Explain how negative feedback mechanisms contribute to the stability of populations within an ecosystem.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Many ecosystems maintain relatively stable population sizes over time despite fluctuations in environmental conditions. This stability arises from regulatory mechanisms that respond to changes in population density.
Model answer (4 marks)
Negative feedback occurs when a deviation from a set point triggers a response that reverses that deviation.
1. If population size rises above the equilibrium, limiting factors such as food scarcity, increased predation or disease spread become stronger.
2. These factors raise mortality or lower the reproductive rate, so the population growth rate falls.
3. If the population falls below the equilibrium, limiting factors relax – more food and fewer predators or pathogens – so birth rates rise and death rates fall.
4. The opposing responses keep the population close to its carrying capacity (K), maintaining a dynamic equilibrium and ecosystem stability.
1. If population size rises above the equilibrium, limiting factors such as food scarcity, increased predation or disease spread become stronger.
2. These factors raise mortality or lower the reproductive rate, so the population growth rate falls.
3. If the population falls below the equilibrium, limiting factors relax – more food and fewer predators or pathogens – so birth rates rise and death rates fall.
4. The opposing responses keep the population close to its carrying capacity (K), maintaining a dynamic equilibrium and ecosystem stability.
Examiner tips
- Use the phrase "negative feedback" and explain the reversal of the deviation.
- Show the two directions – increase and decrease – and link to limiting factors.
- Mention carrying capacity or K as the equilibrium point.
- Keep the answer concise and use exam‑specific terminology.
Common mistakes
- Confusing negative feedback with positive feedback.
- Failing to mention both population increase and decrease scenarios.
- Using vague terms like "balance" without specifying limiting factors or K.
Mark scheme (4 marks)
- Negative feedback occurs when a deviation from a set point (or equilibrium) triggers a response that reverses / opposes that deviation.
- As population size increases, limiting factors such as food availability, predation, or disease intensify, increasing mortality / reducing reproductive rate.
- As population size decreases below equilibrium, limiting factors relax, allowing birth rate to exceed death rate and population numbers to recover.
- These opposing responses maintain the population near a dynamic equilibrium (carrying capacity / K), contributing to ecosystem stability.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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