A threatened species is any type of living thing, such as animals, plants, or fungi, that is at risk of disappearing soon. Scientists sometimes use a method called critical depensation to study how populations grow. This method looks at how the total weight of all the living things in a group affects how quickly their numbers increase. This way of measuring helps determine how endangered a species is without directly considering how humans affect them.

IUCN definition

The International Union for Conservation of Nature (IUCN) is the main organization that studies threatened species. It does not group these species into one category but divides them into three groups based on how much they are at risk:

• Vulnerable species
• Endangered species
• Critically endangered species

Other groups include species that are not as threatened, such as near threatened, least concern, and conservation dependent. Species that have not been studied (NE) or lack enough information (data deficient) are also not considered "threatened" by the IUCN.

The terms "threatened" and "vulnerable" are sometimes used the same way when discussing IUCN categories. However, "threatened" usually refers to all three groups (critically endangered, endangered, and vulnerable), while "vulnerable" refers to the group with the least risk among these three. In most cases, these terms can be used interchangeably because all vulnerable species are also threatened species. Additionally, species in the more at-risk groups (endangered and critically endangered) are, by definition, also considered vulnerable. Therefore, all threatened species can also be called vulnerable.

Threatened species are also known as red-listed species because they are listed in the IUCN Red List of Threatened Species.

Subspecies, populations, and stocks may also be classified as threatened.

By country

The Commonwealth of Australia (federal government) has laws to classify and protect endangered species. These laws are part of the Environment Protection and Biodiversity Conservation Act 1999, often called the EPBC Act. This Act defines six categories of species status in Section 179: extinct, extinct in the wild, critically endangered, endangered, vulnerable, and conservation dependent. These categories are described as follows:

• Extinct: No reasonable doubt that the last member of the species has died.
• Extinct in the wild: Known only to survive in cultivation, and not seen in nature despite thorough searches.
• Critically endangered: Extremely high risk of extinction in the wild soon.
• Endangered: Very high risk of extinction in the wild in the near future.
• Vulnerable: High risk of extinction in the wild over a medium-term period.
• Conservation dependent: Requires specific conservation programs to avoid being classified as one of the other categories.

The EPBC Act also protects threatened ecosystems, such as plant communities and Ramsar Convention wetlands used by migratory birds.

Lists of threatened species are created under the Act and serve as the main reference for identifying threatened species in Australia. The Species Profile and Threats Database (SPRAT) is an online tool that provides details about species and ecological communities listed under the EPBC Act. It includes information about the species’ appearance, population, habitat, behavior, and taxonomy.

The Threatened Mammal Index, launched in April 2020, was combined with the Threatened Bird Index (created in 2018) to form the Threatened Species Index. This project is a collaboration between the National Environmental Science Program’s Threatened Species Recovery Hub, the University of Queensland, and BirdLife Australia. It shows overall trends in species populations rather than detailed data on individual species. Users can access downloadable data through a web-app to compare trends by species group or region. The Index uses visual tools, such as graphs and maps, to display data clearly. In April 2020, the Mammal Index reported a decline of more than one-third in the numbers of threatened mammals between 1995 and 2016. However, the data also showed that targeted conservation efforts have helped some species. The Index includes data from over 400,000 surveys and covers population trends for 57 threatened or near-threatened mammal species in Australia.

Individual states and territories in Australia follow the EPBC Act but may also have their own laws for additional protection. For example, Western Australia has the Wildlife Conservation Act 1950. Some species, like Lewin’s rail (Lewinia pectoralis), are not listed as threatened under the EPBC Act but may be recognized as threatened by specific states or territories.

In New South Wales, the Department of Planning, Industry and Environment identifies key threats to native plants and animals, including pests, weeds, climate change, and habitat loss.

The German Federal Agency for Nature Conservation (Bundesamt für Naturschutz, BfN) maintains a regional Red List for Germany that includes at least 48,000 animal and 24,000 plant and fungus species. This system is similar to the IUCN’s categories but adds a “warning list” for species with uncertain extinction risks and rare species that are not endangered but face high risks due to small populations.

In the United States, the Endangered Species Act defines “threatened” as a species likely to become endangered in the foreseeable future across all or part of its range. This is the less protected category compared to “endangered.” An example of a threatened species under this law is the Bay checkerspot butterfly (Euphydryas editha bayensis).

In the U.S., state wildlife agencies manage species that are endangered or threatened within their state but not nationally. For example, the trumpeter swan (Cygnus buccinator) is threatened in Minnesota, though large populations still exist in Canada and Alaska.