Population pyramid

Definition

A graphical representation of the age structure of a population, with cohorts arranged by age class (usually youngest at the base) and sex or phenotype distinguished on opposing sides. The shape reveals growth status: a broad base indicates high recruitment and expanding population; a column or inverted profile signals stable or declining numbers. In arthropod ecology, age pyramids track cohort survival through instars, estimate generation time, and project population trajectories for pest management or conservation monitoring.

Etymology

From population (Latin populus, 'people') + pyramid (Greek pyramis, 'pyramidal shape'), describing the typical triangular profile of rapidly growing populations.

Example

A population pyramid for a univoltine butterfly might show a single broad cohort of first-instar larvae in spring, narrowing through pupal and adult stages, with no overlap between generations—contrasting with overlapping cohorts in multivoltine species.

Synonyms

• age-sex pyramid
• age structure diagram

Related Terms

• cohort
• instar
• Life table
• demography
• generation time
• Survivorship curve
• r-selection
• K-selection

Usage Notes

In entomology, 'population pyramid' is sometimes used loosely for any age-frequency histogram, though strict demographers reserve it for paired-sex or class displays. For insects with indeterminate adult aging or cryptic pupal stages, cohort separation may require developmental staging rather than calendar age. Contrast with static life table, which presents mortality data without graphic cohort structure.