Natality and Mortality: Birth Rate and Death Rate

Natality, commonly called the birth rate, refers to the number of births occurring within a population during a given time period, usually expressed as a per capita rate -- the number of births per individual (or per some standard number of individuals, such as per thousand) already present in the population. Natality is one of the two processes that add new individuals to a population.

Mortality, or the death rate, is the corresponding measure of the number of deaths occurring within a population over a given time period, again usually expressed as a per capita rate. Mortality is the principal process that removes individuals from a population.

Besides births and deaths, two further processes can change the size of a population: immigration, the arrival of new individuals into the population from an outside source (for example, individuals moving in from a neighbouring population of the same species), and emigration, the departure of individuals from the population to settle elsewhere. Immigration adds individuals to the population in the same way that births do, while emigration removes individuals in the same way that deaths do.

Taken together, these four processes determine how a population's size changes from one time period to the next. In words: if natality (births, B) and immigration (I) exceed mortality (deaths, D) and emigration (E) over a given interval, the population grows in size; if the reverse is true, the population shrinks; and if the two are balanced, the population size stays roughly constant. This relationship can be summarised as the population size at a later time being equal to the population size at an earlier time, plus the number of individuals added by births and immigration, minus the number removed by deaths and emigration, over the intervening period. …