Flexi Says: Population growth rate (r) is how fast a population changes in size over time. A positive growth rate means a population is increasing. A negative growth rate means it is decreasing. The two main factors affecting population growth are the birth rate (b) and death rate (d). Population growth may also be affected by people coming into the population from somewhere else (immigration, i) or leaving the population for another area (emigration, e). The formula for population growth takes all these factors into account. r = (b + i) - (d + e) r = population growth rate b = birth rate i = immigration rate d = death rate e = emigration rate. Migration is another type of movement that changes population size. 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. For example, many northern hemisphere birds migrate thousands of miles south each fall. They go to areas where the weather is warmer and more resources are available. Then they return north in the spring to nest. Some animals, such as elk, migrate vertically. They go up the sides of mountains in spring as snow melts. They go back down the mountain sides in fall as snow returns.