Threatened species, as a conservation status, are animal and plant species which are at risk of being extinct. The best-known worldwide conservation status listing and ranking system, is the IUCN Red List. Other more specialised lists and systems exist, such as The Nature Conservancy's conservation status ranking system and CITES. By monitoring these species, the ecology of an area is assessed.
When discussing the IUCN Red List, the official term "threatened" is a grouping of three categories: Critically Endangered, Endangered, and Vulnerable. Critically endangered is the most threatened of the three classifications and Vulnerable is the least.
The criteria (highly simplified) are the following:
Ø Population size must have decreased by 90% or more over the last 10 years.
Ø Extent of occurrence, is less than 100km² and the area of occupancy is less than 10km² with a fragmented range or it is only found at a single location.
Ø Population size is estimated to number less than 250 mature individuals and continuing decline is projected.
Ø In addition there are fluctuations in numbers of individuals and sub-populations contain less than 250 mature individuals.
Ø Population size is estimated to be less than 50 mature individuals.
Ø Quantitative analysis and modelling show the probability of extinction in the wild is at least 50% within 10 years.