The concept of physiological density is one that attempts to quantify the extent to which a nation's agricultural resources are being exploited. Physiological density is determined by dividing the nation's population by its units of arable land. This shows us how many people are being supported by each unit of farm land and can tell us something about how intensively the country is being farmed. A country with a high physiological density may (if it is farming efficiently) be closer to running out of resources than one with a lower density.
This measure of population density should not be confused with what we might call arithmetic density, which simply measures how many people there are for each unit of total land area (as opposed to the area of arable land) in a country.