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Most people have a general idea of what an endangered, threatened, or extinct species is, but biologists have rather precise definitions for each term. An endangered species is a type of animal or plant that is in immediate danger of extinction. The species usually has a small population and needs protection in order to survive. The mountain gorilla, the Indian python, the lady slipper orchid, and thousands of other plant and animal species throughout the world are endangered.
Biologists use the word threatened to describe species that face serious problems, but whose populations are not in immediate danger of becoming extinct. Some examples of threatened species are the African elephant, the northern spotted owl, and the eastern indigo snake.
Extinct species no longer exist or live anywhere in the world. The dodo, the passenger pigeon, and the dinosaurs are examples of extinct species.
An Old Phenomenon
Extinction is not a new phenomenon. For hundreds of millions of years, extinction has been occurring naturally, as part of the evolutionary process. Some cases of extinction have been caused by natural disasters, such as volcanic eruptions. Others have been the result of environmental changes, such as shifts in climate. Sometimes extinction occurs on a very large scale, with hundreds or thousands of species becoming extinct over a relatively short period of time. An example of this is the dinosaurs and their contemporaries, victims of a mass extinction that took place at least 65 million years ago.
An Increasing Rate of Extinction
Although extinction itself is not an old phenomenon, the current rate of extinction is something new. Biologists say that at least three animal and plant species become extinct every day, a rate much higher than anything in the past 65 million years.
Why Species Become Endangered
Species become endangered for a wide variety of reasons. However, when individual cases are grouped and studied, the same broad causes appear again and again:
Rapid habitat destruction is the main reason that species become endangered. Natural changes usually occur at a slow rate, so the effects on individual species are usually slight, at least over the short term. When the rate of change is greatly speeded up, there may be no time for individual species to adapt to new conditions. The results can be disastrous. This increase in the rate of habitat destruction is directly linked to the rise in human population. As more people use more space--for homes, farms, shopping centers, and so on--there is less living space for species that cannot adapt to changing conditions. People also affect plant and animal habitats when they take wood, oil, and other products from the land.
Another people-related problem that harms wildlife is the introduction of exotic species - foreign species that are deliberately or accidentally introduced into new habitats by human activities. Sometimes an introduced species causes no obvious harm, but in other cases the introduced species causes serious problems. The worst of these problems is when introduced species begin to prey on
native species and cause them harm.
Overexploitation is another reason species become endangered. One example of this is the case of the great whales, many of which were reduced to extremely low populations sizes in the mid-20th century because of unrestricted whaling. In 1982 a number of countries agrees to put a ban on commercial whaling. As a result, some whale species that used to be endangered have made great comebacks. Many other species, however, are still at risk. Some other animal species experience high rates of exploitation because of the trade in animal parts. Currently, this trade is centered in several parts of Asia where there is a strong market for traditional medicines made from items like tiger bone and rhino horn. Other people-related problems that put plant and animal species at risk include poaching, pollution, and overcollecting.
Asian elephants used to live in the forests from Iraq to southern China. Since these forests were cut down to make room for farms and villages, the elephants have been confined to small, hilly regions where they have little contact with humans. These tiny areas of land cannot supply enough food for the elephants. An adult elephant eats about 330 pounds (150 kg) of grasses, leaves, and other vegetation each day. When forests were larger, Asian elephants migrated with the seasons. In this way, they found fresh food supplies. The plants and trees could also regenerate after the elephants left.
Today there is nowhere for the elephants to go. Experts say that the Asian elephant population is about 55,000, living on a habitat of about 190,00 square miles (494 sq km). In contrast to this, the African elephant population is about 10 times this size and lives on almost 3 million square miles (7.7 million sq. km) of available habitat.
Black Lace Cactus
This colorful plant is a favorite of collectors around the world. It is a tiny plant, only 6 inches tall. It grows alone or in small groups in desert areas near the coast of southern Texas in the United States. It is called "black lace" because the pattern of spines on each stem looks like lace.
One reason the black lace cactus is endangered is because its habitat has been destroyed. In areas where the land has been cleared to plant grass for cattle, the cacti have disappeared. Another problem is overcollecting. The plant's large pink and purple flowers are very pretty. For this reason, many people dig up the plants and take them home for their private collections. Other people dig up them up and sell them.
Many birds sing or whistle. Others--such as myna birds and many parrots--talk. The kagu is a bird that barks! These barking birds live in the forests of New Caledonia, an island about 900 miles (1,450 km) east of Australia.
Kagus are big birds. They are 20-24 inches (51-61 cm) long and weigh about 1.9 pounds (0.9 kg). Their loud barking noise is becoming rare because only about 650 kagus are alive today.
One problem for kagus is the animals that people have brought to the New Caledonia. These dogs, pigs, cats, and rats eat kagus or their eggs. Another problem is hunting. Some people kill kagus for their meat. But, the biggest problem for kagus is the loss of habitat. The forests of New Caledonia have been cleared for mining and agriculture, leaving only a few small valleys where the kagus can live.
Manus Island Tree Snail
Manus Island, north of New Guinea, is covered with rain forest. The Manus Island tree snail, a small animal with a bright green shell, lives in the tops of the trees in this forest.
Overcollecting has been a serious problem for these small animals. Many people like to collect the shells of Manus Island tree snails because of their beautiful color. The 1.6-inch long (4 cm) shells are often used for jewelry. Another big problem for these snails is the loss of the forests where they live. Loggers are cutting down more and more trees of the Manus Island rain forest.
Little is known about the habits of this little animal. If the logging and collecting continue, soon there will be no Manus Island tree snails left to study.
Leatherback Sea Turtle
Picture a turtle that is six feet (1.8 m) long and weighs 1400 pounds (636 kg)! That's the size of a large Leatherback sea turtle, the largest turtle on earth. It is called "leatherback" because its shell is covered with a leathery-type skin.
Leatherbacks live in the warm waters of the Atlantic, Indian, and Pacific Oceans. Males spend all of their time at sea, and females come on land only when it is time to lay their eggs.
Loss of nesting habitats is a serious problem for Leatherbacks. Females build their nests on remote sandy areas along the coast. Because many coasts are being made into beaches, leatherbacks often cannot find a safe place to lay their eggs. Other problems are fishing and hunting. Leatherbacks get caught in fishing nets, and in some parts of Asia they are hunted for food and oil. Only about 100,000 females are alive today. It is hard to know the number of males since they never come ashore.
Karner Blue Butterfly
With a wingspan of about one inch (2.5 cm), Karner Blue butterflies are among the smallest of all butterflies. They are also among the rarest. They are found in the midwestern and northeastern parts of the United States.
Many people like to collect Karner Blue butterflies because they are so beautiful. However, because numbers of Karner Blue butterflies are so low, the collection of even a few can seriously harm their population.
An even bigger problem for these butterflies is habitat loss. The only known food of the Karner blue butterfly is the wild lupine, a small blue flowering plant. Wild lupine grows best in sandy soils, in areas that are occasionally cleared by wildfires. Land development and lack of wildfire have reduced the growth of this plant. Without the wild lupine, Karner Blue butterflies cannot exist.