:''(If you were linked from a country article: the density there is based on land area, see below)''
Population density can be used as a measurement of any tangible item. However it is most frequently applied to living organisms, humans in particular. Population density is usually expressed in terms of items or organisms per unit area.
Definitions of population density
Population density, according to the definition above, depends on the scale of the sampling area used, and is difficult to define as a real-valued
continuous function over the area in question. If the items concerned are modelled as discrete points, the population density will jump up and down as the edge of the sampling area passes over individuals. Modelling the individuals as spatially extended objects has other problems, as the scale of the sampling area approaches the scale of an object (for example, a person's scale may be regarded as the size of the grounds of their dwelling place: for some people, this will be a large area).
To resolve some of these problems, population
density may be regarded, like coastline distance, as a scale-dependent
fractal quantity.
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'').
Human population density
For
human beings, 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.
It is frequently measured in persons per square mile or persons per square kilometre or hectare, which can be obtained simply by dividing the number of persons by the land area measured in square miles or in square kilometers or hectares.
Commonly this may be calculated for a county, city, country or the entire world. In the country articles the density is based on land area. However, the
list of countries by population density is based on total area, including inland water.
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 km². The Indonesian island of Java has a similar density, with 114 million people, resulting in about 856 persons per sq km. World overall population density presently averages 42 persons per km².
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.
See also
External links
af:Bevolkingsdigtheid
als:Bevölkerungsdichte
ast:Densidá de población
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be:Шчыльнасьцьнасельніцтва
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de:Bevölkerungsdichte
el:Πυκνότηταπληθυσμού
eo:Loĝdenso
es:Densidad de población
fi:Väestötiheys
fr:Densité de population
he:צפיפות אוכלוסיה
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