High Quality Content by WIKIPEDIA articles! Population density (in agriculture standing stock and standing crop) is a measurement of population per unit area or unit volume. It is frequently applied to living organisms, and particularly to humans. It is a key geographic term. 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 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 the scientist who identified it.