Population density
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Population density by country, 2006 |
Population density is a measurement of
population per unit area or unit volume. It is frequently applied to
living organisms,
humans in particular.
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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A street in Hong Kong, one of the most densely-populated places in the world. |
For
humans, population density is the number of persons per unit of area (which may include or exclude cultivated or potentially productive area). Commonly this may be calculated for a
county,
city,
country, another
territory, or the
entire world.
For example, the
world population is 6.5 billion humans, and
Earth's area is 510 million square kilometers (200 million square miles). Therefore the world-wide human population density is 6500 million / 510 million = 13 per km² (33 per mi²), or 43 per km² (112 per mi²) considering that humans live on land, which forms 150 million km² of the earth. This density rises with the population growth, and some people think there is a limit of what the Earth can support.
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.
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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Population density map of the world in 1994. |
While the arithmetic density is the most common way of measuring population density, several other methods have been developed which aim to provide a more accurate measure of population density over a specific area.
Arithmetic density â€" The total number of people / area of land measured in km².
Physiological density â€" The total population divided by the amount of arable land.
Agricultural density â€" The total rural population to the amount of agricultural land.
Residential density â€" The number of people living in an urban area divided by the amount of residential land.
Ecological optimum â€" The density of population which can be supported by the area's natural resources.
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List of countries by population density*
List of countries by population*
List of religious populations*
List of selected cities by population density*
Human geography*
Idealised population*
Optimum population*
Population bottleneck*
Population coding*
Population genetics*
Population health*
Population momentum*
Population pyramid*
Small population size*
CityRanks.com combines Google Maps and 2000 Census data to show the population densities of U.S. zip codes on an interactive map.
*
Selected Current and Historic City, Ward & Neighborhood Densities*
Population density world-mapnds-nl:Bevolkingsdichheid
