Population is studied in a wide variety of ways and disciplines. In population dynamics, size, age and sex structure, mortality, reproductive behaviour, and growth of a population are studied. Demography is the study of human population dynamics. Other aspects are studied in sociology, economics, and geography. Plant and animal populations are studied in biology, in particular in that branch of ecology known as population biology , and in population genetics. In biology, a population denotes a breeding group whose members breed mostly or solely among themselves, usually as a result of physical isolation, although biologically they could breed with any members of the species.
Population density is a measure of the number of people or organisms per unit of area. Variants may express the population per unit of habitable, inhabited, productive (or potentially productive), or cultivated area. A particular geographic area of land is said to have a carrying capacity, representing the maximum population which it can support. Some observers of human societies believe that the concept of carrying capacity also applies to human population, and that unchecked population growth can result in a "Malthusian catastrophe". Others dispute this view.
Population may also mean the process of populating a geographic area, as by procreation or immigration.