The three important characteristics of a population are :
(a) Density : The number of individuals per unit area or volume. For example, the number of frogs per $\mathrm{m}^3$ of forest region.
(b) Natality or Birth rate: The birth rate is determined by the number of individuals born to a given population during a given period of time.
(c) Mortality or Death rate: The death rate or mortality represents a decrease in a given population during a given period of time. Generally, the death of individuals in a population are expressed by specific mortality which is described as the mortality for given age group.