1. StatisticsPopulation density (in agriculture: standing stock or plant density) is a measurement of population per unit land area. It is mostly applied to humans, but sometimes to other living organisms too. It is a key geographical term. == Biological population densities == Population density is population divided by total land area, sometimes including seas and oceans, as appropriate. Low densities may cause an extinction vortex and further reduce fertility. This is called the Allee effect after the scientist who identified it. Examples of the causes of reduced fertility in low population densities are: Increased problems with locating sexual mates Increased inbreeding === Human densities === Population density is the number of people per unit of area, usually transcribed as "per square kilometre" or square mile, and which may include or exclude, for example, areas of water or glaciers. Commonly this is calculated for a county, city, country, another territory or the entire world. World population... from wikipedia.org