Explain how detritivores and decomposers contribute to the cycling of matter in an ecosystem, and distinguish their roles from those of producers and consumers.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Detritivores physically fragment dead organic matter, increasing surface area for decomposers.
Decomposers (bacteria, fungi) secrete extracellular enzymes that chemically break down the fragments, mineralising organic compounds and releasing inorganic nutrients such as nitrate, phosphate and CO₂ back into the abiotic environment.
Producers (autotrophs) fix inorganic CO₂ and nutrients into organic molecules via photosynthesis, moving matter from the abiotic to the biotic component.
Consumers obtain organic matter by feeding on other organisms; they transfer matter up the food chain but do not return it to the inorganic pool, whereas detritivores and decomposers ultimately recycle matter back to the abiotic environment, closing the cycle.
Decomposers (bacteria, fungi) secrete extracellular enzymes that chemically break down the fragments, mineralising organic compounds and releasing inorganic nutrients such as nitrate, phosphate and CO₂ back into the abiotic environment.
Producers (autotrophs) fix inorganic CO₂ and nutrients into organic molecules via photosynthesis, moving matter from the abiotic to the biotic component.
Consumers obtain organic matter by feeding on other organisms; they transfer matter up the food chain but do not return it to the inorganic pool, whereas detritivores and decomposers ultimately recycle matter back to the abiotic environment, closing the cycle.
Examiner tips
- Use the word ‘physically fragment’ and ‘chemically digest/mineralise’ to show understanding of the processes. Show the direction of matter flow for each group. Highlight that only detritivores and decomposers return matter to the abiotic pool.
- Use concise, exam‑style sentences and avoid unnecessary words.
Common mistakes
- Confusing detritivores with decomposers; they perform different steps. Failing to mention that producers move matter from abiotic to biotic. Using vague terms like ‘break down’ without specifying physical vs chemical action.
Mark scheme (4 marks)
- Detritivores physically break down (fragment) dead organic matter / detritus into smaller pieces, increasing the surface area available for decomposer action.
- Decomposers (bacteria and fungi) chemically digest / mineralise organic compounds via extracellular enzymes, releasing inorganic nutrients (e.g. nitrate, phosphate, carbon dioxide) back into the abiotic environment.
- Producers (autotrophs) incorporate inorganic nutrients / CO₂ into organic compounds via photosynthesis, thereby moving matter from the abiotic to the biotic component of the ecosystem.
- Consumers obtain organic matter by feeding on other organisms (heterotrophically), transferring matter along the food chain but not returning it to the inorganic pool, unlike detritivores and decomposers which ultimately return matter to the abiotic environment / close the cycle.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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