Natural Population Change is calculated by subtracting the death rate from the birth rate. It is the difference between the number of live births and deaths during a given time period. It can be either positive or negative. The three main causes of population change are: - Births that are usually measured using the birth rate (number of live births per 1,000 of the population per year). - Deaths that are usually measured using the death rate (number of deaths per 1,000 of the population per year). - Migration is the movement of people in and out of an area.