An endangered species is a plant, animal or other organism that is threatened by extinction. Species become endangered for two main reasons: loss of habitat and loss of genetic variation.

Loss of Habitat

A loss of habitat can happen naturally. Nonavian dinosaurs, for instance, lost their habitat about 65 million years ago. The hot, dry climate of the Cretaceous period changed very quickly, most likely because of an asteroid striking Earth. The impact blasted debris into the atmosphere, reducing the amount of heat and light that reached Earth's surface. The nonavian dinosaurs were unable to adapt to this new, cooler habitat. They eventually became extinct.

Human activity can also contribute to a loss of habitat. Development for housing, industry, and agriculture reduces the habitat of native organisms.

Development can eliminate habitat and native species directly. In the Amazon rainforest of South America, for example, developers have cleared hundreds of thousands of acres. To "clear" a piece of land is to remove all trees and vegetation from it. The Amazon rainforest is cleared for cattle ranches, logging, and urban use.

Development can also endanger species indirectly. Some species, such as fig trees of the rainforest, may provide habitat for other species. As trees are destroyed, species that depend on that tree may also become endangered. Trees provide habitat in the canopy, or top layer, of a rainforest. Plants such as vines, fungi such as mushrooms and insects such as butterflies live in the rainforest canopy. So do hundreds of species of tropical birds and mammals such as monkeys. As trees are cut down, this habitat is lost. Species have less room to live and reproduce.

Loss of Genetic Variation

Genetic variation is the diversity found within a species. It's why human beings may have blond, red, brown, or black hair. Genetic variation allows species to adapt to changes in the environment. Usually, the greater the population of a species, the greater its genetic variation.

Inbreeding is when close family members reproduce. Groups of species that inbreed usually have less genetic variation, because no new genetic information is introduced to the group. Disease is much more common, and much more deadly, among inbred groups. Inbred species do not have the genetic variation to develop resistance to the disease. For this reason, fewer offspring of inbred groups survive to maturity.

Loss of genetic variation can occur naturally. Cheetahs (Acinonyx jubatus) are a threatened species native to Africa and Asia. These big cats have very little genetic variation. They cannot adapt to changes in the environment as quickly as other animals, and fewer cheetahs survive to maturity.

Human activity can also lead to a loss of genetic variation. Overhunting and overfishing have reduced the populations of many animals. Reduced population usually means there is less genetic variation.

The Red List

The International Union for Conservation of Nature (IUCN) keeps a "Red List of Threatened Species." The list alerts organizations and governments about species that require greater conservation efforts. The Red List defines the seriousness and causes of a species' risk of extinction.

The threat is usually calculated by the size of a population, or if the population has experienced a sudden sharp decline. Rapid destruction of its habitat is also an important factor. The Red List has seven levels of conservation: least concern, near threatened, vulnerable, endangered, critically endangered, extinct in the wild and extinct. Each category represents a different threat level.

Least Concern and Near Threatened

Least concern is the lowest level of conservation. A species of least concern is one that has a widespread and abundant population. Human beings are a species of least concern, for example, along with most domestic animals, such as dogs (Canis familiaris) and cats (Felis catus). Many wild animals, such as pigeons and houseflies (Musca domestica), are also classified as least concern.

A near threatened species is one that may face a significant threat in the near future. Many species of violets, native to tropical jungles in South America and Africa, are near threatened, for instance. They have healthy populations. However, their habitat is disappearing at a fast pace. 

Vulnerable, Endangered and Critically Endangered Species

These three categories — vulnerable, endangered and critically endangered — include species facing potential threats. As their population and habitat decreases, these species become more threatened.

Vulnerable Species: Ethiopian Banana Frog

This small frog is native to southern Ethiopia in Africa. It is a vulnerable species because the size and quality of its forest habitat are in decline. 

Critically Endangered Species: Bolivian Chinchilla Rat

The Bolivian chinchilla rat is a rodent found in a small part of Bolivia, in South America. It is critically endangered because its habitat has shrunk to less than 100 square kilometers (39 square miles). The major threat to this species is loss of its forest habitat.

Extinct in the Wild and Extinct

A species is extinct in the wild when it no longer lives in its natural habitat. However, it may survive in zoos (animals) or be cultivated in greenhouses (plants). A species is extinct when the last individual of that species has died.

Extinct: Cuban Macaw

The Cuban macaw (Ara tricolor) was a tropical parrot native to Cuba. Hunting and collecting the birds for pets led to the bird's extinction in the 1800s.

Protecting Endangered Species

When a species is classified as endangered, governments and international organizations can work to protect it. Laws may limit hunting and destruction of the species' habitat. Individuals and companies that break these laws may face huge fines. Because of such actions, many species have recovered from their endangered status.

The brown pelican (Pelecanus occidentalis) was taken off the endangered species list in 2009, for instance. This seabird is native to the coasts of North America and South America, as well as the islands of the Caribbean Sea. In 1970, the number of brown pelicans in the wild was estimated at 10,000. The bird was classified as vulnerable.

During the 1970s and 1980s, governments and conservation groups worked to help the brown pelican recover. Young chicks were reared in hatching sites, then released into the wild. The pesticide DDT, which damaged the eggs of the brown pelican, was banned. During the 1980s, the number of brown pelicans soared. The bird, whose population is now in the hundreds of thousands, has been moved to the category of least concern. Today, it is considered safe from extinction, thanks to the efforts of people who worked to save it.