It would be useful to be able to predict future populations to be prepared to care for the living. Studying population growth also helps scientists understand what causes changes in population sizes and growth rates. ... Finally, studying population growth gives scientists insight into how organisms interact with each other and with their environments. The population of a country or area grows or declines through the interaction of three demographic factors: fertility, mortality, and migration. To project future population, demographers make assumptions about how the current rates of births, deaths, and immigration and emigration will change in the future.