4.2Ecology & the Environment

Feeding Relationships — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions

5 min read 8 parts Lesson 2 of 7 1 quiz

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Lesson summary

Feeding Relationships Edexcel IGCSE Biology 4BI1 notes: 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 ÷ ener…

Food chains
Pyramids of number
Transfers along a food chain
Trophic levels in a food web
Drawing pyramids
Feeding Relationships Edexcel IGCSE Biology 4BI1: what to do next

Part 1 of 8Key ideas

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 or 5 trophic levels because too little energy remains.

A change in one population in a food web affects others.

Part 2 of 8Learning objectives

Learning objectives

Name the trophic levels in a food chain.

Construct and interpret food chains, food webs and pyramids.

Explain energy transfers and calculate efficiency along a food chain.

Part 3 of 8Essential knowledge

Trophic levels

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

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

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

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

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.

Trophic levels in a food web

An organism can occupy different trophic levels in different food chains; for example, an animal eating both plants and herbivores is a primary consumer in one chain and a secondary consumer in another.

Drawing pyramids

Put producers at the bottom and each higher trophic level above in order. Label every bar, keep the bars the same height, and make each bar's width proportional to the number, biomass or energy, drawn to scale when possible.

Part 4 of 8Process & 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.
1. Sunlight captured by producer
2. Eaten by primary consumer
3. Energy lost as heat and waste
4. About 10% passed on
5. Decomposers recycle nutrients

Part 5 of 8Worked 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.

Part 6 of 8Summary

- Trophic levels describe feeding position.
- Food chains join into food webs.
- Pyramids of energy are always pyramid-shaped.
- Only about 10% of energy passes to the next level.

Part 7 of 8Keep going

Feeding Relationships Edexcel IGCSE Biology 4BI₁: what to do next
These Feeding Relationships Edexcel IGCSE Biology 4BI₁ notes covered the key ideas, worked examples and exam technique for this topic. Check your Feeding Relationships Edexcel IGCSE Biology 4BI₁ understanding with the practice questions below, then reread any Feeding Relationships Edexcel IGCSE Biology 4BI₁ point you were unsure about before moving on.
Feeding Relationships Edexcel IGCSE Biology 4BI₁: revision checklist
- Revise the main Feeding Relationships Edexcel IGCSE Biology 4BI₁ definitions until you can write them from memory.
- Practice: Feeding Relationships Edexcel IGCSE Biology 4BI1 practice questions
- Previous lesson: 4.1 The Organism in the Environment — Notes
- Next lesson: 4.3 Cycles within Ecosystems — Notes
- Course: Edexcel International Gcse Biology 4BI1 — all units
- Official syllabus: Pearson Edexcel International GCSEs

Part 8 of 8Compare: Compare

Types of ecological pyramid
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

Energy losses between trophic levels
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

Key terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Exam technique

Keywords the mark scheme looks for in this lesson

Producer · Primary consumer · Secondary consumer · Tertiary consumer · Decomposer · Trophic level · Food chain · Food web · Pyramid of biomass

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).

Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

Avoid vague words

“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.

Do not extrapolate a graph line beyond the data or to zero unless asked.

Six-mark extended responses are marked on content AND a logical, linked line of reasoning.

Last updated: 14 October 2026