Population dynamics

Birth, death, immigration and emigration; age structure and population pyramids.

Environmental Studies5 questions
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How populations change

A population is a group of individuals of the same species living in an area at a given time. Its size changes through four processes:

  • Natality (births) and immigration (individuals coming in) add to it.
  • Mortality (deaths) and emigration (individuals leaving) reduce it.

So: N(t+1) = N(t) + (B + I) − (D + E).

Worked example: a deer population of 500 has 60 births, 20 deaths, 10 immigrants and 5 emigrants in a year. New size = 500 + 60 + 10 − 20 − 5 = 545. Growth = 45, so the growth rate is 45 ÷ 500 × 100 = 9% per year.

Population density = number of individuals ÷ area. For example, 2,400 trees in 40 hectares gives 60 trees per hectare. For humans, the crude birth rate is births per 1,000 people per year: 30,000 births in a population of 15,00,000 gives 20 per 1,000.

Age structure and population pyramids

Individuals are grouped into pre-reproductive, reproductive and post-reproductive ages. A graph of these groups, with males on one side and females on the other, is an age pyramid.

  • Expanding (triangular) pyramid: very broad base, many young people → population will grow fast (typical of high-birth-rate countries).
  • Stable (bell-shaped) pyramid: young and reproductive groups about equal → little change.
  • Declining (urn-shaped) pyramid: narrow base, fewer young than adults → population will shrink (for example Japan and parts of Europe).

Key points

  • Additions: births + immigration; losses: deaths + emigration.
  • Density = number ÷ area.
  • Broad base = expanding; bell = stable; narrow base (urn) = declining.
  • The sex ratio (in India: females per 1,000 males) is another population characteristic; Census 2011 recorded 943.

Exam tips

Numerical questions ask you to apply the B + I − D − E formula; write each term with its sign before adding. Pyramid questions describe the shape and ask for the future trend.

Common mistakes

  • Adding emigration instead of subtracting it.
  • Thinking a broad-based pyramid means a declining population; it means growth.
  • Mixing up birth rate (per 1,000) with growth rate (per cent).

Practice questions

Which pair of processes increases the size of a population?Easy
A population of 500 rabbits has 60 births, 20 deaths, 10 immigrants and 5 emigrants in one year. What is its size at th…Medium
An age pyramid with a very broad base and a narrow top indicates a population that is:Easy
An urn-shaped age pyramid, with fewer young than middle-aged individuals, suggests that the population will:Medium
A town of 15,00,000 people records 30,000 births in a year. Its crude birth rate (per 1,000 people) is:Hard
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