The density of a species population can be expressed either with reference to total area (crude density) or with reference to the actual area of habitat available to the species (ecological density).
Thus, we might have a crude density of 500 rabbits per square mile but if only half of the square mile had suitable habitat for rabbits, the ecological density would be 1000 per square mile of rabbit habitat. When the size of individuals in the population is relatively uniform, as mammals, birds or insects, then density is expressed in terms of number of individuals (numerical density).
But, when the size of individuals is variable, such as true of trees or mixed populations (biomass density) to measure density of a population in terms of measurement of biomass will be satisfactory as a measure of density biomass one can take wet weight, dry weight, volume and carbon and nitrogen weight.
Thus, population density is the total number of the species within some natural habitat. It can be easily calculated by using density formula as follows:
D = n/a / t
Where, D is population density; n is a number of individuals; a is area and t is time unit.
The density of organisms on any area varies. It may change with the seasons, with weather conditions, with food supply, and with many other influences. There is, however, an upper limit to the density of a population within a unit area, imposed by size and tropic level.
Generally the smaller the organism, the greater its abundance per unit area. A 100 acre forest will support a greater number woodland mice then deer. A forest stand will contain many more trees 5 to 8 cm dbh (diameter breast height) than trees 30 to 35 cm dbh.
The larger its size or the higher its position on the tropic levels, the less is the numerical density of an organism. Further, the existence of a population in a particular area for a given time depends upon its rate of reproduction and mortality or death rate, besides the rates of immigration or ingress and emigration or egress. These phenomena exhibit structural attribute of a population and are intimately correlated with the functional aspects such as maintenance, growth and expansion of population.