Biology · Organisms and Populations · NEET
Of a POPULATION. NCERT is clear: an individual has an age (a single number), but only a population has an age DISTRIBUTION the proportion of different age groups. Along with birth rate, death rate and sex ratio, age distribution is one of the attributes a population has that a single organism cannot have. NEET loves this individual-vs-population distinction.
Every age pyramid is divided into three horizontal bands. Bottom = PRE-REPRODUCTIVE (young, not yet breeding). Middle = REPRODUCTIVE (currently breeding). Top = POST-REPRODUCTIVE (old, past breeding). Males are shown on one side of the central line and females on the other. The RELATIVE SIZE of these bands, especially the base, decides the shape.
Look at the BASE (pre-reproductive band). Broad base tapering to a narrow top = TRIANGLE = EXPANDING (growing) population, many young ready to breed. Roughly equal bands with slight taper = BELL SHAPE = STABLE (stationary) population, births balance deaths. Narrow base with a wider middle/top = URN SHAPE = DECLINING population, too few young to replace the adults.
Yes. NCERT says the shape indicates whether a population is stationary, growing or declining. 'Stable' and 'stationary' are used interchangeably for the bell-shaped pyramid where the population size stays roughly constant over time. Some diagrams add 'Type' labels, but for NEET just map: expanding=growing=triangle, stable=stationary=bell, declining=urn.
No, it means high BIRTH rate and many young individuals in the pre-reproductive class. A broad base predicts future growth because that huge young group will soon enter the reproductive stage and breed. Do not confuse a broad base (many young = growth) with mortality; mortality would narrow the upper bands, not the base.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the proportion of individuals belonging to different age groups pre-reproductive, reproductive and post-reproductive within a population. It is a key population attribute studied in population ecology.
Expanding (growing) with a broad triangular base, Stable (stationary) with a bell shape, and Declining with a narrow base and broader top (urn-shaped).
Its shape predicts the future of the population growing, stationary or declining without waiting to measure population size directly. It also reinforces that age distribution is a population attribute, a favourite NEET point.
A triangle with a broad base of pre-reproductive individuals. The large young group will soon reproduce, so the population is set to expand.
Yes. A population dominated by young (pre-reproductive) individuals has high growth potential, while one dominated by post-reproductive individuals will tend to decline over time.
