The number of threatened mammal species listed by IUCN as critically endangered, endangered or vulnerable. The IUCN classification uses quantitative criteria, based on population size, rate of decline and area of distribution, to assign species to the above categories. Listing in a higher extinction risk category implies a higher expectation of extinction; and, over the specified time frames, more species listed in a higher category are expected to become extinct than those in a lower one (without effective conservation action).