The document discusses population change and its causes. Population change results from natural changes like births and deaths, as well as migration. The demographic transition model describes the typical pattern of population change that occurs as countries industrialize - death rates initially fall as living conditions improve, followed later by falling birth rates. However, the model has weaknesses in overgeneralizing and not accounting for non-industrializing countries or factors like government policy and cultural attitudes. Migration can also significantly impact population change on both short and long time scales.