Population density

Population density refers to the number of individuals present in a set area. It is commonly refered to as people per square mile (or kilometres,) which can be obtained simply by;

Number of people in the area / Area

Commonly this may be calculated for a county, city, country or the entire world.

Population density does not have to apply only to people but can be used as a measurement of any tangible item. However it is most frequently applied to living organisms.

Population density is a common biological measurement and is often used by conservationists as a more appropriate measure then 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]. Examples of this may include;
1. Increased problems with locating mates in areas of low density.
2. Increased <inbreeding> in areas of low population density.
3. Increased susceptibility to catasrophic events in low population densities.

However it should be noted that different species will have different expected densities. For example <r selected species> commonly 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 polinators, as is found in the <orchid> family, (the Orchidacae.)