How should the four ecosystem papers be used?

Papers one to three drill food chains, pyramids and food webs separately, while paper four mixes them to test real understanding.

Why do reverse questions use division?

Energy is multiplied by the efficiency at each step, so going back up the chain divides by the same value.

S4 Biology: Ecosystems and Energy Flow (Paper 4)

2026-11-01

Tutor Prep Guide · Secondary Biology

The fourth paper pulls the chapter together: the ten per cent rule, energy budgets, reverse calculations and bioaccumulation, ending with a gross versus net productivity error.

Direct answer　Write out the energy at each level, then multiply or divide by the efficiency as required; whenever the word net appears, subtract plant respiration first.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Biology: Ecosystems and Energy Flow
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationGrass holds 80 000 kJ and the rabbit gains one tenth of it. How much does the rabbit gain?
FoundationThe rabbit takes in 8 000 kJ and 800 kJ reaches the fox. What is the transfer efficiency?
FoundationGross primary productivity is 12 000 kJ/m2/yr and plant respiration uses 4 500 kJ/m2/yr. What is net primary productivity?
FoundationCattle take in 6 000 kJ, respire 3 600 kJ and pass 1 200 kJ as faeces. What percentage of intake goes to growth?
IntermediateA five-level chain starts with 500 000 kJ and keeps one tenth at each transfer. How much reaches the fifth level?
IntermediateTwo ecosystems each receive 2 000 kJ from producers, transferring 15 and 6 per cent. How much more does the first pass on?
IntermediateThe fox gains only 300 kJ and the rabbit to fox efficiency is ten per cent. How much did the rabbit hold?
ChallengeDDT in water is 0.0005 ppm and magnifies tenfold at each of three levels. What is the concentration in the top consumer?
ChallengeWhy can the same grassland support more people than cattle?
Error redoAsked for net primary productivity with gross 9 000 and respiration 3 000, a student answers 9 000. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	8 000 kJ	About one tenth passes on, so 80 000 x 10% = 8 000 kJ.
2	10 per cent	800 out of 8 000 is 10 per cent; the other nine tenths is lost as heat from respiration.
3	7 500 kJ/m2/yr	Net primary productivity is gross productivity minus plant respiration, giving 7 500.
4	20 per cent	Subtracting respiration and faeces leaves 1 200 kJ, which is 20 per cent of intake.
5	50 kJ	Four transfers give 500 000 x 0.1 to the fourth power, only 50 kJ, so chains rarely exceed five levels.
6	180 kJ	One yields 300 kJ and the other 120 kJ, a gap of 180 kJ from a nine point efficiency difference.
7	3 000 kJ	Working backwards, 300 divided by 0.10 gives 3 000 kJ, so reverse questions always use division.
8	0.5 ppm	Three tenfold steps give 0.5 ppm, a thousandfold build-up that puts top consumers most at risk.
9	People eating plants sit one level lower, and each extra level loses about nine tenths of the energy	Each extra level loses about nine tenths of the energy, so eating plants directly supports more people.
10	6 000 kJ/m2/yr	Net productivity must subtract plant respiration, so 9 000 minus 3 000 gives 6 000; 9 000 is only the gross figure.

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.

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Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.