When an entire species, or type, of animal dies out, that species is extinct. Once a species becomes extinct, it is gone forever.

Causes of Extinction
The most common cause of extinction is a sudden, serious change in a species’ habitat. A habitat is the surroundings in which an animal lives. Animals can rarely survive such sudden change. Their food supply may be wiped out. They may also lose shelter or other things that they need to survive. Many things can change a species’ habitat. Floods, fires, droughts, volcanoes, and other natural events may be causes. People also change the environment in ways that drastically affect animals. People clear forests and drain wetlands. They build dams that disrupt the flow of rivers. They build cities on land that animals need to survive. They also create harmful pollution.

Some changes that cause extinction affect only a small area. Others are large enough to affect the entire world. A fire or other local event may cause the extinction of animals that live only in that region. A sudden change in the global climate might wipe out an animal species that lives in many parts of the world.

People can cause extinctions more directly as well. Some species have been hunted to extinction. The passenger pigeon is one example. Humans killed millions of the birds over many years. The last one died in the early 1900s.

Mass Extinctions
Sometimes many different species become extinct in a short time. This is called a mass extinction. Several major mass extinctions have occurred in the past. Each time many animal species were wiped out. Some survived, however, and over millions of years new species developed.

The worst mass extinction happened about 248 million years ago. This extinction included mainly animals without backbones that lived in water. Another mass extinction occurred about 65 million years ago. It wiped out many of the planet’s land animals, including the dinosaurs.

Dinosaur Extinction
Dinosaurs first appeared on Earth about 215 million years ago. They were the most important land animals for more than 150 million years. By 65 million years ago, however, the dinosaurs had died out.

Many scientists believe that a large asteroid, or rock from space, caused this mass extinction. When the asteroid hit Earth, the impact caused drastic changes. Thick dust and other materials blocked the sun. Temperatures dropped, and plants could not grow. The dinosaurs could not survive the cold temperatures and lack of food. But early species of birds and mammals did survive.

Large Mammal Extinction
About 10,000 years ago another mysterious animal extinction occurred. This extinction was especially dramatic in North America, where many large mammals disappeared. Among them were woolly mammoths and saber-toothed cats.

Scientists do not know what caused this extinction. Some believe that growing numbers of human beings hunted and killed too many of the animals or their prey. Another theory is that the climate changed and affected the availability of food.

Endangered Species Today
In the past 200 years the world has lost many animal species. Hundreds of others are on the verge of extinction. Animals at risk of dying out are called endangered species.

Governments today are working to protect the world’s endangered species. Laws protect some animals’ habitats from being polluted or destroyed. Other laws make it illegal to hunt endangered animals. Some species respond well to these protective measures and increase in number. Others are not as successful