Threatened species are any species (including animals, plants, fungi, etc.) which are vulnerable to endangerment in the near future. Species that are threatened are sometimes characterised by the population dynamics measure of critical depensation, a mathematical measure of biomass related to population growth rate. This quantitative metric is one method of evaluating the degree of endangerment.
The International Union for Conservation of Nature (IUCN) is the foremost authority on threatened species, and treats threatened species not as a single category, but as a group of three categories, depending on the degree to which they are threatened:
Less-than-threatened categories are near threatened, least concern, and the no longer assigned category of conservation dependent. Species which have not been evaluated (NE), or do not have sufficient data (data deficient) also are not considered "threatened" by the IUCN.
Although threatened and vulnerable may be used interchangeably when discussing IUCN categories, the term threatened is generally used to refer to the three categories (critically endangered, endangered and vulnerable), while vulnerable is used to refer to the least at risk of those three categories. They may be used interchangeably in most contexts however, as all vulnerable species are threatened species (vulnerable is a category of threatened species); and, as the more at-risk categories of threatened species (namely endangered and critically endangered) must, by definition, also qualify as vulnerable species, all threatened species may also be considered vulnerable.
Subspecies, populations and stocks may also be classified as threatened.
Under the Endangered Species Act in the United States, "threatened" is defined as "any species which is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range". It is the less protected of the two protected categories. The Bay checkerspot butterfly (Euphydryas editha bayensis) is an example of a threatened subspecies protected under the ESA.
Within the U.S., state wildlife agencies have the authority under the ESA to manage species which are considered endangered or threatened within their state but not within all states, and which therefore are not included on the national list of endangered and threatened species. For example, the trumpeter swan (Cygnus buccinator) is threatened in the state of Minnesota, while large populations still remain in Canada and Alaska.
The Commonwealth of Australia has legislation for categorising and protecting endangered species, namely the Environment Protection and Biodiversity Conservation Act 1999, which is known in short as the EPBC Act. This act has six categories; extinct, extinct in the wild, critically endangered, endangered, vulnerable, and conservation dependent, defined in Section 179 of the act, and could be summarised as;
The EPBC Act also recognises and protects threatened ecosystems such as plant communities, and Ramsar Convention wetlands used by migratory birds.
Individual states and territories of Australia are bound under the EPBC Act, but may also have legislation which gives further protection to certain species, for example Western Australia's Wildlife Conservation Act 1950. Some species, such as Lewin's rail (Lewinia pectoralis), are not listed as threatened species under the EPBC Act, but they may be recognised as threatened by individual states or territories.