Now, There are Only 4 Northern White Rhino Left on Earth

There is a difference between extinction that occurs as a result of gradual changes in the biosphere and extinction that occurs as a result of human activity. The former is a slow process, one that takes generations. Since it is such a slow process, other species have time to adapt, to find other sources of food. The latter is quick, and may take just a decade (or less). Because it happens so rapidly, it often significantly disrupts (and even destroys) local habitats.