Biology · Ch 11 — Organisms and Populations
Age Distribution and Age Pyramids
Age Distribution and Age Pyramids
Age distribution refers to the proportion of individuals in a population that belong to each of its different age classes, conventionally grouped into three broad categories: the pre-reproductive age group (individuals not yet old enough to reproduce), the reproductive age group (individuals currently capable of reproducing), and the post-reproductive age group (older individuals past their reproductive years).
When these proportions are plotted graphically, with the size of each age class represented as a horizontal bar and the classes stacked from youngest at the bottom to oldest at the top, the resulting figure is called an age pyramid. The overall shape of a population's age pyramid is a useful diagnostic of whether that population is likely to grow, remain stable, or shrink in the near future, independent of its current total size.
Ecologists conventionally recognise three characteristic age-pyramid shapes. An expanding population has a broad base of pre-reproductive individuals and a comparatively narrow apex of older individuals, giving the pyramid a roughly triangular shape; because such a large fraction of the population is still to enter its reproductive years, this population is set to keep growing for some time even if the current birth rate per individual does not change. A stable population shows roughly similar proportions of pre-reproductive and reproductive individuals, giving a more bell-shaped or rectangular profile, and tends to maintain approximately the same size over time. A declining population has a narrow base of pre-reproductive individuals relative to the reproductive and post-reproductive classes above it, giving an urn-like shape that is wider in the middle or top than at the bottom; with relatively few young individuals available to replace those that die or age out of the reproductive class, s …
Worked out. When the proportion of individuals in each age class of a population is plotted as a bar graph stacked from the youngest (pre-reproductive) class at the bottom to the oldest (post-reproductive) class at the top, the resulting figure is called an age pyramid, and its overall shape reveals the likely future trajectory of the population. An expanding population produces a broadly triangular pyramid, with a wide base of pre-reproductive individuals tapering to a narrow apex, because a large proportion of the population has not yet reproduced and will soon add many new births. A stable population produces a pyramid closer to a bell or an elongated diamond shape, with roughly comparable proportions across the pre-reproductive and reproductive classes. A declining population produces an urn-shaped pyramid, narrow at the base and relatively broader higher up, ref …