Age-structured populations

Age-structured models divide a population according to age because survival and reproduction often differ strongly between young, mature and old individuals.

Age classes

A simple model might use classes such as juveniles, reproductive adults and older adults. The state of the population is then described by the number of individuals in each class rather than by one total population size.

Why age structure matters

Two populations with the same total size can have very different future behaviour if one contains many reproductive adults and the other contains mostly non-reproductive individuals.

Example. A population can be represented by an age vector such as \((n_1,n_2,n_3)\), where each component gives the number in a specified age class.

Processes in an age-structured model

The main processes are survival from one age class to the next and production of new individuals by reproductive classes. These ideas lead naturally to matrix population models, including Leslie matrices.

Key idea. Age structure matters whenever demographic rates depend on age, because total population size alone does not contain enough information to predict future change.