Under the Endangered Species Act in the United States, a species or subspecies is regarded as "endangered" when it is in danger of extinction throughout all or a significant part of its normal range.

A species is considered "threatened" when it is likely to become an endangered species in the foreseeable future.

A species does not have to be in danger of global extinction in order to be regarded as endangered. For example, many species that are endangered or threatened in the lower 48 states of the United States still thrive in Alaska, which is largely still wilderness. These include the grizzly bear, bald eagle, and gray wolf, among others.

It makes sense to protect a species before it has declined too far. When its populations have become small or isolated, it is harder and more expensive to help the species recover.

In addition, many plants and animals play an important role in their ecosystems, such as that of a keystone species. It is important to conserve species in many parts of their range so ecosystems remain healthy.

Note: Emphasized words can be found in the glossary.