A dead tree falls in a woodland. Over several years, decomposers break down the wood, returning nutrients to the soil. Explain how the rate of decomposition of the fallen tree would change if the woodland experienced a prolonged period of cold, dry weather. Give three reasons for your answer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A dead tree falls in a woodland. Decomposers begin to break down the wood over several years, returning nutrients to the soil. A prolonged period of cold, dry weather then affects the woodland.
Model answer (5 marks)
The rate of decomposition would decrease.
1. Low temperature slows enzyme activity in bacteria and fungi, so metabolic reactions that break down cellulose and lignin occur more slowly.
2. Dry conditions reduce the amount of water available; water is essential for the diffusion of enzymes and for the survival of decomposers, so their activity and growth are limited.
3. The combined cold and dry environment reduces the growth and reproduction of decomposers, leading to a smaller population that can act on the wood.
Overall, the fallen tree will take longer to decompose.
1. Low temperature slows enzyme activity in bacteria and fungi, so metabolic reactions that break down cellulose and lignin occur more slowly.
2. Dry conditions reduce the amount of water available; water is essential for the diffusion of enzymes and for the survival of decomposers, so their activity and growth are limited.
3. The combined cold and dry environment reduces the growth and reproduction of decomposers, leading to a smaller population that can act on the wood.
Overall, the fallen tree will take longer to decompose.
Examiner tips
- Use the word ‘decrease’ or ‘slow down’ to show the change in rate; link each reason to a specific effect on decomposers; keep the answer concise and use correct biological terms such as ‘enzyme activity’, ‘metabolic reactions’, and ‘decomposers’.
Common mistakes
- Confusing the effect of temperature with that of moisture; stating that cold increases decomposition; giving reasons that are not directly related to decomposer activity (e.g., changes in light).
Mark scheme (5 marks)
- Low temperature reduces the rate of decomposition because enzyme activity in decomposers is reduced / enzymes work more slowly at low temperatures
- Dry conditions reduce water availability, which is needed for the metabolic reactions of decomposers / for decomposers to survive and function
- The combination of cold and dry conditions reduces the growth and reproduction of decomposers (bacteria and fungi), so fewer decomposers are present to break down the wood
- Overall, the rate of decomposition decreases / the fallen tree takes longer to fully decompose
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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