3.2Unit 2 · Ecology & the Environment
Feeding Relationships — Unit 2 — Edexcel IGCSE Biology Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 4 parts Lesson 3 of 5 1 quiz
Name the trophic levels in a food chain.Arrows in a food chain show the direction of energy transfer.About 10% of energy is transferred from one trophic level to the next.Efficiency = (energy transferred to next level ÷ energy available) × 100.Food chains rarely have more than 4 …
- Pyramids of number
- Transfers along a food chain
- Drawing a food pyramid — the conventions that carry the marks
- What happens when one population is removed — work through the web
- Trophic levels in a food web
- Explain why food chains rarely have more than five trophic levels
Section 1 of 4Feeding Relationships — Unit 2
Trophic levels:
- producers (level 1) → primary consumers (level 2) → secondary consumers (level 3) → tertiary consumers (level 4).
- Decomposers feed on dead material from all levels.
Food chains:
- show a single feeding pathway, e.g. grass → grasshopper → frog → snake.
- Arrows point from the organism eaten to the eater, showing energy flow.
Food webs:
- most organisms eat and are eaten by several species, so chains interlink.
- If one species declines, its predators may decline and its prey may increase, and competition may change.
Pyramids of number:
may be inverted, e.g. one tree feeding many caterpillars. Pyramids of biomass use dry mass and are usually pyramid-shaped. Pyramids of energy are always pyramid-shaped because energy is lost at each level.Transfers along a food chain:
- energy is lost as heat from respiration, in faeces and urine, and in parts not eaten.
- So only around 10% is passed on, which limits the length of food chains and explains why there are fewer top predators.
- Eating lower down a food chain (e.g. crops rather than meat) is more energy-efficient for humans because fewer transfers occur.
Drawing a food pyramid — the conventions that carry the marks
- Producers go at the bottom; each bar above is the next trophic level.
- Label every bar with its trophic level, not with one organism's name.
- The bar gets wider as the amount increases, and the bars are the same height.
- The widths must be to scale with the values given.
- Numbersone organism can support many,
- so a pyramid of numbers may be inverted or spindle-shaped.
- (e.g.1one oak tree2many caterpillars3few robins)
- (e.g.
- so a pyramid of numbers may be inverted or spindle-shaped.
- Biomass and energyalways a true pyramid, because the amount available falls at each level.
What happens when one population is removed — work through the web
- Fewer rabbits → grass increases, because less of it is eaten.
- Fewer rabbits → foxes decrease, because a food source has gone.
- More grass → caterpillars and earthworms increase.
- Foxes switch to what is left → frogs and mice decrease as the foxes compensate.
- Describe every change as increase or decrease, never as "die out".
Trophic levels in a food web:
An organism can feed at more than one level, so in a food web it may be, for example, a primary consumer in one food chain and a secondary consumer in another.
Section 2 of 4Process & sequence
- Light energy is captured by producers in photosynthesis.
- Primary consumers eat producers and take in chemical energy.
- At each level, much energy is lost as heat from respiration and in waste.
- Only about 10% passes to the next trophic level as biomass.
- Decomposers break down dead organisms and waste, recycling nutrients.
Energy along a food chainwrite these steps in this order; each step is a marking point.
- Sunlight captured by producer
- Eaten by primary consumer
- Energy lost as heat and waste
- About 10% passed on
- Decomposers recycle nutrients
Section 3 of 4Compare
| Pyramid | Shows | Always pyramid-shaped? |
|---|---|---|
| Numbers | Number of organisms at each level | No (e.g. one oak tree supports many insects) |
| Biomass | Dry mass of organisms at each level | Nearly always |
| Energy | Energy transferred per unit area per year | Always |
| Loss | Explanation |
|---|---|
| Respiration | Energy released for movement and keeping warm, lost as heat |
| Egestion | Undigested material in faeces |
| Excretion | Energy in urea and other waste |
| Parts not eaten | Bones, roots, fur not consumed |
Section 4 of 4Worked examples
Grass contains 20 000 kJ of energy. The rabbits eating it gain 2000 kJ. Calculate the efficiency of transfer.
- Efficiency = (2000 ÷ 20 000) × 100 = 10%.
Explain why food chains rarely have more than five trophic levels.
- At each level most energy is lost as heat from respiration, in waste and uneaten parts.
- After several transfers too little energy remains to support another population of organisms.
- Efficiency of biomass transfer (clover → snails)
- Formula: % efficiency = (biomass at the higher trophic level ÷ biomass at the lower trophic level) × 100.
- Substitute: 138 ÷ 1450 × 100.
- Result: 9.52 % to 3 significant figures.
- The same formula works with energy values instead of biomass — check which one the table gives.
- Method marks are available even if the final figure is wrong, so write the substitution down.
- Why the transfer is never close to 100 %
- Parts are uneaten (bones, fur, teeth, cellulose in plant stems).
- Some material is egested as undigested faeces rather than absorbed.
- Energy leaves as excretion — e.g. urea.
- Most of the absorbed energy is spent in respiration and lost as heat during movement and maintenance.
- Some organisms die without being eaten, so their energy goes to decomposers.
- At each level most energy is lost as heat from respiration, in waste and uneaten parts.
Marked practiceQuizzes for this lesson
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🌳 Student Search Tree & Frequently Asked Exam Questions: feeding relationships
Step-by-step problem-solving guide, mark scheme criteria, and experimental error analysis curated by Dr. Khaled Dabour (DABOURHUB).
1. Identify all given values and convert non-standard units, 2. State the primary formula or physiological/chemical principle, 3. Calculate to 3 significant figures and clearly state units.
Avoid confusing independent vs dependent variables in practical questions. In calculation questions, double-check percentage change formula: (change / original) × 100.
Where can I find feeding relationships past paper questions by topic with the mark scheme?
On DABOURHUB every feeding relationships question is grouped by syllabus point and shown with the mark-scheme wording, so you can see exactly which sentence earns the mark instead of scrolling through whole year papers.
What is the most common feeding relationships exam question and why do students lose marks on it?
The repeated pattern on Edexcel papers is a short stem that asks you to apply feeding relationships and then justify it with a command word (explain, suggest, compare). Marks are lost by stating the fact without linking it back to the wording of the question, so each feeding relationships item here is written as a full-mark answer.
Is there an alternative to Save My Exams or Physics and Maths Tutor for feeding relationships?
Yes: the feeding relationships notes, topical questions and MCQ quizzes on this site follow the same syllabus headings as the official specification, and they open straight from the search result with no download step.
How do I revise feeding relationships in two weeks before the exam?
Three passes: read the feeding relationships notes once, then do the topical questions and mark them against the mark scheme, then redo only the ones you missed. The quiz on the page gives instant feedback so the third pass takes minutes rather than an evening.