Genetic drift
Infections and extreme climate events are frequent causes. Occasionally, invasions by more competitive species can be devastating. The extinction of many Australian marsupials by foreign species is a famous example. Although the population has rebounded, its genetic variability is much less than that of the Southern elephant seal. Cheetahs have very little variation. We think the species was reduced to a small number at some recent time. Because it lacks genetic variation, it is in danger from infectious diseases.