Population density

Population density

Overview


Population density is a measurement of population
Population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

 per unit area or unit volume. It is frequently applied to living organisms, and particularly to human
Human
Humans are the only living species in the Homo genus...

s. It is a key geographic term.

Population density is population divided by total land area.

Low densities may cause an extinction vortex
Extinction Vortex
Extinction vortices are a class of models through which conservation biologists, geneticists and ecologists can understand the dynamics of and categorize extinctions in the context of their causes. Developed by M. E. Gilpin and M. E...

 and lead to further reduced fertility. This is called the Allee effect
Allee effect
The Allee effect is a phenomenon in biology characterized by a positive correlation between population density and the per capita population growth rate in very small populations.-Description:...

 after the scientist who identified it.
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Encyclopedia


Population density is a measurement of population
Population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

 per unit area or unit volume. It is frequently applied to living organisms, and particularly to human
Human
Humans are the only living species in the Homo genus...

s. It is a key geographic term.

Biological population densities


Population density is population divided by total land area.

Low densities may cause an extinction vortex
Extinction Vortex
Extinction vortices are a class of models through which conservation biologists, geneticists and ecologists can understand the dynamics of and categorize extinctions in the context of their causes. Developed by M. E. Gilpin and M. E...

 and lead to further reduced fertility. This is called the Allee effect
Allee effect
The Allee effect is a phenomenon in biology characterized by a positive correlation between population density and the per capita population growth rate in very small populations.-Description:...

 after the scientist who identified it. Examples of the causes in low population densities include:
  • Increased problems with locating mates
  • Increased inbreeding
    Inbreeding
    Inbreeding is the reproduction from the mating of two genetically related parents. Inbreeding results in increased homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is...



Different species have different expected densities. R-selected species
R/K selection theory
In ecology, r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity or quality of offspring...

 commonly have high population densities, while K-selected species may have lower densities. Low densities may be associated with specialized mate location adaptations such as specialized 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 kilometer or mile (which may include or exclude cultivated or potentially productive area). Commonly this may be calculated for a county
County
A county is a jurisdiction of local government in certain modern nations. Historically in mainland Europe, the original French term, comté, and its equivalents in other languages denoted a jurisdiction under the sovereignty of a count A county is a jurisdiction of local government in certain...

, city
City
A city is a relatively large and permanent settlement. Although there is no agreement on how a city is distinguished from a town within general English language meanings, many cities have a particular administrative, legal, or historical status based on local law.For example, in the U.S...

, country
Country
A country is a region legally identified as a distinct entity in political geography. A country may be an independent sovereign state or one that is occupied by another state, as a non-sovereign or formerly sovereign political division, or a geographic region associated with a previously...

, another territory, or the entire world
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

.

The world's population is 7 billion, and Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's total area (including land and water) is 510 million square kilometers (197 million square miles). Therefore the worldwide human population density is 6.8 billion ÷ 510 million = 13.3 per km² (34.5 per sq. mile). If only the Earth's land area of 150 million km² (58 million sq. miles) is taken into account, then human population density increases to 45.3 per km² (117.2 per sq. mile). This calculation includes all continental and island land area, including Antarctica. If Antarctica is also excluded, then population density rises to 50 people per km² (129.28 per sq. mile). 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
Dependent territory
A dependent territory, dependent area or dependency is a territory that does not possess full political independence or sovereignty as a State, and remains politically outside of the controlling state's integral area....

. These territories share a relatively small area and a high urbanization
Urbanization
Urbanization, urbanisation or urban drift is the physical growth of urban areas as a result of global change. The United Nations projected that half of the world's population would live in urban areas at the end of 2008....

 level, with an economically specialized city
City
A city is a relatively large and permanent settlement. Although there is no agreement on how a city is distinguished from a town within general English language meanings, many cities have a particular administrative, legal, or historical status based on local law.For example, in the U.S...

 population drawing also on rural resources outside the area, illustrating the difference between high population density and overpopulation
Overpopulation
Overpopulation is a condition where an organism's numbers exceed the carrying capacity of its habitat. The term often refers to the relationship between the human population and its environment, the Earth...

.

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 and access to resources. Most of the most densely-populated cities are in southern and eastern Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

, though Cairo
Cairo
Cairo , is the capital of Egypt and the largest city in the Arab world and Africa, and the 16th largest metropolitan area in the world. Nicknamed "The City of a Thousand Minarets" for its preponderance of Islamic architecture, Cairo has long been a centre of the region's political and cultural life...

 and Lagos
Lagos
Lagos is a port and the most populous conurbation in Nigeria. With a population of 7,937,932, it is currently the third most populous city in Africa after Cairo and Kinshasa, and currently estimated to be the second fastest growing city in Africa...

 in Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...

 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 separate suburban communities are included, as in the concepts of agglomeration
Agglomeration
In the study of human settlements, an urban agglomeration is an extended city or town area comprising the built-up area of a central place and any suburbs linked by continuous urban area. In France, INSEE the French Statistical Institute, translate it as "Unité urbaine" which means continuous...

 or metropolitan area
Metropolitan area
The term metropolitan area refers to a region consisting of a densely populated urban core and its less-populated surrounding territories, sharing industry, infrastructure, and housing. A metropolitan area usually encompasses multiple jurisdictions and municipalities: neighborhoods, townships,...

, 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.

As a comparison, based on a world population of seven billion, the world's inhabitants would, as a loose crowd taking up ten square feet per person (Jacobs Method), would occupy a space roughly the size of Fiji
Fiji
Fiji , officially the Republic of Fiji , is an island nation in Melanesia in the South Pacific Ocean about northeast of New Zealand's North Island...

's land area.

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 people / area of land (measured in km² or sq miles).
  • Physiological density
    Physiological density
    The physiological density or real population density is the number of people per unit area of arable land.A higher physiological density suggests that the available agricultural land is being used by more and may reach its output limit sooner than a nation that has a lower physiological...

    : The total population / area of arable land
    Arable land
    In geography and agriculture, arable land is land that can be used for growing crops. It includes all land under temporary crops , temporary meadows for mowing or pasture, land under market and kitchen gardens and land temporarily fallow...

    .
  • Agricultural density: The total rural population / area of arable land
    Arable land
    In geography and agriculture, arable land is land that can be used for growing crops. It includes all land under temporary crops , temporary meadows for mowing or pasture, land under market and kitchen gardens and land temporarily fallow...

    .
  • Residential density : The number of people living in an urban area / area of residential land.
  • Urban density
    Urban density
    Urban density is a term used in urban planning and urban design to refer to the number of people inhabiting a given urbanized area. As such it is to be distinguished from other measures of population density. Urban density is considered an important factor in understanding how cities function...

    : The number of people inhabiting an urban area / total area of urban land.
  • Ecological optimum: The density of population which can be supported by the natural resources.

See also

  • Human geography
    Human geography
    Human geography is one of the two major sub-fields of the discipline of geography. Human geography is the study of the world, its people, communities, and cultures. Human geography differs from physical geography mainly in that it has a greater focus on studying human activities and is more...

  • Idealized population
  • Optimum population
    Optimum population
    The optimum population is the size of a population that is optimal. There are, however, various opinions on what "optimal" means in this usage, resulting in various end-targets for it, but estimations usually take ecological sustainability and carrying capacity more or less into account...

  • Population bottleneck
    Population bottleneck
    A population bottleneck is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing....

  • Population genetics
    Population genetics
    Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...

  • Population health
    Population health
    Population health has been defined as “the health outcomes of a group of individuals, including the distribution of such outcomes within the group.” It is an approach to health that aims to improve the health of an entire population. One major step in achieving this aim is to reduce health...

  • Population momentum
    Population momentum
    Population momentum refers to population growth at the national level which would occur even if levels of childbearing immediately declined to replacement level. For countries with above-replacement fertility , population momentum represents natural increase to the population...

  • Population pyramid
    Population pyramid
    A population pyramid, also called an age structure diagram, is a graphical illustration that shows the distribution of various age groups in a population , which forms the shape of a pyramid when the population is growing...

  • Rural transport problem
    Rural transport problem
    The rural transport problem refers to difficulties experienced in providing transport links to rural communities. Low rural population density makes viable public transport difficult, though people in rural areas usually have a greater need for transport than urban dwellers...

  • Small population size
    Small population size
    Small populations behave differently from larger populations. They often result in population bottlenecks, which have harmful consequences for the survival of that population.-Demographic effects:...



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