Population density can be used as a measurement of any tangible item. However it is most frequently applied to living organisms. Population density is usually expressed in terms of items or organisms per unit area.
Note that population density, according to this definition, depends on the scale of the sampling area used, and is hard to define as a real-valued continuous function over the area in question. If the items concerned are modelled as discrete points, the population density will jump up and down as the edge of the sampling area passes over individuals. Modelling the individuals as spatially extended objects has other problems, as the scale of the sampling area approaches the scale of an object (for example, a person's scale may be regarded as the size of the grounds of their dwelling place: for some people, this will be a large area).
To resolve some of these problems, population density may be regarded, like coastline distance, as a scale-dependent fractal quantity.
Population density is a common biological measurement and is often used by conservationists as a more appropriate measure than 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[?], named after W. C. Allee[?], who first identified it. Examples of this may include;
However it should be noted that different species will have different expected densities. For example r selected species[?] commonly have 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 pollinators, as is found in the orchid family, (the Orchidaceae.)
For human beings, population density is the number of persons per unit of land area. It is frequently measured in persons per square mile or persons per square kilometre which can be obtained simply by dividing the number of persons by the land area measured in square miles or in square kilometers.
Commonly this may be calculated for a county, city, country or the entire world, see List of countries by population density. Cities with very high population densities include Macau, Monaco, Hong Kong, Singapore, and Gibraltar.
Cities with high population density are usually considered in the condition of overpopulation. Notice cities that are usually considered overpopulated are not the same as cities with high population density. Overpopulation is rather relative concept that is, Tokyo or NYC is usually considered overpopulated because other cities in their countries have less population and less population density.