Three things that will cause a population to decrease are:
- High mortality rates due to disease, famine, war, or civil unrest. Examples include pandemics like the Black Death, famines like the Great Irish Famine, and wars such as the Mongol invasion of Europe.
- Low birth rates and fertility rates falling below the replacement level (sub-replacement fertility). This can happen due to social, economic, or political factors such as government population policies, urbanization, delayed marriage, and lifestyle changes.
- Emigration or net migration loss, where more people move out of a region than move in. This can be driven by economic opportunities elsewhere, conflicts, or lack of facilities leading to depopulation.
These factors can act separately or in combination to cause population decline in a region or country.
