Biology • Year 11 • Module 4 • Lesson 5
Ecological Pyramids: Numbers, Biomass and Energy
Apply your understanding of the three pyramid types to real ecosystem data, calculate trophic efficiencies, and explain the difference between standing crop and productivity.
1. Analyse data from an Australian temperate grassland
The table below shows data collected from 1 m2 of Australian temperate grassland over one year. Use the data to answer all parts. 10 marks
| Trophic level | Organism | Number of individuals | Biomass (g m−2) | Energy (kJ m−2 yr−1) |
|---|---|---|---|---|
| T1 (Producer) | Native grasses | 500 | 800 | 15,000 |
| T2 (Primary consumer) | Grasshoppers | 120 | 60 | 1,500 |
| T3 (Secondary consumer) | Blue-tongue skinks | 15 | 18 | 150 |
| T4 (Tertiary consumer) | Brown falcon | 1 | 4 | 15 |
1.1 Describe the shape of the pyramid of numbers for this grassland. Is it upright or inverted? Justify your answer using specific values from the table. 2 marks
1.2 Describe the shape of the pyramid of biomass for this grassland. Is it upright or inverted? Justify your answer using specific values. 2 marks
1.3 Calculate the trophic efficiency (%) from T1 to T2, and from T2 to T3. Show your working. 2 marks
1.4 A student claims: “Because the number of grasshoppers (120) is less than the number of grass plants (500), this ecosystem must be structured efficiently.” Evaluate this claim. 2 marks
1.5 Which of the three pyramid types (numbers, biomass, energy) best represents the actual energy structure of this grassland? Explain why. 2 marks
2. Interpret and compare biomass data
The graph below shows biomass standing crop (g m−2) and daily productivity (g m−2 day−1) for two ecosystems sampled on the same day. 8 marks
Figure 2. Standing crop and daily productivity for producers and primary consumers in two ecosystems (same day). Grassland standing crop values are much larger — see annotation.
2.1 In the open ocean, which level has the higher standing crop: phytoplankton or zooplankton? By how much? 1 mark
2.2 Explain why the open ocean shows an inverted pyramid of biomass, using the concepts of standing crop, productivity and lifespan from the graph. 3 marks
2.3 A student looks at the graph and says: “The ocean must be less productive than the grassland because its phytoplankton standing crop is only 4 g m−2.” Evaluate this claim using data from the graph. 2 marks
2.4 Explain why a pyramid of energy for the open ocean would be upright, despite the inverted biomass pyramid. 2 marks
3. Apply to a new scenario — Australian eucalypt woodland
An ecologist surveys a 1 m2 patch of Australian eucalypt woodland and records the following data: 6 marks
| Trophic level | Organism | Number | Biomass (g) | Energy (kJ yr−1) |
|---|---|---|---|---|
| T1 | Eucalyptus seedlings | 50 | 2,000 | 30,000 |
| T2 | Leaf beetles | 2,000 | 80 | 3,000 |
| T3 | Spiders | 200 | 25 | 300 |
| T4 | Kookaburra | 2 | 8 | 30 |
3.1 Is the pyramid of numbers for this woodland upright or inverted? Explain using specific values. 2 marks
3.2 Calculate the trophic efficiency from T1 to T2 and from T2 to T3 using the energy data. Show your working. 2 marks
3.3 Despite the inverted pyramid of numbers, the pyramid of biomass is upright. Explain why, using values from the table. 2 marks
Q1.1 — Numbers pyramid shape (2 marks)
The pyramid of numbers is upright. Numbers decrease at each level: 500 grasses > 120 grasshoppers > 15 skinks > 1 falcon. Each level has fewer individuals than the one below it. [1 mark for stating upright + 1 mark for using specific values in the correct direction.]
Q1.2 — Biomass pyramid shape (2 marks)
The pyramid of biomass is upright. Producer biomass (800 g) exceeds herbivore biomass (60 g), which exceeds skink biomass (18 g), which exceeds falcon biomass (4 g). Biomass decreases at every level. [1 mark for stating upright + 1 mark for using specific values correctly.]
Q1.3 — Trophic efficiency (2 marks)
T1 → T2: (1,500 ÷ 15,000) × 100 = 10%. [1 mark]
T2 → T3: (150 ÷ 1,500) × 100 = 10%. [1 mark]
Both calculations must show working. Units not required but values must be correct.
Q1.4 — Evaluate claim (2 marks)
The claim is partially correct but incomplete. It is true that there are fewer grasshoppers (120) than grasses (500), giving an upright pyramid of numbers. However, this does not indicate efficiency — the pyramid of numbers only reflects individual counts, not energy flow. Trophic efficiency is measured using energy data: T1→T2 efficiency is 10%, meaning 90% of the energy entering T1 is lost before reaching T2. [1 mark for identifying a flaw in the claim + 1 mark for using energy data to support.]
Q1.5 — Best pyramid type (2 marks)
The pyramid of energy best represents the actual energy structure [1 mark]. It measures energy flow through each level over time (kJ m−2 yr−1), is independent of organism body size and lifespan, and is always upright. Numbers and biomass pyramids can be distorted by organism size — for example, one large organism can invert the numbers pyramid even when the energy structure is upright [1 mark].
Q2.1 — Open ocean standing crop (1 mark)
Zooplankton have the higher standing crop: 20 g m−2 compared with 4 g m−2 for phytoplankton — 5 times greater. [1 mark; accept values within reading error.]
Q2.2 — Inverted ocean biomass pyramid (3 marks)
Phytoplankton have very short lifespans (days) and are consumed almost as fast as they reproduce [1 mark]. Their daily productivity (20 g m−2 day−1) is very high, but because they do not accumulate, their standing crop at any moment is only 4 g m−2 [1 mark]. Zooplankton live longer and accumulate biomass, so their standing crop (20 g m−2) exceeds that of phytoplankton at this moment, inverting the biomass pyramid [1 mark].
Q2.3 — Evaluate productivity claim (2 marks)
The claim is incorrect. Phytoplankton daily productivity (20 g m−2 day−1) greatly exceeds grass daily productivity (2 g m−2 day−1) — 10 times higher [1 mark]. The low standing crop of phytoplankton does not indicate low productivity; it reflects the rapid consumption and short lifespan of phytoplankton. Standing crop and productivity are different measures and must not be conflated [1 mark].
Q2.4 — Energy pyramid upright in ocean (2 marks)
Energy pyramids measure total energy flowing through each level per unit time, not standing crop at one moment [1 mark]. Energy is always lost as heat via cellular respiration at each transfer, so total energy entering T1 (phytoplankton) always exceeds total energy passing to T2 (zooplankton), regardless of their standing crops. The second law of thermodynamics ensures this is true in all ecosystems [1 mark].
Q3.1 — Woodland numbers pyramid (2 marks)
The pyramid of numbers is inverted. T2 has the most individuals (2,000 beetles) while T1 has only 50 seedlings. The producer level has the fewest individuals [1 mark], because each large eucalyptus seedling supports many small leaf beetles — organism size, not energy, drives the inversion [1 mark].
Q3.2 — Trophic efficiencies (2 marks)
T1 → T2: (3,000 ÷ 30,000) × 100 = 10%. [1 mark]
T2 → T3: (300 ÷ 3,000) × 100 = 10%. [1 mark]
Q3.3 — Biomass upright despite inverted numbers (2 marks)
The biomass pyramid is upright because T1 (eucalyptus seedlings) has a total biomass of 2,000 g, far exceeding T2 leaf beetles (80 g) and T3 spiders (25 g) [1 mark]. Even though there are far fewer trees than beetles, each tree is much larger in mass. Biomass pyramids measure total dry mass per level, not organism count, so large individual producers can give an upright biomass pyramid even when the numbers pyramid is inverted [1 mark].