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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

📚 Edexcel International GCSE Biology 4BI1 Hub / Topical Revision
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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

Section 1 of 8Feeding Relationships

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.

Section 2 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

Section 3 of 8Compare

Types of ecological pyramid
PyramidShowsAlways pyramid-shaped?
NumbersNumber of organisms at each levelNo (e.g. one oak tree supports many insects)
BiomassDry mass of organisms at each levelNearly always
EnergyEnergy transferred per unit area per yearAlways
Energy losses between trophic levels
LossExplanation
RespirationEnergy released for movement and keeping warm, lost as heat
EgestionUndigested material in faeces
ExcretionEnergy in urea and other waste
Parts not eatenBones, roots, fur not consumed

Section 4 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.

Section 5 of 8Keep going

  1. 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.

Section 6 of 8Key terms

Section 7 of 8Command 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.

Section 8 of 8Exam technique

Keywords the mark scheme looks for in this lesson

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)

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.

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