What are two reasons that populations will decrease?

A reduction over time in a region's population can be caused by sudden adverse events such as outbursts of infectious disease, famine, and war or by long-term trends, for example sub-replacement fertility, persistently low birth rates, high mortality rates, and continued emigration.

The two factors that decrease the size of a population are mortality, which is the number of individual deaths in a population over a period of time, and emigration, which is the migration of an individual from a place