A population of bacteria is introduced to a nutrient-rich culture medium. Initially the population grows slowly, then increases very rapidly, before eventually levelling off. Explain why the population growth eventually levels off and remains relatively stable.

Cambridge International IGCSE Biology (0610) — 19.4 Populations · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The population grows slowly at first because the number of cells is small and resources are abundant. As the population increases, the limited nutrients become depleted and competition for food rises, which increases the death rate. Consequently the birth rate falls and eventually equals the death rate. When the birth and death rates are equal the population size no longer changes, so it stabilises at the carrying capacity of the environment.

Examiner tips

  • Use the terms 'resources become limited', 'competition', 'death rate', 'birth rate', and 'carrying capacity'; link each to the next step.
  • Show the logical sequence: resource limitation → competition → death rate ↑, birth rate ↓ → rates equalise → stable population.
  • Use the word 'stabilises' or 'remains stable' to match the mark scheme.

Common mistakes

  • Confusing the initial slow growth with the later rapid growth; the answer should focus on why it levels off.
  • Using vague phrases like 'the population stops growing' without explaining the underlying causes.
  • Failing to mention the carrying capacity explicitly.

Mark scheme (4 marks)

  1. Resources (e.g. food/nutrients) become limited as population size increases
  2. Increased competition leads to a higher death rate
  3. Birth rate falls or equals death rate
  4. Population stabilises at the carrying capacity of the environment

Key terms in this question

population

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