Population density (people per
) by country, 2006
Population density (people per
) map of the world in 1994.
and standing crop
) is a measurement of population
per unit area or unit volume. It is
frequently applied to living
, and particularly to humans
is a key geographic term.
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
measure than population size.
Low densities may cause an extinction
and lead to further reduced fertility. This is called
the Allee effect
after the scientist
whoidentified it. Examples of the causes in low population
- 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
have lower densities. Low densities may be associated with
specialized mate location adaptations such as specialized
pollinators; as found in the orchid
Human population density
, population density is the number
of people per unit of area usually per square kilometer
include or exclude cultivated or potentially productive area).
Commonly this may be calculated for a county
, or the
The world population
is 6.8 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. mile), or 44.7 per km² (115.5 per sq. miles) if only
the Earth's land area of 150 million km² (58 million sq. miles) is
taken into account. This density rises when the population grows.
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
Several of the most densely-populated territories in the world are
, or dependencies
. These territories share a
relatively small area and a high urbanization
level, with an economically
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
, 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
- Arithmetic density: The total number of people
/ area of land (measured in km² or sq miles).
- Physiological density: The total population /
area of arable land.
- Agricultural density: The total rural
population / area of agricultural land.
- Residential density : The number of people
living in an urban area / area of residential land.
- Urban density :
The number of people inhabiting an urban area / total area of urban
- Ecological optimum: The density of population
which can be supported by the natural resources.