Ecology is the discipline of biology that is concerned primarily with the interaction between 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 are the population, community, ecosystem, and biosphere.
A population is a group of individuals belonging to one species usually occupying a defined area. 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, the environment is said to have reached its carrying capacity. The growth of a population cannot exceed the carrying capacity of its environment for long.
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, an example of a density‐dependent population. Density‐dependent curbs include disease, competition, predation, and territoriality. Another curb, occurring in a density‐independent population, takes place when there is an external limiting factor. Density‐independent curbs result from climatic fluctuations, such as temperature changes.