Comparison of Red list classes above
and NatureServe status below
The World Conservation Union (IUCN) are the experts on threatened species. They put threatened species into three groups, each group in more danger than the next:
One way scientists can work out how threatened a species is, is to measure the birth and death rates. If a species becomes small in numbers, it can get to a critical point. This happens when there are not enough individuals left alive to continue the species (critical depensation).
Species that are not a threatened species, but are still a concern, can be put into groups of Near Threatened, Least Concern, and Conservation Dependent. Conservation dependent is no longer used a group. When the species has not been studied, Not Evaluated, or there is not enough information, Data Deficient, the IUCN does not put them on the threatened species list.
Threatened species are also called "red listed" species, because they are in the IUCN Red List of Threatened Species.
United States definition change
Under the Endangered Species Act in the United States, "threatened" means any species which is likely to become endangered in the near future, in all, or a large part of its range. The Bay checkerspot butterfly (Euphydryas editha bayensis) is an example of a threatened species protected by the Endangered Species Act.