Population density

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Population density is a measurement of population per unit area or unit volume. It is frequently applied to living organisms, humans in particular.

1 Biological population densities
2 Human population density
3 See also
4 External links

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Biological population densities

Population density is a common biological measurement and is often used by conservationists as a more appropriate measure than absolute numbers. Low population densities may cause an extinction vortex, where low densities lead to further reduced fertility. This is referred to as the Allee effect, named after W. C. Allee, who first identified it. Examples of this may include:

Increased problems with locating mates in areas of low density.
Increased inbreeding in areas of low population density.
Increased susceptibility to catastrophic events in low population densities.

Different species will have different expected densities. For example R-selected species commonly have high population densities, while K-selected species may have lower population densities. Low population densities may be associated with specialised mate location adaptations such as specialised pollinators; as found in the orchid family (Orchidaceae).

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Human population density

For humans, population density is the number of persons per unit of area (which may include or exclude inland water), though it may also be expressed in relation to habitable, inhabited, productive (or potentially productive) or cultivated area. Commonly this may be calculated for a county, city, country, another territory, or the entire world.

In the country articles in Wikipedia, the density is typically based on land area. However, the list of countries by population density is based on total area, including inland water.

Several of the highest-density territories in the world are very small city-states, micronations or dependencies. These territories share a relatively small area and an exceptionally 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.

The most densely populated large state is Bangladesh, where 134 million people live in a highly agricultural area around the lower Ganges river, with a national population density in excess of 900 persons per square kilometre. The Indonesian island of Java has a similar density, with 114 million people, resulting in about 856 people per square kilometre. Overall world population density presently averages 42 people per square kilometre.

Cities with exceptionally high population densities are often considered to be overpopulated, though the extent to which this is the case depends on factors like quality of housing and infrastructure or access to resources. Most of the largest densely-populated cities are in southern and eastern Asia, though Cairo and Lagos in Africa also fall into the category.

City population is however, heavily dependent on the definition used for the urban area: densities will be far higher for the central municipality than when more recently-developed and as yet administratively unincorporated suburbs are included, as in the concepts of agglomeration or metropolitan area, the latter including sometimes neighbouring cities.

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