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Population density (in agriculture standing stock and standing crop) is a measurement of population per unit area or unit volume. It is frequently applied to living organisms, and particularly to humans. It is a key term used in geography.
Biological population densities
Population density refers to the number of individuals per square kilometer of land area. It is a common biological measurement and is more often used by conservationists as a measure than population size.
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 in low population densities include:
- Increased problems with locating mates
- Increased inbreeding
Different species have different expected densities. R-selected species commonly have high population densities, while K-selected species may have lower densities. Low densities may be associated with specialised mate location adaptations such as specialised pollinators; as found in the orchid family (Orchidaceae).
Human population density
For humans, population density is the number of people per unit of area usually per square kilometre (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.
The world population is 6.7 billion , and Earth's area is 510 million square kilometers (197 million square miles)  . Therefore the worldwide human population density is 6.7 billion ÷ 510 million = 13.1 per km² (34.0 per sq mi), or 44.7 per km² (115.5 per sq mi) if only the Earth's land area of 150 million km² (58 million sq mi) is taken into account. This density rises when the population grows. It also includes all continental and island land area, including Antarctica. Considering that over half of the Earth's land mass consists of areas inhospitable to human inhabitation, such as deserts and high mountains, and that population tends to cluster around seaports and fresh water sources, this number by itself does not give any meaningful measurement of human population density.
Several of the most densely-populated territories in the world are city-states, microstates, micronations, or dependencies. These territories share 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.
Cities with high population densities are, by some, 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 most densely-populated cities are in southern and eastern Asia, though Cairo and Lagos in Africa also fall into this category.
City population is, however, heavily dependent on the definition of "urban area" used: densities are often higher for the central municipality itself, than when more recently-developed and administratively unincorporated suburban communities are included, as in the concepts of agglomeration or metropolitan area, the latter including sometimes neighboring cities. For instance, Milwaukee has a greater population density when just the inner city is measured, and not the surrounding suburbs as well.
Other methods of measurement
While 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: The total number of peoples / area of land measured in km² or sq miles.
- Physiological density: The total population / the amount of arable land.
- Agricultural density: The total rural population / amount of agricultural land.
- Residential density : The number of people living in an urban area / the area of residential land.
- Urban density : The number of people inhabiting an urban area / the total area of urban land.
- Ecological optimum: The density of population which can be supported by the natural resources.
- Human geography
- Idealised population
- Optimum population
- Population bottleneck
- Population genetics
- Population health
- Population momentum
- Population pyramid
- Rural transport problem
- Small population size
- List of countries by population density
- List of countries by population
- List of cities by population
- List of cities by population density
- List of islands by population density
- List of religious populations
- City Ranks 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