If the birth rate is less than the death rate over time, extinction results.
Extinction is a natural result of evolution.
Species go extinct when they are unable to adapt to changes in the environment or compete effectively with other organisms. Well over 99 per cent of the species that have ever lived have gone extinct. A graph of invertebrate extinctions over the last 600 million years. The mass extinctions appear as periodic peaks rising above the background extinction levels. This data is from the work of D. M. Raup and J. J. Sepkoski.