Population density per square kilometre 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.
 Biological population densities
Population density 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 W. C. Allee who first identified it. Examples of the causes in low population densities include:
- Increased problems with locating mates
- Increased inbreeding
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