Why can a biomass pyramid be inverted?

Biomass is a standing stock at one moment; fast-turnover phytoplankton stay small yet produce a lot.

How is net primary productivity found?

Gross primary productivity minus the energy used by plant respiration.

S4 Biology: Ecosystems and Energy Flow (Paper 2)

2026-11-01

The second paper compares pyramids of numbers, biomass and energy, and uses intake, respiration and faecal data to find energy left for growth.

Direct answer　Identify which pyramid is being asked about, then for an energy budget subtract respiration and egested material from intake to leave growth.

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)

FoundationWhat normally forms the base of a pyramid of biomass?
FoundationWhich kind of pyramid can be inverted, and why?
FoundationState three main routes by which energy is lost between trophic levels.
FoundationGross primary productivity is 5 000 kJ/m2/yr and plant respiration uses 2 000 kJ/m2/yr. What is net primary productivity?
IntermediateGrass has 50 000 kJ, the cattle gain 5 000 kJ and people gain 500 kJ. What percentage of the grass energy do people obtain?
IntermediateCattle take in 8 000 kJ, respire 4 500 kJ and pass 1 500 kJ as faeces. What percentage of intake is used for growth?
IntermediateWhy do food chains usually have only four or five levels?
ChallengeLevels A, B and C hold 12 000, 1 200 and 120 kJ. What percentage of A is C, and what is the B to C efficiency?
ChallengeA marine pyramid of biomass may be inverted yet the energy pyramid never is. Why?
Error redoA student confuses biomass with energy. How do their units differ?

03　Answers and one-line notes

#	Answer	One-line note
1	The producers, holding the largest biomass	Producers support the whole system and usually hold the greatest biomass, so the base is widest.
2	A pyramid of numbers, because one large tree can support very many insects	Counted as numbers of individuals, one producer can support very many primary consumers.
3	Heat from respiration, uneaten parts, and excreted or egested material	Together these account for roughly nine tenths of the energy never passed on.
4	3 000 kJ/m2/yr	Net primary productivity is gross productivity minus the plants own respiration, giving 3 000.
5	1 per cent	500 out of 50 000 is 1 per cent, each transfer losing about nine tenths.
6	25 per cent	The remainder is 2 000 kJ, which is 25 per cent of intake.
7	Only about one tenth passes each time, so the next level would have too little to support consumers	Energy falls by a factor of ten each step and soon drops below what another level needs.
8	1 per cent and 10 per cent	120 out of 12 000 is 1 per cent, while 120 out of 1 200 is 10 per cent.
9	Phytoplankton turn over very fast, so a small standing biomass still supports the levels above	Biomass is a standing stock at one moment, while the energy pyramid measures flow over time and so stays upright.
10	Biomass is measured in g/m2 or kg/m2 while energy is measured in kJ/m2	Biomass measures mass while energy measures flow, so the units are not interchangeable.

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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