An endangered species is a plant, animal, fungus, microbe, 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. Dinosaurs, for instance, lost their habitat about 66 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 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 of housing, industry and agriculture reduces the habitat of native organisms.

Development can eliminate habitats and native species directly. In South America, for example, developers have cleared about 20 percent of the Amazon rainforest in the last 50 years. When a piece of land is "cleared," all trees and vegetation are removed. The Amazon rainforest has been cleared mainly for cattle ranches to meet the worldwide demand for beef.

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

A species may also be important to the people of an area. Fig trees, for example, are important to some indigenous groups of the rainforest. Indigenous groups believe fig trees are important. They have religious and cultural beliefs about the trees. These beliefs have created rules against harming or cutting down fig trees. These rules end up protecting the trees.

Loss of habitat also happens because of climate change. Climate change refers to changes to Earth’s weather patterns over many years. Earth’s climate changes naturally over time. But since the 19th century, humans have caused rapid climate change. Humans affect the climate by burning fossil fuels, like gas and oil, to power machines. Burning these fossil fuels releases gases, like carbon dioxide, which traps additional heat from the sun. This creates warmer temperatures.

Many animals have lost areas in their habitats due to changing climate. Polar bears have lost part of their range because icebergs are melting. The golden toad of Costa Rica went extinct because the amount of rainfall decreased in their environment. They could not survive in the drier environment.

Loss of Genetic Variation

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

Inbreeding occurs when close family members reproduce with each other. 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 disease. For this reason, inbred offspring are less likely to survive to maturity.

Loss of genetic variation can occur naturally. Cheetahs 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 cause a loss of genetic variation. For example, overfishing has reduced the populations of many fish species. Worldwide, fish eating has gone up in the last 50 years. The pressure on the fishing industry has reduced the number of healthy adult fish in the wild. Fewer fish are reproducing overall, which lessens the genetic variation in the fish population.

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 determined based on the size of a population or if the population has experienced a sudden sharp decline. Rapid destruction of habitat is also an important factor.

The Red List has nine levels of conservation. Seven of these levels rank how close a species is to extinction. The seven levels are: least concern, near threatened, vulnerable, endangered, critically endangered, extinct in the wild and extinct. Each category represents a different threat level. The last two levels are “data deficient” and “not evaluated.” “Data deficient” means that scientists have not been able to gather enough information to rank the animal. Often, scientists have trouble collecting data about a species because there are not enough animals for them to observe. This means that data deficient species are likely to be close to extinction. The last category is “not evaluated.” This means that scientists have not yet studied how close the animal is to extinction.

Least Concern and Near Threatened

Least concern is the lowest level of conservation. A species of least concern is one that has a large, widespread population. Examples of species of least concern are the arctic fox, the harbor seal and the brown bear.

A near threatened species is one that may face a significant threat in the near future. American bison, for example, are considered near threatened. They almost went extinct in the 19th century because of hunting and habitat loss. Their habitats were largely taken away by the U.S. government for farmland. But conservation efforts brought them back to near threatened status. Many of these conservation efforts were led by indigenous groups, like the Eastern Shoshone. The tribe is caring for a growing herd that has been released onto protected land. Without conservation programs working to protect them, the bison population would likely decrease much further.

Vulnerable, Endangered and Critically Endangered Species

These three categories—vulnerable, endangered and critically endangered—include species facing potential threats. As their populations and habitats decrease, these species become more threatened.

Vulnerable Species: Ethiopian Banana Frog

The Ethiopian banana frog is a small frog native to southern Ethiopia in Africa. It is a vulnerable species because the size and quality of its forest habitat are in decline. This is because of developers clearing the forest to make room for housing and agriculture.

Endangered: Big Leaf Mahogany

A big leaf mahogany is a tall tree that grows in the rainforests of Central and South America. People have been cutting down mahogany trees for many years to use in cabinetry and flooring. Overharvesting has severely limited its range and lessened the number of mature trees. Once harvested, it is almost impossible to grow a new tree in the same spot. This is because mahogany trees need a lot of sunlight to grow. The remaining forest trees block the young mahogany tree from the light. However, there are people working to conserve the mahogany trees. For example, the Wampis Nation, an indigenous group in Peru, have a legal right to manage the tropical forest. They protect mahogany trees against illegal logging.

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. People are clearing the forest for agriculture.

Extinct in the Wild and Extinct

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

Extinct in the Wild: Wyoming Toads

Wyoming toads lived only in small areas of Wyoming and Colorado. The Wyoming toad is a small amphibian that lived in shallow ponds and lakes formed by rainwater, melted snow and groundwater.

Wyoming toads died off in the wild because of pollution and disease. When it rains, rainwater washes pesticides from farms into nearby waterways. When it rained near the Wyoming toads’ habitat, the pesticides got into the water where the toads lived. Many of the toads also got sick from a fungus that killed amphibians around the world. Despite these challenges, there is a small group of Wyoming toads living in the Mortenson Lake National Wildlife Refuge. That population is too small and too dependent on human assistance to be reintroduced into the wild. In the future, however, the species might be successfully reintroduced.

Extinct: Cuban Macaw

The Cuban macaw 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, such as the Endangered Species Act in the United States, may limit hunting of these species and destruction of their habitats. Individuals and organizations that break these laws may face huge fines. Internationally, the Convention on International Trade in Endangered Species of Wild Fauna and Flora controls the trade of animals across borders and stops the trade of endangered species. Local and regional organizations also work to increase the populations of threatened or once-threatened species. Because of these actions, some species have recovered from their endangered status.

The brown pelican, for instance, was taken off the endangered species list in 2009. This seabird is native to the coasts of North and South America, as well as the islands in the Caribbean Sea. In 1970, the bird was listed as federally endangered because of its low population and low birthrate, which were caused by pollution. A pesticide called DDT had been polluting the pelicans’ habitats and damaging their eggs. DDT made the pelican’s eggshells thinner. This made them more vulnerable to breaking before chicks could hatch.

During the 1970s and 1980s, governments and conservation groups worked to help the brown pelican recover. Young chicks were reared in hatching sites and then released into the wild. The U.S. government also banned the pesticide DDT. The bird, whose population is about 300,000, is now considered of least concern.

There are also ways for individuals to help protect endangered species. National Geographic Explorer Joel Sartore uses the power of photography to inspire people to help protect at-risk species before it is too late. He is the founder of the National Geographic Photo Ark, which aims to document every species living in the world's zoos, aquariums and wildlife sanctuaries.

People can donate to organizations that help endangered animals. They can create native plant gardens that encourage local wildlife to live in their yards. Visiting a national park or wildlife conservation area is another way to support endangered species. People can also vote or sign petitions to advocate for government action to help endangered species.