 As the IUCN Red List does not consider a species extinct until extensive, targeted surveys have been conducted, species that are possibly extinct are still listed as critically endangered.IUCN maintains a list of "possibly extinct" "CR(PE)" and "possibly extinct in the wild" "CR(PEW)" species, modelled on categories used by BirdLife International to categorize these taxa.To be defined as critically endangered in the Red List, a species must meet any of the following criteria (A–E) ("3G/10Y" signifies three generations or ten years—whichever is longer—over a maximum of 100 years; "MI" signifies Mature Individuals)