[prescriptivist]

Sounds like the limit is discretionary. From the NYT:

"Scientists do not declare extinctions lightly. It often takes decades of fruitless searching. About half of the species in this group were already considered extinct by the International Union for Conservation of Nature, the global authority on the status of animals and plants. The Fish and Wildlife Service moved slower in part because it is working through a backlog, officials said, and tends to prioritize providing protection for species that need it over removing protection for those that don’t."