Biology • Year 11 • Module 4 • Lesson 5
Ecological Pyramids: Numbers, Biomass and Energy
Lock in the three pyramid types, what each measures, when they are upright or inverted, and why energy pyramids are never inverted.
1. Complete the key term definitions
Fill in each blank using the word bank below. Each term is used once. 6 marks
| Term | Definition — fill in the blank |
|---|---|
| Pyramid of numbers | Shows the number of individual organisms at each _________________________, and can be upright or _________________________. |
| Pyramid of biomass | Shows the total _________________________ of all organisms at each trophic level at a single point in time — also called the _________________________. |
| Pyramid of energy | Shows the total _________________________ passing through each trophic level per unit area per unit time. Always upright because _________________________ is always lost at each transfer. |
2. True or false — with correction
Circle T or F. If the statement is false, write the corrected version on the line provided. 8 marks (1 mark T/F + 1 mark correction where needed)
2.1 A pyramid of energy is always upright because energy is lost at each trophic level, primarily as heat. T / F
2.2 A pyramid of numbers can be inverted, for example when one large eucalyptus tree supports thousands of insects. T / F
2.3 Pyramids of biomass are always upright in every ecosystem because producer biomass is always greater than consumer biomass. T / F
2.4 An inverted biomass pyramid in the open ocean means energy is flowing backwards from zooplankton to phytoplankton. T / F
3. Match pyramid type to description
In the right-hand column, write Numbers, Biomass or Energy to identify which pyramid type is being described. Some types may appear more than once. 6 marks
| # | Description | Pyramid type |
|---|---|---|
| 3.1 | Measured in kJ m−2 yr−1; always upright; most reliable ecological model. | |
| 3.2 | Measures standing crop; can be inverted in the open ocean due to rapid phytoplankton turnover. | |
| 3.3 | Can be inverted in a forest ecosystem when one large tree supports many small insects. | |
| 3.4 | Shows total dry mass at each trophic level; usually upright in terrestrial ecosystems. | |
| 3.5 | The least reliable ecological model because it is highly sensitive to organism size. | |
| 3.6 | Measures what flows through each level over time, not what is present at one moment. |
4. Concept recall — answer in 1–2 sentences
Use precise lesson terms in each answer. 8 marks (2 each)
4.1 What is the difference between standing crop and productivity?
4.2 Why are pyramids of energy the most reliable ecological model?
4.3 Give one Australian ecosystem example where the pyramid of numbers is inverted. Name the producer and explain why inversion occurs.
4.4 Explain why a pyramid of energy is always upright. Your answer must refer to the second law of thermodynamics.
5. Link the pyramid types
Draw labelled arrows between the five terms below to show how they connect. Each arrow must carry a linking phrase. Aim for at least 5 labelled arrows. 5 marks
Supplied terms: pyramid of numbers · pyramid of biomass · pyramid of energy · standing crop · trophic efficiency.
Q1 — Key term definitions (6 marks)
Row 1 (pyramid of numbers): trophic level; inverted. Row 2 (pyramid of biomass): dry mass; standing crop. Row 3 (pyramid of energy): energy; energy.
Marking: 1 mark per correct blank (6 total).
Q2 — True / false with correction (8 marks)
2.1 True. Energy is lost primarily as heat via cellular respiration at each level; the second law of thermodynamics means no transfer is 100% efficient.
2.2 True. One large tree (producer) supports thousands of small insects (primary consumers), placing the narrow base at the producer level.
2.3 False. Correction: pyramids of biomass can be inverted in aquatic ecosystems (e.g. the open ocean) where phytoplankton have very short lifespans and are consumed as fast as they reproduce, so their standing crop at any moment is smaller than the zooplankton biomass above them.
2.4 False. Correction: an inverted biomass pyramid in the ocean does not indicate backward energy flow. Energy still flows from phytoplankton to zooplankton. The inversion occurs because phytoplankton are consumed almost as fast as they are produced, so their standing crop is momentarily low despite their high productivity. The pyramid of energy remains upright.
Marking: 1 mark per T/F; 1 mark per correction where needed (statements 2.3 and 2.4 require a correction).
Q3 — Match pyramid type (6 marks)
3.1 Energy | 3.2 Biomass | 3.3 Numbers | 3.4 Biomass | 3.5 Numbers | 3.6 Energy
Marking: 1 mark per correct answer.
Q4.1 — Standing crop vs productivity (2 marks)
Standing crop is the total biomass of organisms present in an ecosystem at a single moment in time. Productivity is the rate at which new biomass is produced per unit area per unit time (e.g. g m−2 yr−1). An organism can have low standing crop but high productivity (e.g. phytoplankton) if it is consumed as fast as it grows. [1 mark for each correct definition; must contrast the two concepts for 2 marks.]
Q4.2 — Why energy pyramids are most reliable (2 marks)
Energy pyramids measure the total energy flowing through each trophic level per unit area per unit time, rather than what is present at one instant. This makes them independent of organism body size and lifespan [1 mark] — unlike numbers and biomass pyramids, which can be distorted (inverted) by a single large organism or by rapid turnover. Only energy pyramids are always upright in every ecosystem [1 mark].
Q4.3 — Inverted numbers example (2 marks)
In an Australian eucalypt forest, a single large eucalyptus tree (the producer, T1) supports thousands of leaf-eating insects (T2), hundreds of spiders (T3), and a small number of kookaburras (T4) [1 mark]. The pyramid of numbers is inverted because the producer level has the fewest individuals — the inversion is caused by the large body size of the producer relative to the consumers, not by the energy structure [1 mark].
Q4.4 — Why energy pyramids are always upright (2 marks)
Energy is lost at every trophic level, primarily as heat via cellular respiration [1 mark]. The second law of thermodynamics states that no energy transfer is 100% efficient — some energy is always dissipated as thermal energy. Therefore, the total energy entering any trophic level must always exceed the total energy passed to the next level, making the pyramid necessarily upright [1 mark].
Q5 — Sample concept map (5 marks)
Correct arrows should include at least five of the following:
- pyramid of biomass — measures → standing crop
- pyramid of numbers — and pyramid of biomass can be → inverted
- pyramid of energy — is always → upright (never inverted)
- trophic efficiency — determines the width of each level in → pyramid of energy
- pyramid of energy — is the most reliable because it measures flow, not → standing crop
- All three pyramid types — display → trophic levels
Award 1 mark per correctly labelled, directionally valid arrow (max 5).