(covers information from several alternate timelines)
An endangered species was a species which was on the brink of extinction, whether caused naturally or by the actions of sentient beings.
In an alternate timeline the Xindi succeeded in destroying Earth in 2154, rendering Humans an endangered species of roughly six thousand. (ENT: "Twilight")
Josiah, a member of the xenophobic organization Terra Prime, claimed in 2155 that Humans were becoming an endangered species due to the increasing presence of aliens on Earth. (ENT: "Demons")
Endangered species within the Federation, including space-borne species, were listed on the Endangered Species List and protected by the Endangered Species Act. Starfleet captains were required to act to protect them on threat of court martial. (DIS: "Magic to Make the Sanest Man Go Mad")
In 2258 of an alternate reality, Spock considered himself part of an endangered species when Vulcan was destroyed by Nero, leaving roughly 10,000 Vulcan survivors, in his estimation. (Star Trek)
Corvan gilvos were endangered by the pollution of their native habitat on Corvan II. In 2368, there were only fourteen remaining. (TNG: "New Ground")
