Population ecology measures the size of a population: all the living things from one species that live in an place. A population gets bigger because of birth and movement into a place, and it gets smaller because of death and movement out of a place. Growth rate is the change in population size divided by the current population size. When a population is small, growth rate does not change, so the population shows exponential growth. Rate of exponential growth depends on how a living thing reproduces. If it has only a few offspring (children) which grow slowly, like a human, the rate will be low. If it has a lot of offspring which grow quickly, like a fruit fly, the rate will be high. Any environment only has enough resources, such as food, water, or space, for a certain size of population. This size is called the carrying capacity. When population size is near the carrying capacity, growth rate will become less. The graph of population growth will be an S-shape, called logistic growth.