Age structure of population — expanding, stable, declining pyramids

Question

A population ecologist studies three different countries and draws their age pyramids. Country A has a broad base that narrows sharply upward, Country B has nearly equal width at all age classes, and Country C has a narrower base than middle. Identify the population growth status of each country and predict which will face the biggest resource pressure in 20 years.

Quick Concept Map

flowchart TD
    A["Age Structure Pyramid"] --> B["Broad base?"]
    B -->|Yes| C["Expanding Population"]
    B -->|No| D["Base = Middle?"]
    D -->|Yes| E["Stable Population"]
    D -->|No| F["Base narrower than middle?"]
    F -->|Yes| G["Declining Population"]
    C --> H["High pre-reproductive proportion"]
    E --> I["All age groups balanced"]
    G --> J["High post-reproductive proportion"]

Solution — Step by Step

The base of an age pyramid represents the pre-reproductive age group (0-14 years). A wide base means lots of young individuals who will eventually reproduce — so the population is set to grow.

Country A (broad base, narrow top) = Expanding population. Many young people, fewer old. Think of India in the 1990s — high birth rate, growing population.
Country B (uniform width) = Stable population. Birth rate roughly equals death rate. Think of countries like Sweden or Australia.
Country C (narrow base) = Declining population. Fewer children being born than the number of adults. Japan and Italy are classic examples today.

Country A will face the greatest resource pressure in 20 years. Why? All those children in the base will move into the reproductive age group — and if fertility rates stay high, the population grows exponentially. Meanwhile, Country C will face a different problem: an ageing population needing more healthcare and pensions, but fewer working-age people to support them.

Why This Works

Age pyramids are essentially a snapshot of a population’s reproductive potential. The shape tells us whether the population is “loaded” with future parents (expanding), balanced (stable), or running low on future parents (declining).

The key insight is that population growth has momentum. Even if Country A reduces its birth rate today, the large cohort of young people already born will reproduce, causing growth to continue for 1-2 generations. This is called population momentum — and it is why India’s population continued growing even after fertility rates dropped below replacement in some states.

NEET loves asking you to identify pyramid types from diagrams. The trick is simple: look at the base width relative to the middle. Broad base = expanding, equal = stable, narrow base = declining. They have also asked about population momentum in recent years.

Alternative Method

Instead of visual analysis, you can use the dependency ratio approach:

\text{Dependency Ratio} = \frac{\text{Pre-reproductive} + \text{Post-reproductive}}{\text{Reproductive age group}} \times 100

High dependency ratio with more young dependents = expanding
Low dependency ratio = stable
High dependency ratio with more old dependents = declining

Common Mistake

Students often confuse a declining pyramid with a population that is dying out. A declining pyramid simply means the birth rate is below replacement level — the population shrinks slowly over decades, not overnight. Also, do not confuse “stable” with “zero population” — a stable population still has births and deaths, they just balance out.