Flexi Says: Populations gain individuals through births and immigration. They lose individuals through deaths and emigration. These factors together determine how fast a population grows. Migration is the regular movement of individuals or populations each year during certain seasons. The purpose of migration usually is to find food, mates, or other resources. So a zebra population may grow under any of the following conditions: if more individuals are born and survive; if more individuals immigrate into the population; if fewer individuals emigrate out of the population, or if fewer individuals die. The latter may occur if a population of predators decreases, leading to fewer zebras taken by predators.