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Ecosystems and Energy Flow Paper 2: Intermediate
Ecosystems and Energy Flow Paper 2: Intermediate
Adding gross and net production and transfer efficiency, comparing short and long chains, and handling the inverted biomass pyramid.
Direct answers Answers and explanations for all ten questions
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: DSE Biology Ecosystems and Energy Flow |
| Level | Paper 2 (intermediate) |
02 Questions (10 items)
- FoundationAn ecosystem has gross primary production of 30000 kJ m⁻² yr⁻¹ and producers respire 18000. What is net primary production?
- FoundationGrassland producers hold 50 000 kJ. Passing through three consumer levels at a tenth each, how much reaches the third-level consumer?
- FoundationIf transfer efficiency is 12 per cent rather than 10 per cent, how much of 40 000 kJ reaches the next level?
- IntermediateFrom 100 000 kJ of plant energy, eating plants directly gives 10000.0 kJ while eating beef from cattle fed on them gives 1000.0 kJ. How many times more is the first?
- IntermediateWhy can a biomass pyramid in a marine ecosystem be inverted?
- IntermediateWhat is the relation between net and gross production?
- ChallengeWhere does the energy in consumers faeces and remains end up?
- ChallengeTo support a secondary consumer population holding 240 kJ, how much energy must producers fix at a tenth per transfer?
- ChallengeWhat happens to net primary productivity after a grassland fire?
- Error redoError: a student infers energy transfer efficiency from the shape of a biomass pyramid. What is the problem?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 12000 kJ m⁻² yr⁻¹, which is 40.0% of gross production. | Net primary production is gross production minus respiration, and only that remainder is available to the next level. |
| 2 | 50.0 kJ. | Three transfers multiply by 0.1 three times, leaving one thousandth. |
| 3 | 4800.0 kJ. | Efficiency is measured, not fixed: ten per cent is a working estimate, not a law. |
| 4 | 10.0 times more: the shorter the food chain, the more energy is available to people. | Each extra level loses another nine tenths, so short chains are more efficient for food supply. |
| 5 | Phytoplankton reproduce and turn over fast and are grazed quickly, so standing biomass is small while production still supports the levels above. | A biomass pyramid shows standing stock while an energy pyramid shows flow, so the two must not be mixed. |
| 6 | Net production equals gross production minus respiration, measured over the same time. | Forgetting to subtract respiration is the commonest lost mark, especially in productivity questions. |
| 7 | With decomposers, which break the organic matter into inorganic nutrients while the energy is lost as heat. | Decomposers are not a trophic level, yet without them nutrients could not cycle. |
| 8 | 24000.0 kJ. | Working backwards means multiplying by ten at each level, not multiplying by three. |
| 9 | It falls close to zero in the short term and recovers gradually as succession restores plant cover. | Fire removes biomass and nutrients; recovery depends on water and nutrient supply. |
| 10 | Biomass is a standing stock affected by turnover and can be inverted; efficiency must come from an energy pyramid or measured transfers. | The two pyramids measure different things, so using the wrong one gives the opposite conclusion. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
Subtract respiration first, then apply the efficiency; neither step can be skipped.Tutor Registration →
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