Animals can become endangered for a number of different reasons, but many experts believe that loss of habitat is frequently the main cause. Pollution can also damage a population, as can overhunting and overfishing. In other words, the activities of human beings are the biggest problems. Predation and the introduction of new animals, disease and food chain disruption also contribute, however, and it is common for more than one issue to put creatures at risk.
Some animals become endangered when people take over their habitats, which often happens because individuals or groups want one or more resources those areas have. Commercial deforestation in the Amazon rainforest, for example, provides lumber that is used for things like construction, paper and furniture, but it has seriously reduced the population of many species and led to the extinction of others. Land itself is a important resource to humans, because it provides living, building and farming space.
In some instances, loss of habitat occurs naturally, such as with forest fires or flooding of land over time. These cases are much harder to predict, but people sometimes can reduce or even reverse the effects through simple techniques like planting trees. Even these methods take time, however, so environmentalists and zoologists are frequently in a race against the clock in order to keep numbers of populations from declining too severely. They sometimes have to use alternate methods, such as capturing or raising animals in captivity to be released in a specific location.
Sometimes, people influence natural phenomena, indirectly causing habitat loss. A good example are polar bears, which are slowly losing habitat as rising global temperatures cause sea ice to melt. Experts believe that individuals are making the problem worse primarily through the emission of greenhouse gases, such as carbon dioxide. It is not always clear how much influence people have, however. The greenhouse effect can influence weather patterns that affect animals, for instance, but it's difficult to measure the percentage of global warming that relates to human activities.
Commercial and industrial operations may reduce land or water for animals and pollute an animal’s environment. The baiji, a fresh water dolphin species native to China, lived in a major river that became very polluted; it has been declared functionally extinct, meaning that it’s unlikely the population can recover. Even noise 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 these animals beaching themselves.
Overhunting and Overfishing
Another factor that makes animals become endangered is overfishing or overhunting. Some environmental groups point to hunting as an immediate threat to many large animals, such as jaguars, and claim that as much as 25% of species are declining because of human activity. Experts believe that the extinction of the American passenger pigeon is due almost entirely to over-hunting. In the 1600s, these birds made up between 25 - 40% of the bird population in the United States. At the time, no laws governed how many of them a hunter could take, so people killed millions for meat over the next 300 years. The last passenger pigeon died in captivity in 1921. Similarly, commercial whaling led to near extinction of many types of whales before most, but not all, countries agreed to ban this practice.
New Species and Predation
The introduction of an non-native species to an area might contribute to others becoming endangered. Animals that are native to the environment might not have any defense against a new predator, making them easy to kill. 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. Even if the creatures that are introduced aren't predatory, they can compete for food, water and living space.
Disease has the potential to devastate populations of different animals. As an example, an epidemic of bovine spongiform encephalopathy (BSE), better known as mad cow disease, killed hundreds of thousands of cattle worldwide starting around 1970. Some of these diseases can be attributed to genetics or affect only one species, but many can pass from one type of animal to another. The introduction of new species to an environment, including people, sometimes is problematic for this reason.
Problems in the Food Chain
When animals become endangered, the entire food chain can be affected, and even the status of the smallest of animals can have a profound effect. Scientists believe, for example, that a disease caused by a mite-transmitted virus has seriously threatened honeybee populations world wide. These insects are largely responsible for pollinating flowering plants, including many fruits and vegetables. Without their pollination, thousands of species could lose a source of food and shelter, which further affects the higher-level creatures that consume those animals.
Combination of Problems
Species can become endangered due to a combination of several of these factors. A species that's fighting both habitat loss and pollution, like the baiji, for example, may die off more quickly than either alone would cause. The combination of factors also makes it much more difficult to save animals from extinction. Doing so usually requires a collective effort from many people, which is sometimes difficult to achieve because of cultural and economic differences between regions.