Explain how the concept of carrying capacity influences the growth dynamics of a population experiencing logistic growth.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A bacterial population is introduced into a nutrient-rich culture medium. Initially, the population grows rapidly, but growth rate slows and eventually stabilises at a maximum population size.
Model answer (4 marks)
Carrying capacity (K) is the maximum population size that an environment can sustainably support, limited by available resources.
When the population is well below K, resources are plentiful and the per‑capita birth rate exceeds the death rate, so the population grows rapidly (approximately exponential or J‑shaped growth).
As the population size approaches K, intraspecific competition for limiting resources increases, reducing the per‑capita birth rate and/or increasing the death rate, so the rate of population growth declines.
At K, the birth rate equals the death rate, giving a net growth rate of zero; the population size stabilises, producing the characteristic S‑shaped (sigmoidal / logistic) growth curve.
When the population is well below K, resources are plentiful and the per‑capita birth rate exceeds the death rate, so the population grows rapidly (approximately exponential or J‑shaped growth).
As the population size approaches K, intraspecific competition for limiting resources increases, reducing the per‑capita birth rate and/or increasing the death rate, so the rate of population growth declines.
At K, the birth rate equals the death rate, giving a net growth rate of zero; the population size stabilises, producing the characteristic S‑shaped (sigmoidal / logistic) growth curve.
Examiner tips
- Use the term ‘carrying capacity (K)’ early. Show the change from exponential to logistic growth. Explain the role of resource limitation and competition. Link the zero net growth at K to the stabilisation of the population.
Common mistakes
- Confusing carrying capacity with maximum population size. Forgetting to mention that birth rate decreases or death rate increases as K is approached. Using vague terms like ‘resources’ without linking to competition or per‑capita rates.
Mark scheme (4 marks)
- Carrying capacity (K) is the maximum population size that an environment can sustainably support, limited by available resources.
- When population size is well below K, resources are plentiful and per capita birth rate exceeds death rate, so the population grows rapidly (approximately exponential / J-shaped growth).
- As population size approaches K, intraspecific competition for limiting resources intensifies, reducing per capita birth rate and/or increasing death rate, so the rate of population growth declines.
- At K, birth rate equals death rate, so net population growth rate is zero and population size stabilises, producing the characteristic S-shaped (sigmoidal / logistic) growth curve.
Key terms in this question
carrying capacity · logistic growth
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