An endangered species is an organism, such as a plant, fungus, microbe or animal, 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 of the asteroid forced 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. Dinosaurs became endangered and then extinct.

Human activity can also contribute to a loss of habitat. Development of housing, industry and agriculture reduces the habitat of native organisms. This can happen in a number of different ways.

Development can eliminate habitats and native species directly. In South America, human activities have resulted in the clearing of about 20% 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 primarily for cattle ranches in order 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, species that depend on that tree for their habitat may also become endangered. Tree crowns provide a habitat in the canopy, or top layer, of a rainforest. Plants and insects, such as vines and butterflies, live in the rainforest canopy, as do hundreds of species of tropical birds and mammals, such as monkeys. When trees are cut down, this habitat is lost, and species have less room to live and reproduce.

A species may also be important to the people of an area. Strangler fig trees (Ficus species), found in rainforests around the world, are important to some Indigenous groups. For centuries, Indigenous people have been preserving fig trees due to their religious and cultural beliefs surrounding the trees. These beliefs have created taboos, or restrictions, that end up protecting the trees by forbidding harm to them. Though Indigenous groups have taken steps to protect fig trees, developers often still destroy these trees to build more cattle ranches.

Habitat loss can also occur when humans introduce an invasive species to an area. Invasive species are non-native organisms that have been introduced into an area and cause harm to the environment and its native species. Some non-native species do not cause a negative impact to native organisms or the ecosystem around them, so they are generally not considered invasive. Human activities play a large role in spreading invasive species. Sometimes it is by accident, such as when animals get into shipping containers and end up in a new country. Other times, humans introduce new species on purpose, like when they import exotic pets and then set them free.

Loss of habitat may happen as development takes place in a species’ range. Many animals have a range that spans hundreds to thousands of square kilometers. Male mountain lions (Puma concolor) have a range of more than 260 square kilometers (100 square miles), while females have a range anywhere between 52 and 155 square kilometers (between 20 and 60 square miles). In North America, mountain lions used to have free rein. However, as urban areas grew, such as Los Angeles, California, people expanded into the mountain lion’s range, making their habitat smaller. That means the habitat can support fewer mountain lions, overall.

Currently, despite the threat to their habitat, mountain lions are not endangered in California because there is still enough wilderness to support them. However, that may change if wildfires continue to shrink their habitat. Wildfires are a normal part of life in the California wilderness, but climate change has led to drier conditions in the area, which can cause bigger and more frequent fires.

Loss of habitat can also lead to increased encounters between wild species and people. As human development encroaches further into the wilderness, people become more exposed to wild species. These animals are simply patrolling their range, but interactions with people can be deadly. Polar bears (Ursus maritimus), mountain lions and alligators (Alligator mississippiensis and Alligator sinensis) are all predators that have been brought into close contact with people as they have lost their habitats to homes, farms and businesses. Increased closeness between humans and animals is generally more dangerous to the animal. People hunting or accidentally killing these animals can also contribute to a species’ endangered status. For example, freeways cut off populations of mountain lions from each other, and they often get hit by cars, contributing to their declining numbers.

Habitat loss also happens because of climate change. Climate change refers to changes to Earth’s weather and temperature patterns over many years. Natural changes to Earth affect climate, but since the 19th century, humans have caused very rapid climate change by burning fossil fuels like gas and oil. Burning fossil fuels releases carbon dioxide into the atmosphere, which traps additional heat from the sun. Many animals have lost parts of their habitats due to the changing climate. Polar bears, for example, have lost a portion of their range because icebergs are melting. The golden toad (Incilius periglenes) of Costa Rica went extinct because the amount of rainfall decreased, and they could not adapt quickly enough to survive in the changed environment.

Loss of Genetic Variation

Genetic variation is the diversity found within a species. It is why human beings can 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 one another. Species that tend to inbreed usually have little 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 survive to maturity.

Loss of genetic variation can occur naturally. Cheetahs (Acinonyx jubatus)—a threatened species native to Africa and Asia—have very little genetic variation. Biologists say that after a mass extinction of large mammals around 12,000 years ago, cheetahs went through a long period of inbreeding. As a result, there are very few genetic differences within cheetah populations. They cannot adapt to changes in the environment as quickly as other animals, and fewer cheetahs survive to maturity. Cheetahs are also much more difficult to breed in captivity than other big cats, such as lions.

Human activity can also cause a loss of genetic variation. For example, as fish consumption worldwide has increased in the last 50 years, overfishing has reduced the populations of many animals. The pressure on the fishing industry to catch more and more fish has led to a decrease in the number of healthy adult fish in the wild that can reproduce. This reduced population means there are fewer breeding pairs. A breeding pair is made up of two mature members of the species that are not closely related and can produce healthy offspring. With fewer breeding pairs, genetic variation shrinks.

Monoculture, the agricultural method of growing a single crop, can also reduce genetic variation. Modern agribusiness relies on monocultures because it is more efficient than growing a variety of crops. Focusing on a single crop can save time and money, because, for example, the farmer only needs specialized equipment for one crop. Many of the potatoes cultivated, sold and consumed in the United States are from a single species, the Russet Burbank potato (Solanum tuberosum). The Russet Burbank potato has a longer shelf life than many other potato species, which increases profit for farmers. But potatoes, native to the Andes Mountains of South America, have dozens of natural varieties. The genetic variation of wild potatoes allows them to adapt to climate change and disease. For Russet Burbank potatoes, however, farmers must use fertilizers and pesticides to ensure healthy crops, because the plant is susceptible to disease.

To combat this lack of genetic variation, plant breeders often go back to wild species to collect genes that help cultivated plants resist disease, pests and drought. This can also help plants adapt to climate change. However, climate change is also threatening wild varieties due to loss of habitat. That means domesticated plants may lose an important source of traits that help them overcome new threats.

The Red List

The International Union for Conservation of Nature (IUCN) keeps a “Red List of Threatened Species.” The Red List defines the severity and specific causes of a species’ threat of extinction. The Red List has nine levels of conservation. Seven of these levels rank how close a species is to extinction: least concern, near threatened, vulnerable, endangered, critically endangered, extinct in the wild and extinct. Of the two additional levels used by the IUCN, one is “data deficient,” which means that scientists have not been able to gather enough information about a species to determine which category they belong. The other level is “not evaluated,” which means scientists have not gathered information for that species.

Species that are not threatened by extinction at the time of the research are placed within the first two categories: least concern and near threatened. Those that are most threatened are placed within the next three categories, known as the threatened categories: vulnerable, endangered and critically endangered. Species that are extinct in some form are placed within the last two categories: extinct in the wild and extinct.

Classifying a species as endangered has to do with its range and habitat, as well as its actual population. For this reason, a species can be of least concern in one area and endangered in another area. The gray whale (Eschrichtius robustus), for instance, has a healthy population in the eastern Pacific Ocean along the coast of North and South America. The population in the western Pacific, however, is critically endangered.

While the Red List is an important tool for conservation and awareness, it does have some limitations. For example, some experts criticize the IUCN for grouping the “extinct in the wild” category with “extinct,” instead of grouping it in one of the threatened categories. The Red List focuses on animal populations in the wild, which is why it groups “extinct” and “extinct in the wild” together. However, some of these plants and animals are thriving in captivity, and people are making efforts to reintroduce them to their natural habitats. Experts say these species need further assessment to determine how close to extinction they are. Additionally, scientific research suggests that species in the “data deficient” designation are very likely to be close to extinction. However, because species in that category are not considered at risk, they are generally not included in legislation designed to protect threatened species.

Least Concern

Least concern is the lowest level of conservation. A species of least concern is one that has a widespread and abundant population. Examples of species of least concern are the arctic fox (Vulpes lagopus), the harbor seal (Phoca vitulina) and the brown bear (Ursus arctos).

Near Threatened

A near threatened species is one that is likely to qualify for a threatened category in the near future.

American bison (Bison bison) nearly went extinct due to the hunting practices of White frontier settlers in the 19th century, but they are now in the near-threatened category. This is due to conservation efforts, many of which have been 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 such as these working to protect them, the bison population would likely decrease much further.

However, bison remain near threatened. Their habitats have been largely taken away by the U.S. government for farmland. Because of this, few bison live in their original habitat, which contributes to their near-threatened status.

Vulnerable Species

The definitions of the three threatened categories (vulnerable, endangered and critically endangered) are based on five criteria: population reduction rate, geographic range, population size, population restrictions and probability of extinction. Threatened categories have different thresholds for these criteria. As the population and range of the species decreases, the species becomes more threatened.

1) Population reduction rate

A species is classified as vulnerable if its population has declined between 30% and 50%. This decline is measured over 10 years or three generations of the species, whichever is longer. A generation is the period of time between the birth of an animal and the time it is able to reproduce. Mice are able to reproduce when they are about six weeks old. Because this time is so short, mouse populations are mostly tracked over 10-year periods. An Asian elephant's generation lasts about 14 years, so Asian elephant (Elephas maximus) populations are measured over 45-year periods.

2) Geographic range

A species is characterized as vulnerable if its “extent of occurrence” is estimated to be less than 20,000 square kilometers (7,722 square miles). An extent of occurrence is the smallest area that could contain all sites of a species’ population. If all members of a species could survive in a single area, the size of that area is the species’ extent of occurrence.

A species is also classified as vulnerable if its “area of occupancy” is estimated to be less than 2,000 square kilometers (772 square miles). An area of occupancy is where a specific population of that species resides. This area is often a breeding or nesting site in a species’ range.

3) Population size

Species with fewer than 10,000 mature individuals are considered vulnerable. A species is also categorized as vulnerable if its population declines by at least 10% within 10 years or three generations, whichever is longer.

4) Population restrictions

Population restriction is a combination of population and area of occupancy. A species is considered vulnerable if it is restricted to less than 1,000 mature individuals.

5) Probability of extinction in the wild for a vulnerable species is at least 10% within 100 years.

Vulnerable Species: Ethiopian Banana Frog

The Ethiopian banana frog (Afrixalus enseticola) is a small frog native to high-altitude areas in southern Ethiopia. It is a vulnerable species, because the extent and quality of its forest habitat are in decline. One of the main threats to this habitat is forest clearance, mostly for housing and agriculture.

Vulnerable Species: Tahiti Reed-warbler

The Tahiti reed-warbler (Acrocephalus caffer) is a songbird found on the Pacific island of Tahiti. It is a vulnerable species because it has a very small population. The bird is found only on a single island, meaning both its extent of occurrence and area of occupancy are small, under 500 square kilometers (193 square miles).

The Tahiti reed-warbler is also endangered because of human activity. When an electricity plant opened up in the bamboo forests where the birds live, the new roads and tracks through the forests disturbed the birds’ space and further degraded their habitat. There is also an invasive weed called Miconia that is crowding out the native bamboo and preventing it from growing.

Vulnerable Species: Galápagos Kelp

Galápagos kelp (Eisenia galapagensis) is a type of seaweed only found near the Galápagos Islands in the Pacific Ocean. Galápagos kelp is classified as vulnerable because its population has declined by 30% and 50% in 10 years.

Climate change is the leading cause of decline among Galápagos kelp. Galápagos kelp is a cold-water species, and it does not adapt quickly to changes in water temperature. Climate change as a result of rising greenhouse gases has made oceans warmer, harming the kelp. There is also a natural weather pattern called El Niño that brings unusually warm water to the Galápagos, which worsens the problem.

Endangered Species

1) Population reduction rate

A species is considered endangered when its population has declined between 50% and 70%. This decline is measured over 10 years or three generations of the species, whichever is longer.

A species is classified as endangered when its population has declined by at least 70% and the cause of the decline is known. A species is also classified as endangered when its population has declined by at least 50% and the cause of the decline is unknown.

2) Geographic range

An endangered species’ extent of occurrence is less than 5,000 square kilometers (1,930 square miles). An endangered species’ area of occupancy is less than 500 square kilometers (193 square miles).

3) Population size

A species is classified as endangered when there are fewer than 2,500 mature individuals. When a species population declines by at least 20% within five years or two generations, it is also classified as endangered.

4) Population restrictions

A species is categorized as endangered when its population is restricted to less than 250 mature individuals. When a species’ population is this low, its area of occupancy is not considered.

5) Probability of extinction for endangered species in the wild is at least 20% within 20 years or five generations, whichever is longer.

Endangered: Scimitar-horned Oryx

The scimitar-horned oryx (Oryx dammah)—a species of antelope with long horns—once extended across northern Africa. Previously, the scimitar-horned oryx was listed as extinct in the wild due to overhunting (for meat and leather) and habitat loss (for livestock). However, populations still existed on animal reserves. In 2016, one of these herds was reintroduced into the wild. Research has shown that the animals are living in the wild successfully without human intervention. Though the population of wild scimitar-horned oryx is still small, it has grown enough to move the species to the endangered designation.

Endangered: Indus River Dolphin

The Indus River dolphin (Platanista minor) is a dolphin that lives in the upper portion of the Indus River in Pakistan. It has poor eyesight and lives in murky water, so it uses echolocation to navigate and catch food. The Indus River dolphin is endangered because human development has broken up its river habitat. Pakistan—a largely hot, dry country—has built dams in the Indus River to help irrigate agricultural fields, lowering the river’s water levels. These dams have also kept the dolphins apart and made mating difficult, causing the population to drop. Pakistan is likely to build more irrigation dams to keep crops alive as climate change results in more droughts. Should this occur, the Indus River dolphin many face even worse outcomes.

Endangered: Big Leaf Mahogany

A big leaf mahogany (Swietenia macrophylla) is a very tall tree that grows in the rainforests of Central and South America. Historically, there has been a strong international demand for mahogany because of its use in cabinetry and flooring. This has resulted in overharvesting that has severely limited the range of mahogany trees. Once harvested, it is difficult to grow more trees in the same forest. The tree needs a lot of sunlight, and adult mahogany trees block mahogany saplings from sprouting. However, there are people working to conserve big leaf mahogany trees. For example, the Wampis Nation, an Indigenous group in Peru, have a legal right to manage the tropical forest, and they protect mahogany trees from illegal logging.

Critically Endangered Species

1) Population reduction rate

A critically endangered species’ population has declined between 80% and 90%. This decline is measured over 10 years or three generations of the species, whichever is longer.

2) Geographic range

A critically endangered species’ extent of occurrence is less than 100 square kilometers (39 square miles). A critically endangered species’ area of occupancy is estimated to be less than 10 square kilometers (4 square miles).

3) Population size

A species is classified as critically endangered when there are fewer than 250 mature individuals. A species is also classified as critically endangered when the number of mature individuals declines by at least 25% within three years or one generation, whichever is longer.

4) Population restrictions

A species is considered critically endangered when subpopulations are restricted to less than 50 mature individuals. When a species’ population is this low, its area of occupancy is not considered.

5) Probability of extinction for critically endangered species in the wild is at least 50% within 10 years or three generations, whichever is longer.

Critically Endangered Species: Bolivian Chinchilla Rat

The Bolivian chinchilla rat (Abrocoma boliviensis) is a rodent found in a small section of the Manuel M. Caballero Province of Bolivia. It is critically endangered because its extent of occurrence is less than 100 square kilometers (39 square miles). The major threat to this species is loss of its cloud forest habitat. People are clearing forests to create more land for agriculture. Scientists do not know a lot about these rodents, in part because they are so rare.

Critically Endangered: Galápagos Pink Land Iguana

The Galápagos pink land iguana (Conolophus marthae) is only found on one small island in the Galápagos. As the name suggests, these lizards have a distinctive pink hue. Scientists had assumed that these pink lizards were part of the same species as other land iguanas until the late 2000s, when research revealed that it was a unique species. The entirety of their small population lives on the side of an active volcano. Predators (like invasive rats), periodic droughts and volcanic eruptions have contributed to the iguana’s critically endangered status. There are approximately 200 left in the world.

Critically Endangered: Sri Lankan Legume Tree

The Sri Lankan legume tree (Crudia zeylanica), native only to the island of Sri Lanka in the Indian Ocean, is a giant species of legume, a group that includes peas and peanuts. However, people do not eat the legumes of the Sri Lankan tree.

Until 2019, scientists and conservationists believed that the Sri Lankan legume tree was extinct. That year, a group of researchers found six adult trees in new locations in Sri Lanka. This tree is still at high risk, however, as a highway construction project threatens the species’ survival. For example, one tree was removed to complete an expressway, despite the efforts of environmentalists and even Buddhist monks who blessed the tree in an effort to save it from being cut down. The Royal Botanic Gardens in Peradeniya, Sri Lanka is growing several specimens of the tree and hopes to reintroduce them into the wild.

Extinct in the Wild

A species is categorized as extinct in the wild when it only survives in cultivation (if a plant), in captivity (if an animal) or as a population well outside its established range. A species may be listed as extinct in the wild only after years of surveys have failed to record an individual in its native or expected habitat.

Extinct in the Wild: Alagoas Curassow

The Alagoas curassows (Mitu mitu) is a black bird native to Brazil. Like other curassows, the Alagoas curassow has glossy feathers, but they also have a spot of gray, bare skin near their ears, which distinguishes them from other curassow species.

Alagoas curassows have been extinct in the wild since the late 1980s. They once lived in a lowland forest habitat that was systematically cleared by the Brazilian government so the land could be used to grow sugarcane. The remaining Alagoas curassows that live in captivity were bred from only three birds, so they are currently experiencing a genetic bottleneck similar to the one experienced by ancient cheetahs. They are, however, breeding successfully enough in specialized programs that a few were reintroduced in the wild in 2019, though it is still unclear if the birds will be able to live independently in the wild.

Extinct in the Wild: Wyoming Toads

Wyoming toads (Anaxyrus baxteri) lived only in the Laramie Basin in the U.S. states of Wyoming and Colorado. The Wyoming toad is a small amphibian that lived in nonpermanent, shallow ponds and lakes that are formed by rainwater, melted snow and groundwater.

Wyoming toads died off in the wild because of pollution and disease. Pesticides that run off of nearby farms polluted the water where the toads once lived. There is also a fungus that has depleted amphibian species around the world, including in the local Laramie waters. Despite these challenges, there is a small group of Wyoming toads living in the Mortenson Lake National Wildlife Refuge. At this time, the 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 in the Wild: Yellow Fatu

The yellow fatu (Abutilon pitcairnense), a plant with pale yellow flowers, is endemic to Pitcairn Island. Though it was discovered on the island in 1898, sightings were not common. It was believed to be extinct prior to 2003, when one specimen was found in the wild. After that discovery, organizations like the Royal Botanic Gardens housed some of its seeds to try and grow more specimens and reintroduce them in the wild. Unfortunately, the wild plant was killed by a landslide in 2005.

In addition to landslides and other types of land degradation, invasive species have made it difficult for this plant to grow. One of these species is the rose-apple tree (Syzgium jambos), which was brought to the island as a wood fuel source. This tree not only grows tall to crowd out other species, but it also spreads its roots laterally and stops other seeds from taking hold.

Extinct

A species is considered extinct when there is no reasonable doubt that the last remaining individual of that species has died.

Extinct: Cuban Macaw

The Cuban macaw (Ara tricolor) was a tropical parrot native to Cuba and a small Cuban island, Isla de la Juventud. Hunting and collecting the birds for pets led to the bird’s extinction. The last specimen of the Cuban macaw was collected in 1864.

Extinct: Bramble Cay melomys

The Bramble Cay melomys (Melomys rubicola) was a rat-like rodent endemic to Bramble Cay, a small Australian island. Climate change was the major factor in its extinction, which occurred in the early 2010s. Climate change has caused sea levels to rise and made harsh storms more devastating. Flooding repeatedly damaged the melomys’ seaside habitat, making it difficult for the rodents to get food and find shelter. These floods also directly killed many individuals of the species. Bramble Cay melomys are considered the first mammal to become extinct due to the cumulative effects of climate change.

Extinct: St. Helena Olive

The St. Helena olive (Nesiota elliptica) was found only on a small volcanic island in the Atlantic Ocean. The British made the island a colony in the mid-1600s, and colonists cleared much of the island for farming during the 19th century, greatly reducing the population of the olive tree. The last tree living in the wild died in 1994, and the last cultivated specimen died in 2003. The St. Helena olive tree is extinct with no seeds or other material to revive it.

Endangered Species and People

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 and destruction of the species’ habitats. Individuals and organizations that break these laws may face huge fines. 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 such actions, some species have recovered from their endangered status.

The brown pelican (Pelecanus occidentalis), 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 damaging the pelicans’ eggs. DDT thinned the eggshells, making 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, human access to nesting sites was severely restricted, and the U.S. government banned the pesticide DDT. During the 1980s, the number of brown pelicans steadily increased. 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.