The endangered species list is a result of the Endangered Species Act of 1973. According to this law, endangered species are those that are likely to become extinct as a result of human activities or natural causes in all or a major portion of their range. Threatened species are those that are likely to become endangered. The Secretary of the Interior and the Secretary or commerce, acting through the National Marine Fisheries Service (NMFS), can add and remove species from the list.
The law requires government agencies to consult with the Fish and Wildlife Service and the NMFS to make sure that their activities do not negatively affect the species on the list. It also protects these species from individuals and businesses.
There aren't any strict rules regarding numbers, but studies have been done to determine what species actually make it onto the list.
Most species on the list are plants or vertebrates, even though most species in the world are invertebrates. (this does not mean that invertebrates are less threatened, just that they are less known and less appreciated).
Animals listed generally had about 1,000 individuals left at the time they were listed.
Plants generally had fewer than 120 individuals when they were listed.
There were 39 species with 10 or fewer individuals by the time they were listed (the small amount of genetic diversity left in such a small population makes them unlikely to recover due to difficulty adapting and to genetic diseases).
In 1998, there were more than 900 species on the list and almost 4000 species being considered for it. It is a difficult list to get on, mostly because it restricts economic activity.
Some species have gone extinct while being considered for the list. Partly this is due to the time it takes. Partly this is due to landowners killing the species on their property while it is being considered so that, if it becomes listed, they won't have to deal with the restrictions
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As of 1998, only 21 species had been delisted.
