Interpreting a Population Pyramid Under Migration Pressure
A coastal city of 480,000 people has a population pyramid showing a narrow base (0–14 years = 16% of population), a bulging 25–39 cohort (31%), and a small elderly cohort (65+ = 11%). The city's total fertility rate is 1.6, below replacement. Each year roughly 9,000 young adults arrive for work in a new port and 4,000 leave, while 2,800 residents die and 4,200 are born. A councillor claims 'the city is ageing and will soon shrink, so we should close schools and build retirement villages.' A planner disagrees, arguing the pyramid's shape reflects migration, not just ageing.
1. Calculate the city's natural increase and net migration, and use these to evaluate the councillor's claim.
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Natural increase = births − deaths = 4,200 − 2,800 = +1,400 per year. Net migration = arrivals − departures = 9,000 − 4,000 = +5,000 per year. Total annual change = 1,400 + 5,000 = +6,400, or about 1.3% growth.
The councillor's claim is partly wrong. The city is not shrinking: net migration is more than triple natural increase, so total population is growing. The narrow base reflects low fertility (TFR 1.6), which does signal future ageing, but the migration-driven bulge in the 25–39 cohort means the working-age population is expanding now.
Closing schools may still be defensible if the 0–14 cohort keeps shrinking, but it is premature to assume shrinkage overall. The planner's point is stronger: the pyramid's shape is dominated by in-migration of young adults, so policy should focus on housing, services and jobs for that cohort, while planning for ageing over the longer term as the bulge moves upward.
2. Explain how the shape of this pyramid would change over 25 years if migration stopped tomorrow, and what that implies for the councillor's retirement-village proposal.
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If migration stopped, the +5,000 annual inflow would vanish, leaving only natural increase of +1,400, and that too would fade as the low-fertility base works through. The 25–39 bulge would age into the 50–64 range, then into 65+, progressively widening the top of the pyramid. Meanwhile the narrow base would keep the young cohort small, so the pyramid would become top-heavy and column-like rather than the current broad-middle shape.
This supports the councillor's long-term logic: demand for aged care and retirement housing would rise substantially in 20–25 years as the bulge reaches retirement age. But the timing matters. Building retirement villages now would serve a cohort that is still 25–40 years from peak need, while the immediate pressure is housing and services for working-age migrants. A defensible plan phases aged-care investment later and maintains school capacity only where the residual child cohort justifies it, rather than closing schools on the basis of an ageing claim that the migration data contradict.