 2004, many plant and animal species are disappearing—with extinction rates 100 to 1,000 times higher than at any other point in history. In the past 20 years alone, 15 species have become extinct, and 12 exist only in captivity. Human-caused factors—such as habitat destruction and global warming—are cited as the most critical threats to the world’s biodiversity.

The Red Data Book

In 1966, the IUCN first produced the Red Data Book, a loose-leaf volume of information on the status of many kinds of animals. As the status of the animals changes, new pages are sent to subscribers. Pink pages indicate critically endangered species, while green pages signify those species that were formerly endangered, but that have recovered to a point where they are no longer threatened. There are pitifully few green pages in the book, and the number of pink pages continues to increase. Since 2000, the IUCN has also published these data online.

Why Species Become Endangered

Species become endangered for various reasons, but today almost all of them can be related directly or indirectly to human activity. Endangered status most frequently results from over-hunting, loss of habitat and food, low population, or poisoning of the environment.

Hunting

Some animals are hunted as food, some as trophies; others are hunted commercially. The polar bear (Ursus maritimus) of the Arctic has long been hunted for subsistence by Inuit (Eskimo) and other native peoples, and until recently was a popular sport-hunting trophy. As long as the bears were chased by hunters on foot, the prey had a reasonable chance to escape. Their numbers declined precipitously once hunters began using airplanes and helicopters. Today, polar bears are protected throughout their range and are hunted only in limited numbers by indigenous peoples. A 2005 study determined that climate change seriously threatens the polar bear. Melting sea ice hampers the animals’ ability to hunt and reproduce, prompting many experts to suggest that polar bears should be declared a vulnerable species.

Although trophy hunters once killed hundreds of tigers (Panthera tigris), the greatest threat to these endangered cats comes from poachers—hunters who supply an illegal trade in body parts used forAsian folk medicines.

Another threat to tigers comes from human encroachment. As the human population of southern Asia increases, the amount of wilderness will diminish, and the cat may disappear. Of the eight tiger subspecies that once roamed Earth, the Caspian, Bali, and Javan tigers became extinct in the past 70 years, leaving only five subspecies. As of 2005, the population of wild tigers was between 5,000 and 7,500—but poaching and habitat loss remain serious threats.

Other animal species are hunted commercially. The spotted cats—especially the cheetah (Acinonyx jubatus), leopard (Panthera pardus), jaguar (Panthera onca), ocelot (Leopardus pardalis), and margay (Leopardus wiedii)—are all threatened. They are usually hunted for their fur, for sport, and to protect livestock. Hunting for sport is most easily controlled. Commercial and protective hunting are difficult to regulate and pose the greatest threats.

Cheetahs are extinct in India, and their range is much reduced in the Middle East and in Africa. The number of leopards has also been greatly reduced. Until the Convention on International Trade in Endangered Species (CITES) in 1973, many of these cats were killed each year for their fur. Populations of South American wildcats—jaguars, ocelots, and margays—have also been impacted by the illegal fur trade. Although these cats have disappeared from many areas where they once lived, sufficient numbers probably still exist for each species to survive if the hunting is stopped.

Commercial hunting is also directly responsible for the endangered status of many species of whales. These mammals are killed mainly for the oil extracted from their blubber, for human and animal food, and for fertilizer. One tragic example is the blue whale (Balaenoptera musculus), which is seriously endangered. In the mid-1950s, there were 30,000 to 40,000 of these huge mammals, some of which reached more than 100 feet (30 meters) in length. In the following years, the whales were widely hunted, and each year more were killed than were born. In 1965, there was a moratorium placed on taking blue whales. International agreements in the 1980s and early 1990s limited the hunting of larger whales. The current blue-whale population is estimated to be about 11,500 animals.

Loss of Habitat and Food Supply

Indirect threats to animals and plants are often more dangerous, more widespread, and less obvious than such direct threats as poaching and commercial hunting. Most threatened species are not hunted. Instead, they are endangered by the loss of their habitat or food supply.

A tragic example of such a species is the ivory-billed woodpecker (Campephilus principalis), which once inhabited the great river-bottom forests from the Mississippi River to North Carolina to the swamps of Florida. As the forests were cut down, the woodpecker became scarce and was ultimately thought to be extinct in this country. In 2005, a team of scientists reported that at least one ivory bill lives in the remote Big Woods region of southeastern Arkansas, an assertion they backed up with audio recordings of the bird’s call.

The American black-footed ferret (Mustela nigripes), a member of the weasel family, has always lived in close association with the black-tailed prairie dog (Cynomys ludovicianus), a grassland rodent. The ferrets not only feed almost exclusively on the prairie dogs, but also live in their burrows. But because prairie dogs eat the grass desired for grazing livestock, and their burrows are sometimes responsible for injuring cattle and horses, they have been exterminated by ranchers throughout most of their range. As the prairie dogs gradually disappeared, so did the ferrets. In the 1980s, the few remaining wild ferrets, threatened by disease, were removed to be bred in captivity. In the 1990s and early 2000s, efforts to replenish the species entailed releasing groups of ferrets into protected areas where the animals could establish small, stable populations.

The loss of habitat threatens many forest species, especially on the island of Madagascar. The golden bamboo lemurs, which live only on this island, may soon become extinct because there is little forest left. The same is true of another native of Madagascar, the aye-aye (Daubentonia madagascariensis).

The aye-ayes, whose current population is unknown, are victims of logging and local superstition—they are considered bad luck. These specialized primates feed on wood-boring insects, which they detect with their excellent hearing. They gnaw the wood with their strong front teeth and remove the grubs or adult insects with a highly specialized, long, thin middle finger. Without large expanses of forests, the aye-aye cannot survive.

Other primates endangered by the destruction of forests include the orangutan (Pongo pygmaeus) and the mountain gorilla (Gorilla gorilla beringei). Both species have also been threatened by wars waged near their habitats.

Population Levels

Some species are threatened with extinction because their numbers, while relatively stable, are so small that they may never be able to increase to a safe level. Any small population, especially if all the animals are found within a single region, can easily be wiped out by one catastrophic occurrence, such as a hurricane, a flood, or a fire.

An example of such a species is the American whooping crane (Grus americana), whose numbers were down to only 15 birds in 1941. Even after intense conservation efforts, there were still only 30 birds in 1963. The number in the wild has increased in recent years, and today there are more than 450 wild and captive birds.

Poisoning of the Environment

Another threat to wildlife has increased since the middle of the 20th century—the poisoning of the environment. As the human population has grown, vast amounts of wild land have been cleared for agriculture, thus depriving wildlife of habitat. New agricultural techniques to make the land more productive have included the widespread use of pesticides.

Herbicides that kill weeds and insecticides that destroy insects are dumped on the land in huge quantities. Such chemicals are beneficial in increasing crop yields and controlling disease-carrying insects. However, many of the poisons undergo chemical breakdown very slowly, allowing them to accumulate in the soil and wash into streams, lakes, and oceans.

Once they enter water, the chemicals are eaten or absorbed by microscopic organisms. As these tiny animals and plants are eaten by larger animals, the poisons accumulate in the bodies of the animals in increasingly high concentrations. Thus, the largest animals—those at the top of the so-called food chain—suffer the greatest damage because they take in the highest concentrations of poisons. For example, contaminated algae may be eaten by a small crustacean, which in turn is eaten by a small fish. The small fish may then be eaten by a larger fish, and the larger fish eaten by a grizzly bear. This progression makes up a food chain. The bear at the top of the chain receives the most poison.

The insecticide DDT moves along the food chain in this manner. Beginning in 1946, DDT was widely used as an effective weapon against agricultural pests. But DDT does not break down easily in nature, causing it to remain in the environment for many years.

DDT can cause physiological and genetic changes in animals and humans. For example, it disrupts the calcium-producing mechanism in birds. This means that their eggs are laid with an inadequate, thin, flaky shell that usually breaks before the embryos reach maturity. As a result, populations of the American bald eagle (Haliaeëtus leucocephalus), the osprey (Pandion haliaëtus), the peregrine falcon (Falco peregrinus), the brown pelican (Pelecanus occidentalis), and the Bermuda petrel (Pterodroma cahow) declined sharply. DDT also reaches humans—as analysis of the milk of some nursing mothers shows.

The governments of several countries, including the United States, Canada, and Sweden, restricted the use of DDT in the late 1960s and early 1970s. Since then, many of the previously declining bird species have made a dramatic recovery. However, there are many other defoliant and pesticide substances still in use whose long-range effects need to be carefully evaluated.

Mercury, for instance, is used by agriculture and industry to kill slime mold and fungus. When this poisonous heavy metal finds its way into the sea, it accumulates in living organisms. In some areas, certain fish have been rendered at least temporarily inedible by the high concentrations of mercury in their bodies. Some of the sea mammals that feed heavily on fish are beginning to show high concentrations of mercury and insecticides. California sea lions (Zalophus californianus), Northern fur seals (Callorhinus ursinus), and even polar bears in the Arctic and penguins in the Antarctic have been affected by poisonous heavy metals. The poisons circulate throughout the world and are present in the cells of marine plants and animals in all the oceans. A recent Canadian study suggests that a toxic mercury rain falls on the Arctic each spring.

Polychlorinated biphenyls (PCBs)—chemicals that are used in insulating fluids, paints, plastics, and rubbers—have also found their way into the sea.

The indiscriminate use of poisonous chemicals may now represent the greatest threat to plant and animal life, including humans. The most serious aspect of this situation is that even if all use of these poisons were to cease today, they would continue to pollute the environment for many years to come.

Protective Measures

The protection of animals has been practiced for centuries. Private hunting preserves, in fact, were among the first conservation methods. Landowning nobles sought to protect and nurture certain animals to ensure a supply for sport. There are a number of species, especially in the deer family, that owe their existence to protection by the very people who hunted them.

Since the 1600s, local laws have protected native species. The first such laws in the New World were likely those passed by the Bermuda government in 1621 to protect a bird called the cahow, or Bermuda petrel. It appeared that, despite the law, this bird had become extinct. However, in 1951, a few live specimens were discovered nesting on small rocky islets. They are now carefully protected. In 1694, Massachusetts established a closed season on white-tailed deer (Odocoileus virginianus) to protect their diminishing numbers. The whitetail deer is now the most abundant large mammal in North America.

Today the Survival Service of the IUCN is the main international agency for calling attention to endangered species. At its headquarters in Morges, Switzerland, the organization receives reports from naturalists throughout the world about the status of various species. When someone informs the agency of a threat to a species, the Survival Service contacts other scientists who have knowledge of the animals or of the area involved to try to learn more about the species. If the information received indicates that there is a real threat to the species, a page for the Red Data Book is issued. Even when there is not enough information to ascertain that a species is seriously threatened, the IUCN may still list the species in the book.

The IUCN and other agencies, such as the World Wildlife Fund and the Wildlife Conservation Society, attempt to have species studied in detail to determine their population, the main threats to their existence, and the possible steps that can be taken to preserve them.

Interested individuals and organizations in various countries attempt to pass laws to protect the species. All these activities advertise the plight of endangered species, and thus awaken greater public support for conservation.

Even when laws are passed to protect species, there still remains the problem of enforcement. Many endangered species live in remote areas, often in dense vegetation, where it is difficult to apprehend poachers. Also, some of the protective laws that have been passed have no provision for enforcement. Without enforcement of the laws, there is little real protection for the species.

In the mid-1960s, concern over endangered species greatly increased because the condition of so many was rapidly becoming critical—a situation driven home by Rare and Endangered Fish and Wildlife of the United States, a book published by the Fish and Wildlife Service of the U. S. Department of the Interior. Finally, the Endangered Species Act was passed in 1973. According to this law, species that are on the endangered list cannot be brought into the United States, and species that are not on the list but are protected by law in other countries are also forbidden entry.

One of the problems of protection is that a species may be relatively abundant in one place but very rare in another, and thus can be protected in a country where it is rare and not protected elsewhere. Many animals are illegally killed in a country where they are protected, and the skins are smuggled into a country where such killing is legal. It is difficult, and often impossible, to tell where the animal came from by looking at the skin alone.

Many skins that entered the United States were listed as having originated in places where the species was abundant and unprotected. However, they were in fact illegally killed in a protected country. For example, alligators were protected by law in the United States, but many were nonetheless killed by poachers, smuggled to Europe, and then imported back into the United States as crocodiles.

In 1970, New York State passed endangered-species legislation known as the Mason Act. Designed to stop the type of circumvention of law described above, the Mason Act banned commerce in any crocodilians—alligators, crocodiles, and their relatives—whether endangered or not. The law also banned all trade in other species—such as leopards, cheetahs, polar bears, red wolves, vicuñas, tigers, and snow leopards—no matter what country they came from originally.

Since New York is the center of the fur and fashion industries, the Mason Act has proved to be one of the most potent pieces of legislation ever passed to protect endangered species. Some of the most important points in the Mason Act were incorporated into the Endangered Species Act.

The Endangered Species Act also included the first legislation to protect a previously overlooked group: endangered insects. In 1975, 41 species of butterflies were placed on the U.S. list of threatened and endangered species, protecting them from interstate shipment, commercial sale, and mass collection.

In 1977, the United States signed the Convention of International Trade on Endangered Species (CITES). This treaty seeks to protect threatened animals in all countries by making it illegal to sell them across international boundaries. But by 2005, President George W. Bush and his administration had proposed an amendment that would loosen restrictions on hunting, capturing, and importing endangered animals. And the Endangered Species Act itself is now being challenged by landowners, developers, industry, and the U.S. Congress.

The Future

The outlook for endangered species is not without its bright spots. In 2005, the population of critically endangered California condors (Gymnogyps californianus) reached 279, thanks to the breeding of chicks in captivity for release into the wild. After years of protection, nearly every species of whale has posted a gain in population. Red wolves (Canis lupus niger) reintroduced into the North Carolina lowlands have produced pups. In Florida, the once-endangered alligator (Alligator mississippiensis) has rebounded in great numbers. Protection laws and the banning of DDT in the United States helped the bald eagle recover. In 2005, there were more than 7,000 breeding pairs in the wild. The bird may even be removed from the list of threatened and endangered species. Similar success with other species may depend on legislation, habitat restoration, and reintroduction programs.