Introduction (cont.)

Properties of stable populations

A population will become stable if it experiences constant age-specific fertility and mortality rates over a long period of time. As a consequence, the population will develop a constant rate of growth and will maintain a constant age structure. The growth rate that is generated by the particular combination of a fertility and mortality schedule is formally referred to as the intrinsic rate of natural increase, or the stable population growth rate. For brevity, in the text that follows we simply refer to it as the growth rate and in algebraic relationships we designate it using the letter r.

A constant age structure implies that the proportion of the population in each age group does not change over time. The total size of the population may change, growing or shrinking at a constant rate, but the number at every age changes at exactly the same rate, so that when it is expressed as a fraction of the total this proportion does not change over time.

Stable populations with positive growth rates (r>0) grow steadily over time, negative growth rates (r<0) imply that the population is shrinking steadily. The special case of the stable population with zero growth (r=0) is called a stationary population - in such a theoretical population neither the proportions nor the numbers in each age group change over time. The age structure of a stationary population is the same as that of the life table defining its mortality.