The IUCN (1996) has defined criteria to classify species into critically
endangered, endangered, vulnerable and lower risk. These are based
on population biology principles developed largely by Mace & Lande
(1991). They defined a threatened species as one with a high risk of
extinction within a short time frame. For example, a critically endangered
species has a risk of extinction of 50% within 10 years or three generations,
whichever is longer (Table 1.3).