<P> Population density (in agriculture: standing stock and standing crop) is a measurement of population per unit area or unit volume; it is a quantity of type number density . It is frequently applied to living organisms, and most of the time to humans . It is a key geographical term . </P> <P> Population density is population divided by total land area or water volume, as appropriate . </P> <P> 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 . Examples of the causes in low population densities include: </P>

Which continent has the highest ratio of available fresh water to population size