Extinction may occur when a specie's total number falls below the number required for successful breeding, because its members cannot locate mating partners or their genetic pool loses the needed variety. According to the International Union For Conservation Of Nature, the categories of the risk of extinction are (1) Endangered: unlike to survive if the causal factors continue to operate. (2) Vulnerable: will soon be endangered if the causal factors continue to operate. (3) Rare: not endangered or vulnerable but at risk.