A population is a group composed of all members of the same species that live in a specific geographical area at a particular time. An example of a population might include all the gray squirrels that live in a certain urban park. The areas occupied by a population could include the small area (measured in square millimeters) occupied by bacteria in a rotting apple to the vast areas of ocean (square kilometers) that include the territory of migrating sperm whales. Population ecology is the branch of ecology that studies the structure and changes within a population. Studies of specific populations will indicate the dynamics of the population, in terms of active, ongoing growth; declining growth; or stability.