Ecological Pyramids — Numbers, Biomass and Energy
In 1960, ecologists Hairston, Smith and Slobodkin measured biomass across trophic levels in 40 temperate ecosystems. They found producer biomass averaged 820 g/m², herbivore biomass averaged only 11 g/m², and carnivore biomass just 2.1 g/m² — ratios consistent with roughly 1–2% energy transfer per level. Their data, published as the landmark 'Green World Hypothesis', provided the first empirical foundation for ecological pyramids and explained why herbivores are always rarer than plants, and carnivores rarer still.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
Q1. A student examines a single eucalyptus tree in the Australian bush and counts 2,000 leaf-eating beetles, 200 spiders that eat the beetles, and 20 kookaburras that eat the spiders. Sketch the rough shape of the pyramid of numbers for this food chain. Is it upright or inverted? Explain your reasoning.
Q2. In the open ocean, the total mass of zooplankton (tiny drifting animals) at any moment can exceed the total mass of phytoplankton (microscopic drifting plants). A student concludes from this that energy must flow from zooplankton to phytoplankton. Is this conclusion valid? What else could explain the observation?
Core Content
Counts individuals — can be upright or inverted depending on organism size
In 1960, Hairston, Smith and Slobodkin weighed every organism across 40 temperate ecosystems and found producers averaging 820 g/m², herbivores 11 g/m², and carnivores 2.1 g/m². If you drew a bar for each level scaled to its value, you'd get a staircase narrowing toward the top — an ecological pyramid. The simplest version counts individual organisms rather than weighing them. In a grassland, there might be 10,000 grasses, 1,000 grasshoppers, 100 frogs, and 1 hawk — each level narrower than the one below, reflecting the energy losses Hairston's team measured.
Upright pyramid of numbers: In a grassland, there might be 10,000 grass plants, 1,000 grasshoppers, 100 frogs, 10 snakes and 1 hawk. Each level has fewer individuals than the level below.
Inverted pyramid of numbers: In a forest, one large eucalyptus tree might support 2,000 leaf-eating beetles, 200 spiders and 20 kookaburras. Here the producer level has the fewest individuals, so the pyramid is inverted.
Key insight: Pyramid of numbers reflects individual count, not energy or mass. A single large producer can support thousands of small consumers, inverting the pyramid. This is why numbers pyramids are the least reliable ecological model.
Pyramid of numbers: shows individual count at each trophic level. Upright: grassland (many small grasses > fewer large herbivores). Inverted: forest with one large tree supporting thousands of insects. Least reliable model — distorted by organism size.
Pause — copy the highlighted point into your book before the check below.
A forest pyramid of numbers shows: 1 eucalyptus tree → 2,000 leaf beetles → 200 spiders → 20 kookaburras. What shape is this pyramid?
Shows standing crop (dry mass at one moment) — usually upright on land, can be inverted in oceans
We just saw that counting individuals can give misleading shapes depending on organism size. That raises a question: does measuring total mass instead solve the problem? This card answers it → mostly yes on land, but in the ocean, biomass pyramids can still be inverted — and understanding why reveals the key distinction between standing crop and productivity.
A pyramid of biomass shows the total dry mass of all organisms at each trophic level at a single point in time — the standing crop. Biomass is usually measured in grams per square metre (g m⁻²).
Terrestrial ecosystems — always upright: In most land ecosystems, producer biomass exceeds herbivore biomass, which exceeds carnivore biomass. A square metre of Australian woodland contains far more grass mass than kangaroo mass, and far more kangaroo mass than dingo mass.
Aquatic ecosystems — can be inverted: In the open ocean, phytoplankton are microscopic and have very short lifespans (they reproduce, are eaten and die within days). At any single moment, the total mass of phytoplankton present may be less than the total mass of zooplankton feeding on them. However, the productivity (rate of production) of phytoplankton is enormous.
Critical distinction: Biomass pyramids measure standing crop (what is present right now), not productivity (how fast new biomass is being made). An inverted biomass pyramid in the ocean does not mean there is insufficient producer biomass — it means producers are being consumed as fast as they grow.
| Ecosystem | Pyramid Type | Shape | Explanation |
|---|---|---|---|
| Australian grassland | Numbers | Upright | Many small grasses, fewer large herbivores, fewest carnivores |
| Australian eucalypt forest | Numbers | Inverted | One large tree supports thousands of insects |
| Australian woodland | Biomass | Upright | Standing crop of grass > kangaroos > dingoes |
| Open ocean (pelagic) | Biomass | Inverted | Phytoplankton have very short lifespans; zooplankton standing crop can exceed producer standing crop at a given moment |
| Any ecosystem | Energy | Always upright | Energy is always lost at each transfer; no exceptions |
Pyramid of biomass: shows standing crop (dry mass at one moment) at each trophic level. Terrestrial: usually upright (grass mass > kangaroo mass > dingo mass). Aquatic: can be inverted because phytoplankton have very short lifespans and high turnover. Standing crop ≠ productivity — phytoplankton have tiny standing crop but enormous productivity.
Pause — copy the highlighted biomass pyramid summary, especially the standing crop vs productivity distinction, before the check below.
In the open ocean, the standing crop biomass of zooplankton exceeds that of phytoplankton. Which statement best explains this observation?
The most fundamental and reliable ecological model
We just saw that both numbers and biomass pyramids can be inverted in specific cases. That raises a question: is there any pyramid that is always reliable? This card answers it → the pyramid of energy is the only model guaranteed to be upright in every ecosystem, because the second law of thermodynamics has no exceptions.
The pyramid of energy shows the total energy passing through each trophic level per unit area per unit time — typically measured in kilojoules per square metre per year (kJ m⁻² yr⁻¹).
Unlike numbers and biomass, which measure what is present at a single moment, energy pyramids measure what flows through each level over time. This makes them immune to the distortions caused by body size or lifespan.
Why energy pyramids are always upright:
- Energy is lost at every trophic level, primarily as heat via cellular respiration
- No organism is 100% efficient at converting food into biomass
- The second law of thermodynamics dictates that energy transfers are never 100% efficient — some energy is always dissipated as heat
- Therefore, the total energy entering any trophic level must always exceed the total energy leaving it to the next level
HSC exam tip: If a question asks you to "explain why energy pyramids are always upright," you must refer to energy loss (respiration/heat) and the second law of thermodynamics. Simply stating "because energy decreases" is insufficient for Band 5–6.
Pyramid of energy: shows energy flow per unit area per time (kJ m⁻² yr⁻¹). Always upright — energy lost as heat at every level via respiration. Second law of thermodynamics: no energy transfer is 100% efficient. Most reliable model — not distorted by body size or lifespan.
Pause — copy the highlighted energy pyramid rule and the thermodynamics reason into your book before the check below.
Which of the following pyramids is guaranteed to be upright in every ecosystem, regardless of the organisms present?
In Australian eucalypt woodland, a single ironbark tree can support an extraordinary diversity and abundance of life. Researchers have documented over 300 species of insects living on a single mature eucalypt — including leaf beetles, psyllids, caterpillars, ants and spiders. These insects attract insectivorous birds, lizards and mammals. A single tree becomes an entire ecosystem.
This explains why pyramids of numbers are so often inverted in Australian forests: one massive producer supports thousands of tiny consumers. But if we measure biomass, the pyramid flips upright — that single tree weighs several tonnes, while all the insects combined might weigh only a few kilograms. And if we measure energy, the pyramid is not only upright but reveals the true structure: the tree captures enormous solar energy, but only a small fraction passes to herbivores, and an even smaller fraction to carnivores.
The following data were collected from a square metre of Australian temperate grassland over one year:
| Level | Organism | Number | Biomass (g m⁻²) | Energy (kJ m⁻² yr⁻¹) |
|---|---|---|---|---|
| T1 | Native grasses | 500 | 800 | 15,000 |
| T2 | Grasshoppers | 120 | 60 | 1,500 |
| T3 | Skinks | 15 | 18 | 150 |
| T4 | Brown falcon | 1 | 4 | 15 |
- Using the data, describe the shape of the pyramid of numbers. Is it upright or inverted? Explain using specific values.
- Using the data, describe the shape of the pyramid of biomass. Is it upright or inverted? Explain using specific values.
- Using the data, describe the shape of the pyramid of energy. Calculate the trophic efficiency between each pair of successive levels.
- Which pyramid type provides the most accurate representation of the energy structure of this ecosystem? Justify your answer.
Two ecosystems were sampled on the same day:
- For the open ocean, calculate how many times the phytoplankton standing crop is turned over (replaced) in 30 days. Show your working.
- Explain why the pyramid of biomass is inverted in the open ocean but upright in the grassland, despite energy flowing from producers to consumers in both ecosystems. Use the concepts of standing crop, productivity and lifespan.
- A student claims that because the ocean biomass pyramid is inverted, the ocean must be less productive than the grassland. Evaluate this claim using the productivity data.
- Explain why the pyramid of energy would be upright in BOTH ecosystems, regardless of the biomass data.
An ecologist constructs a pyramid of biomass for a coral reef and finds that the biomass of coral (producers) is less than the combined biomass of all fish. Which conclusion can she validly draw?
An ecological pyramid of energy shows that energy:
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
ApplyBand 4(4 marks) 1. The following data were collected from a square metre of Australian woodland: T1 eucalyptus seedlings — 50 individuals, 2,000 g biomass, 30,000 kJ yr⁻¹; T2 leaf beetles — 2,000 individuals, 80 g biomass, 3,000 kJ yr⁻¹; T3 spiders — 200 individuals, 25 g biomass, 300 kJ yr⁻¹; T4 kookaburra — 2 individuals, 8 g biomass, 30 kJ yr⁻¹. (a) Sketch and describe the shape of the pyramid of numbers. (b) Sketch and describe the shape of the pyramid of biomass. (c) Calculate the trophic efficiency from T1 to T2 and from T2 to T3 using the energy data. Show your working.
AnalyseBand 4–5(5 marks) 2. Explain why a pyramid of energy is always upright, while pyramids of numbers and biomass can be inverted. In your answer, distinguish between standing crop, productivity and energy flow, and give one specific example of an inverted pyramid of numbers and one specific example of an inverted pyramid of biomass.
EvaluateBand 5–6(6 marks) 3. Using the Australian bushland case study from this lesson, evaluate whether protecting a single large old-growth eucalyptus tree could be as ecologically valuable as protecting an entire hectare of grassland. In your answer, compare the pyramid structures (numbers, biomass, energy) that would characterise each ecosystem, and discuss the implications for conservation prioritisation.
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Multiple Choice
MC answers and full explanations are shown inline as you complete each question.
Activity 1 — Construct and Compare Pyramids
(a) Numbers pyramid: Upright. 500 grasses > 120 grasshoppers > 15 skinks > 1 falcon. Each level has fewer individuals than the level below.
(b) Biomass pyramid: Upright. 800 g grasses > 60 g grasshoppers > 18 g skinks > 4 g falcon. Producer biomass exceeds consumer biomass at every level.
(c) Energy pyramid: Upright. T1→T2: (1,500 / 15,000) × 100 = 10%. T2→T3: (150 / 1,500) × 100 = 10%. T3→T4: (15 / 150) × 100 = 10%.
(d) Pyramid of energy is most accurate because it measures energy flow through each level over time, independent of organism size or lifespan. Only energy pyramids always reflect the true energy structure.
Activity 2 — Ocean vs Land
(a) Turnover = productivity × days / standing crop = 20 × 30 / 4 = 150 times in 30 days.
(b) In the ocean, phytoplankton have very short lifespans (1–2 days) and are consumed almost as fast as they reproduce. Their standing crop (4 g) is small because they do not accumulate biomass. However, their productivity (20 g/day) is enormous. Zooplankton live longer (10–30 days) so their biomass accumulates to 20 g. In grassland, grasses are perennial and accumulate biomass over years (800 g).
(c) The claim is incorrect. Ocean productivity (20 g/m²/day = 7,300 g/m²/year) far exceeds grassland productivity (2 g/m²/day = 730 g/m²/year). The inverted biomass pyramid indicates high turnover, not low productivity.
(d) Energy pyramids are upright in both because energy is lost at each transfer via respiration (heat), egestion and excretion. The second law of thermodynamics dictates that no energy transfer is 100% efficient. Therefore, the total energy passing through T1 always exceeds T2.
Short Answer Model Answers
Q1 (4 marks): (a) Inverted pyramid of numbers: T2 (2,000 beetles) > T1 (50 seedlings). The base is narrow because the producers are large individual trees; the second level is wide because many small insects feed on them [1 mark]. (b) Upright pyramid of biomass: T1 (2,000 g) > T2 (80 g) > T3 (25 g) > T4 (8 g). Producer biomass greatly exceeds consumer biomass because the tree is massive compared to the insects [1 mark]. (c) T1→T2: (3,000 / 30,000) × 100 = 10%. T2→T3: (300 / 3,000) × 100 = 10%. [1 mark each]
Q2 (5 marks): Energy pyramids are always upright because energy is lost at each trophic level, primarily as heat via cellular respiration [1 mark]. The second law of thermodynamics states that no energy transfer is 100% efficient [0.5 marks]. Therefore, the total energy entering any trophic level must always exceed the total energy leaving it to the next level [0.5 marks]. Standing crop = biomass present at one moment; productivity = rate of new biomass production; energy flow = total energy passing through a level per unit time [1 mark]. Inverted numbers example: one large eucalyptus tree supporting 2,000 beetles, 200 spiders and 20 birds [0.5 marks]. Inverted biomass example: open ocean where phytoplankton standing crop (4 g/m²) is less than zooplankton standing crop (20 g/m²) because phytoplankton are consumed as fast as they grow [0.5 marks]. Energy pyramids avoid these distortions because they measure energy flow over time, not standing crop [1 mark].
Q3 (6 marks): Old-growth eucalyptus: inverted numbers pyramid (1 tree supports thousands of insects), upright biomass pyramid (tree weighs tonnes; insects weigh grams), upright energy pyramid (tree captures massive solar energy but only small fraction passes to consumers) [1.5 marks]. Grassland: upright numbers pyramid, upright biomass pyramid, upright energy pyramid [1 mark]. Numbers comparison: forest is inverted due to large producer size; grassland is upright because grasses are small and numerous [0.5 marks]. Biomass comparison: both are upright, but the forest has a much wider base relative to upper levels because the tree accumulates enormous biomass [0.5 marks]. Energy comparison: both are upright with similar trophic efficiency (~10%), but the forest has higher total energy capture per unit area due to the tree's large photosynthetic surface [0.5 marks]. Conservation implications: a single old-growth tree can support extraordinary biodiversity (300+ insect species) and acts as a keystone structure for birds, mammals and reptiles. However, grasslands support different communities and are more vulnerable to overgrazing. Both are valuable but protect different species assemblages. Conservation should prioritise based on threatened species, ecosystem rarity and connectivity rather than a single metric. Evaluated conclusion: neither is universally more valuable — they are complementary [1.5 marks].
Five timed questions on ecological pyramids, standing crop vs productivity, and the second law of thermodynamics. Beat the boss to bank a tier.
Enter the arenaHairston, Smith and Slobodkin's 1960 survey of 40 temperate ecosystems found producer biomass averaged 820 g/m², herbivore biomass 11 g/m², and carnivore biomass 2.1 g/m². This is the empirical foundation of the biomass pyramid: each level is smaller because only ~1–2% of the energy from the level below is incorporated into new biomass. The ocean exception — where zooplankton biomass can exceed phytoplankton biomass — is explained by phytoplankton's extremely high turnover rate: a small standing crop produces biomass very rapidly, so the pyramid of standing crop is inverted even though the pyramid of energy production per year is not.
Return to your Think First response. Could you now distinguish between a pyramid of standing crop and a pyramid of energy production — and explain which is always upright and why?