Life tables and the significance of ageing and the age structure of populations
Important properties of a population affected by its age structure are :—the birth rate (which frequently varies with the age of the organism), the death rate (which usually increases with increasing age), the sex ratio (which usually alters because males and females are subject to differing risks throughout life), and behaviour (Which may affect dispersal and the nature of competition).
A life table provides a picture of the age structure of a population. Properly constructed it will supply information about sex ratio, differential mortality between the sexes, expectation of life and the distribution of the age classes. To make useful forecasts about the fate of a population only one other parameter is required, the age specific birth rate. When all these are known a working model of the population may be built. Life tables may be prepared in three ways, and all depend upon the classification of the organisms according to their age at death. The dynamic life table is obtained by following the fate of a single class made up of individuals all of the same age until the last survivor succumbs. This will yield the raw data for the table, and by plotting the number of survivors against age at death a survivorship curve is obtained from which one may readily calculate age-specific death rates and life expectancies and the probability of an organism of anyone age attaining any other. A time-specific life table results from taking a sample of a population and studying the distribution of all the age classes within it at that instant. When it is not possible to draw up either a dynamic or time-specific table, one may use a combination of both, a composite life table. These have been used for estimating annual mortality and productivity when banding and recoveries have been spread over a number of years.