ABSTRACT
A population is a group of organisms of the same species occupying a given space at the same time. Sometimes the population is clearly defined, as in sticklebacks in a pond or mice on an island. Higher plants grow by accumulating modules that usually consist of one leaf, a bud and a piece of stem. It is easy to define the population size when considering unitary organisms – it is simply the number of individuals present in a given area. The age structure describes the number of individuals in each age class as a ratio of one class to another. Age classes can be specific categories, such as years or months, or life history stages, such as eggs, larvae, pupae and instars. A population that is neither increasing nor decreasing has a stationary age distribution, the age structure is stable and the population size constant. Age distributions provide a valuable insight into the history of a population.
