Ecology is the discipline of biology that is concerned primarily with the interaction between and among organisms and their environments. There are many levels of organization in the world of living things, and each level has features not displayed by any other. The important levels of organization are population, community, ecosystem, biosphere, and biome.

A population is a group of individuals belonging to one species usually occupying a defined area at a defined time. Populations of living things interact with other populations of their own kind, with populations of other species, and with physical aspects of their environment.

A population’s growth proceeds until reaching certain environmental limits. When a population has reached the maximum size that the environment can support indefinitely, assuming consistent conditions, the population is said to have reached its carrying capacity. The growth of a population cannot exceed the carrying capacity of its environment forever.

The growth of a population passes through stages. There is an initial lag period of minimal growth, followed by an exponential growth period of maximum population growth. The curbs that limit population growth may include the crowding of a population to the point that resources like food and water become scarce, an example of a density-dependent factor affecting population size. Density-dependent factors include disease, competition, predation, and territoriality. Density-independent factors that affect population size include weather-related events, for example.