An endangered species is a species that is threatened by extinction. There are two main reasons species become endangered; loss of habitat and loss of genetic variation.
A loss of habitat can happen naturally. For example, dinosaurs lost their habitat about 65 million years ago due to an asteroid that hit the the Earth. It blasted debris into the air reducing the amount of heat and sunlight that reached the Earth's surface causing the plants to die. The air also grew cooler. Dinosaurs were unable to adapt to these changes causing them to eventually become extinct.
Human activity can also cause habitat loss. Development for housing, industry, and agriculture cause habitat loss of many native oragnisms. Development can eliminate habitats and native species directly. For example, in the Amazon Rainforest, developers have cleared thousands of acres for cattle ranches, logging, and other urban use. Development can also endanger species indirectly. When developers cut down trees, the species that depend on that tree may also become endangered. Species have less room to live and reproduce. These changes affect species that live there. It may destroy the food some animals need. Without a healthy habitat, some plants and animals can become endangered.
Genetic variation is the diversity found within a species. It's why humans have different colored hair. Genetic variation allows species to adapt to changes in the environment. Usually, when the population of a species if greater, the genetic variation is also greater.
Inbreeding is when close family members reproduce. Groups of species that inbreed usually have less genetic variation due to the fact that no new genetic information is introduced to the group. Disease is much more common, and much more deadly, among inbred groups. Inbred species don't have the gentic variation to develop resistance to the disease, causing fewer offsprings of inbred groups to survive till maturity.
Loss of genetic variation can happen naturally. For example, Cheetahs are a threatened species to native Africa and Asia. Cheetahs have very little genetic variation and cannot adapt to changes in the environment as quickly as other animals. For this reason, fewer cheetahs surive to maturity.
Human activity can also lead to a loss of genetic variation. Overhunting and overifshing have reduced the population of many animals. Reduced population usually means that there is less genetic variation.
The International Union for Conservative 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 and alerts organizations and governments about species that require greater conservation efforts.
The threat of a species is usually calculated by the size of a population, or if the poplation has experienced a sudden decline. Rapid destruction of its habitat is also a very important factor. The Red List has seven levels of conservation: least concern, near threatened, vulnerable, endangered, critically endangered, extinct in the wild, and extinct. There catagories represent different threat levels.
To learn more about the Red List click here
Least concern is the lowest level of conservation. A species of least concern is one that has a widespread and big population. Humans are an example of a species that is classified as least concern along with most domestic animals, like cats and dogs. Many wild animals, like pigeons and houseflies, are also classified as least concern.
A near threatened species is a species that may face a significant threat in the near future. For example, many species of violets, native to tropical jungles in South America and Africa, are near threatened. They have healthy populations but, their habitat is disappearing at a fast pace.
These three catagories (vulnerable, endangered and critically endangered) are based on five criteria: population reduction rate, geographic range, population size, population restrictions, and probability of extinction.
A species is clasified as vulnerable if...
A species is classified as endangered if...
A species is classified as critically endangered if...
A species is extinct in the wild when it no longer lives in its natural habitat. It may be cultivated in greenhouses (plants) or kept in captivity at zoos (animals). 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.
A species is extinct when the last remaining individual of that species has died.