According to the 1973 US Endangered Species Act, animals are defined as endangered if the species is close to extinction throughout all or most of its environment. Animals become endangered for a variety of reasons, most have to do with changes to their natural habitats. In fact, habitat destruction is a primary cause. Urban growth, mining and agricultural operations all can destroy the natural habitats of animal populations.

Some animals become endangered when their habitats contain resources for humans. For example, commercial deforestation in the Amazon Rainforest has seriously reduced the population of many animal species and lead to the extinction of other groups. Land itself is a important resource to humans since it provides us with living space, and when people replace animal habitats with human habitats, it can lead to extensive loss of natural ranges for species.

Commercial and industrial operations may reduce land or water for animals and pollute an animal’s environment. Due to water pollution, the baiji, a fresh water dolphin species native to China, was declared functionally extinct, meaning it’s unlikely the population can recover. Even noise pollution from human activities can lead to the disruption of animal behavior. Some experts point to Navy sonar testing, which can alter migratory patterns of certain whales, as a possible explanation for groups of whales beaching themselves.

Another factor that makes animals become endangered is over-fishing or over-hunting. Experts believe that in the 1600s the passenger pigeon, an American bird, constituted between 25-40% of the bird population in the United States. Because there were no laws governing how many birds a hunter could kill, millions were hunted for meat over the next 300 years. In 1921, the last passenger pigeon died in captivity; the species’ extinction is believed to be due almost entirely to over-hunting. Commercial whaling led to near extinction of many species of whales before most, but not all, countries agreed to ban this practice.

The introduction of an non-native species to a habitat may make animals become endangered. Species that are native to the environment may not have any defense against a new predator, and the effects can be devastating. In Australia, the introduction of the common house cat led to the extinction of the red-fronted parakeet and has seriously damaged the populations of several small mammals, including the bilby and the numbat.

Introduced species can also bring new diseases with them, providing another reason why animals become endangered. Native animals may not have resistance to non-native diseases, and population levels can suffer. Another cause of disease is human-introduced chemicals. The use of Dichloro-Diphenyl-Trichloroethane (DDT) and other insecticides is believed to have had a direct effect on bird populations. In fact, DDT poisoning is believed to be one of the leading factors in driving the California Condor to near extinction.

Species endangerment can also arise from a combination of factors. Animals might face loss of habitat, unfamiliar diseases, poisons in their environment and a variety of other dangers. When this occurs, it can be difficult to save a species from extinction.

When animals become endangered, the entire food chain can be affected. Even the status of the smallest of animals can have a profound effect. For example, in 2008 disease seriously threatened the honeybee, which is largely responsible for pollinating flowering plants, including many fruits and vegetables. Without their pollination, an enormous part of the human diet could be decimated. In fact, should the honeybee become extinct, resulting economic and environmental costs are predicted to run into the billions of dollars. Therefore, many people argue that when animals become endangered it is in man's own self-interest to protect them.