Population density
Population density (in agriculture: standing stock or plant density) is a measurement of population per unit area, or exceptionally unit volume; it is a quantity of type number density. It is frequently applied to living organisms, most of the time to humans. It is a key geographical term. In simple terms, population density refers to the number of people living in an area per square kilometre. Biological population densities. Population density is population divided by total land area or water volume, as appropriate. Low densities may cause an extinction vortex and lead to further reduced fertility. This is  called the Allee effect after the scientist who identified it. Examples of the causes of reduced fertility in low population densities are: Human densities. Population density is the number of people per unit of area, usually quoted per square kilometre or square mile, and which may include or exclude for example areas of water or glaciers. Commonly this may be calculated for a county, city, country, another territory or the entire world. The world's population is around 7,800,000,000 and Earth's total area (including land and water) is 510,000,000 km (197,000,000 sq. mi.). Therefore, from this very crude type  of calculation, the worldwide human population density is approximately 7,800,000,000 ÷ 510,000,000 = 15.3 per km (40 per sq. mi.). However, if only the Earth's land area of 150,000,000 km (58,000,000 sq. mi.) is taken into account, then human population density is 50 per km (129 per sq. mi.). This includes all continental and island land area, including Antarctica. Furthermore, if Antarctica is also excluded, then population density rises to over 55 people per km (over 142 per sq. mi.). However, much of the Earth's land mass consists of areas inhospitable to human habitation, such as deserts and  high mountains, and population tends to cluster around seaports and fresh-water sources. Therefore, additional criteria are needed to make simple population density values meaningful and useful. Several of the most densely populated territories in the world are city-states, microstates and urban dependencies. In fact, 95% of the world's population is concentrated on just 10% of the world's land. These territories have a relatively small area and a high urbanization level, with an economically specialized city population drawing also on rural resources outside the area, illustrating the difference between high population density and overpopulation. Deserts have very  limited potential for growing crops as there is not enough rain to support them. Thus their population density is generally low. However some cities in the Middle East, such as Dubai, have been increasing in population and infrastructure growth at a fast pace. Cities with high population densities are, by some, considered to be overpopulated, though this will depend on factors like quality of housing and infrastructure and access to resources. Very densely populated cities are mostly in Asia (particularly Southeast Asia); Africa's Lagos, Kinshasa, and Cairo; South America's Bogotá, Lima, and São Paulo; and Mexico City and  Saint Petersburg also fall into this category. City population and especially area are, however, heavily dependent on the definition of "urban area" used: densities are almost invariably higher for the centre only than when suburban settlements and intervening rural areas are included, as in the agglomeration or metropolitan area (the latter sometimes including neighboring cities). In comparison, based on a world population of 7.8 billion, the world's inhabitants, if conceptualized as a loose crowd occupying just under 1 m (10 sq. ft) per person (cf. Jacobs Method), would occupy a space a little larger than Delaware's land area . Other methods of measurement. Although arithmetic density is the most common way of measuring population density, several other methods have been developed to provide a more accurate measure of population density over a specific area.