Trophic Levels and Energy Transfer
In 2018 the UN Food and Agriculture Organization published its report The State of Food and Agriculture, revealing that livestock uses 80% of global agricultural land but provides only 18% of global calories. Beef requires 25 kg of grain per kg of meat protein, losing roughly 97% of plant energy along the way. If 10% of global beef consumption shifted to plant protein, the freed land would equal the area of India. These numbers are not policy arguments — they are direct applications of the 10% energy transfer rule that governs every food chain on Earth.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
Trophic pyramid showing energy transfer between trophic levels
Q1. A steer consumes approximately 10,000 kJ of energy stored in grass each day. A student claims the steer will therefore gain 10,000 kJ of body mass energy per day. Is this claim correct? If not, where does the "missing" energy go?
Q2. Could a food chain realistically have 15 trophic levels? Why do natural food chains typically stop at 4 or 5 levels? Use your intuition about energy — not vocabulary you have memorised.
Core Content
Why most ingested energy never becomes biomass at the next level
In 2018, the UN Food and Agriculture Organization measured that beef production requires 25 kg of grain per kg of meat protein — a loss rate of 97%. Livestock globally occupy 80% of agricultural land but deliver only 18% of calories. These are not opinion — they are measurements of trophic efficiency, the fraction of energy at one level that is incorporated into biomass at the next. When a steer eats grass, most of the grass energy vanishes as heat through respiration, as undigested plant matter in faeces, and as urea in urine. Only a small fraction — 3–10% — actually becomes new beef biomass.
- Respiration: The organism breaks down glucose via cellular respiration to release ATP for movement, growth, temperature regulation and reproduction. A large fraction of the chemical energy in food is converted to heat and lost to the environment. This is the single largest energy loss — typically 60–90% of ingested energy.
- Egestion (faeces): Not all food is digestible. Cellulose in plant cell walls, chitin in insect exoskeletons and bone in vertebrates pass through the gut undigested and are egested.
- Excretion (urine): Excess nitrogen from protein metabolism is converted to urea or uric acid and excreted. These waste products contain chemical energy the consumer cannot use.
Only the energy that remains after these three losses is available for growth and reproduction — that is, for building new biomass that can be eaten by the next trophic level.
Three pathways of energy loss: respiration (heat — the largest loss, 60–90%), egestion (faeces — undigested material), and excretion (urine — nitrogenous waste). Only energy stored in new growth and reproduction passes to the next trophic level.
Pause — copy the highlighted point into your book before the check below.
Which of the following represents the LARGEST single loss of energy from an organism to its environment?
Calculating energy available at each trophic level
We just saw that most ingested energy is lost before it becomes new biomass. That raises a question: how much energy is actually left after each trophic transfer? This card answers it → the 10% rule gives us a formula to calculate how energy cascades down the food chain.
Trophic efficiency is calculated as:
In most ecosystems, trophic efficiency averages approximately 10%. This means that if producers (T1) capture 20,000 kJ of solar energy per square metre per year, only about 2,000 kJ will be stored in primary consumer biomass, about 200 kJ in secondary consumer biomass, and about 20 kJ in tertiary consumer biomass.
| Trophic Level | Example Organism (Australian Grassland) | Energy (kJ m⁻² yr⁻¹) | % Transferred |
|---|---|---|---|
| T1 — Producer | Native grasses (e.g. kangaroo grass, Themeda triandra) | 20,000 | — |
| T2 — Primary consumer | Red kangaroo (Osphranter rufus) | 2,000 | 10% |
| T3 — Secondary consumer | Dingo (Canis familiaris dingo) | 200 | 10% |
| T4 — Tertiary consumer | Wedge-tailed eagle (Aquila audax) | 20 | 10% |
Worked calculation: If a square metre of Australian grassland produces 20,000 kJ of grass energy per year, and trophic efficiency is 10% at each step:
- T2 = 20,000 × 0.10 = 2,000 kJ
- T3 = 2,000 × 0.10 = 200 kJ
- T4 = 200 × 0.10 = 20 kJ
After just three transfers, the available energy has fallen from 20,000 kJ to 20 kJ — a 1,000-fold reduction. This is why apex predators require enormous territories.
10% rule formula: Trophic efficiency = (energy at higher level / energy at lower level) × 100. Typically ~10% (range 5–20%). After 3 transfers: 20,000 → 2,000 → 200 → 20 kJ — a 1,000× reduction. Apex predators need enormous territories because so little energy reaches the top.
Pause — copy the highlighted formula and example calculation into your book before the check below.
In a grassland ecosystem, producers contain 40,000 kJ m⁻² yr⁻¹. Assuming 10% trophic efficiency, what is the energy available to tertiary consumers?
One of the most important distinctions in ecology — and tested in almost every HSC exam
We just saw that energy cascades down through trophic levels and is lost at each step. That raises a question: if energy is always being lost, what happens to the atoms (carbon, nitrogen, phosphorus) in organisms when they die? This card answers it → matter behaves completely differently from energy — it is recycled, not lost.
Energy and matter both move through food webs, but they behave very differently. Energy flows in one direction only and is lost at every step. Matter is recycled.
Energy Flow
- Flows in one direction only: sun → producers → consumers → heat lost to space
- Cannot be recycled by ecosystems
- At each transfer, ~90% is lost (mostly as heat via respiration)
- Pyramids of energy are always upright
Matter Cycling
- Atoms (C, N, P, etc.) are recycled between biotic and abiotic components
- Decomposers and detritivores break down dead matter and release inorganic nutrients
- Producers re-absorb these nutrients and rebuild organic compounds
- No net loss of matter from the ecosystem (closed loop)
Energy flows one-way: sun → producers → consumers → heat lost. Matter is cycled: decomposers return nutrients to soil/water; producers reabsorb them. Decomposers recycle MATTER, not energy.
Pause — copy the highlighted energy vs matter distinction into your book before the check below.
A student claims decomposers "recycle energy back to producers." Identify the error in this claim.
Australia dedicates approximately 54% of its total land area to grazing livestock — predominantly cattle and sheep. This makes grazing the largest single land use in the country, far exceeding cropping, forestry and urban areas combined.
Why so much land? Because of trophic efficiency. Cattle are primary consumers (T2). To produce 1 kg of beef requires roughly 10 kg of feed. At 10% efficiency, 90% of the energy in that feed is lost as heat, faeces and urine. A beef steer must consume vast quantities of grass to accumulate relatively little body mass.
This has profound implications for Australian biodiversity. Grazing modifies vegetation structure, reduces ground cover, increases soil compaction, and can lead to woody weed invasion. Understanding the 10% rule helps explain why shifting toward plant-based diets (eating producers directly) reduces land use pressure — you bypass the 90% energy loss at the T1→T2 transfer.
Use the 10% rule to complete the following calculations. Show your working.
- If producers in a square metre of Australian woodland contain 50,000 kJ of energy, how much energy is available to secondary consumers? Show the step-by-step calculation.
- A reef shark contains 500 kJ of energy in its body. Using the 10% rule, calculate the energy that had to pass through the secondary consumer level and the producer level to support this shark.
- If trophic efficiency were 20% instead of 10%, how much energy would be available to T4 consumers given the same T1 value of 50,000 kJ? What does this tell you about why aquatic food chains can sometimes support more trophic levels than terrestrial ones?
A cattle station in northern Queensland produces beef on native pasture. The following data were collected:
- Native pasture productivity: 15,000 kJ m⁻² yr⁻¹
- A mature steer consumes pasture equivalent to 12,000 kJ m⁻² yr⁻¹
- Steer growth efficiency (energy converted to beef): approximately 3% of ingested energy (cattle are less efficient than the theoretical 10% because they are large endotherms with high metabolic costs)
- Average beef energy content: 6,000 kJ per kg
- Calculate how many square metres of pasture are required to produce 1 kg of beef. Show all working.
- If Australia has approximately 7.7 million km² of land and 54% is used for grazing, calculate the total grazing area in km².
- A plant-based diet delivers energy directly from producers (T1). If a human needs 10,000 kJ per day from wheat (T1), and wheat productivity is 20,000 kJ m⁻² yr⁻¹, how many square metres of wheat are needed for one year? Compare this to your answer from (a) and explain the ecological significance.
- Explain why reducing beef consumption is often proposed as a strategy for reducing land-use pressure and protecting native biodiversity.
Which statement correctly describes the relationship between energy and matter in ecosystems?
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 table below shows energy flow through a simplified Australian grassland food chain. Calculate the energy value for T3 (skink), calculate the overall trophic efficiency from T1 to T4, and explain why the overall efficiency from T1 to T4 is much lower than the efficiency at each individual step.
| Trophic Level | Organism | Energy (kJ m⁻² yr⁻¹) |
|---|---|---|
| T1 | Grasses | 25,000 |
| T2 | Grasshopper | 2,500 |
| T3 | Skink | ? |
| T4 | Brown falcon | 25 |
AnalyseBand 4–5(5 marks) 2. Explain why natural food chains rarely exceed five trophic levels. In your answer, refer to the 10% rule, the three energy loss pathways, and explain why a sixth trophic level would be biologically unsustainable. Use a calculation to support your explanation.
EvaluateBand 5–6(6 marks) 3. Using the Australian grazing case study from this lesson, evaluate whether reducing beef consumption would be an effective strategy for decreasing land-use pressure and protecting native biodiversity. Apply the 10% rule to compare land required for a beef-based versus plant-based diet, and discuss at least two ecological consequences of large-scale grazing.
Show all answers
Multiple Choice
MC answers and full explanations are shown inline as you complete each question.
Activity 1 — Trophic Efficiency Calculations
(a) T2 = 50,000 × 0.10 = 5,000 kJ. T3 = 5,000 × 0.10 = 500 kJ.
(b) Shark (T4) = 500 kJ → T3 = 500 × 10 = 5,000 kJ → T2 = 5,000 × 10 = 50,000 kJ → T1 = 50,000 × 10 = 500,000 kJ through T1.
(c) At 20%: T2 = 50,000 × 0.20 = 10,000; T3 = 10,000 × 0.20 = 2,000; T4 = 2,000 × 0.20 = 400 kJ. This is 20 times more than at 10% efficiency. Aquatic ectotherms have lower metabolic costs than terrestrial endotherms, so they convert a higher percentage of ingested energy into biomass.
Activity 2 — Australian Grazing
(a) Beef energy per kg = 6,000 kJ. Energy required in steer = 6,000 / 0.03 = 200,000 kJ. Pasture required = 200,000 / 15,000 = 13.3 m² per kg of beef.
(b) 7,700,000 × 0.54 = 4,158,000 km² (approximately 4.2 million km²).
(c) Annual human need = 10,000 × 365 = 3,650,000 kJ. Wheat area = 3,650,000 / 20,000 = 182.5 m² per year. Compared to 13.3 m² per kg of beef (a single meal), eating plants directly requires approximately 50–100 times less land per unit of energy delivered.
(d) Large-scale grazing modifies vegetation structure, compacts soil, and can lead to woody weed invasion and erosion. It also reduces habitat for ground-dwelling fauna. Reducing beef demand decreases the economic incentive to convert native ecosystems to pasture, thereby protecting biodiversity.
Short Answer Model Answers
Q1 (4 marks): (a) T3 = 2,500 × 0.10 = 250 kJ [1 mark]. (b) Overall efficiency = (25 / 25,000) × 100 = 0.1% [1 mark]. (c) The T1→T2 efficiency is ~10%. However, overall efficiency compounds the loss at each step: 10% × 10% × 10% = 0.1% [1 mark]. Each trophic level loses ~90% of the energy it receives, so after three transfers only 0.1% of the original energy remains [1 mark].
Q2 (5 marks): The 10% rule states that approximately 10% of energy is transferred between successive trophic levels [1 mark]. Three loss pathways: respiration (heat), egestion (faeces), excretion (urine) [1.5 marks]. A sixth trophic level would contain only 20,000 × (0.10)⁵ = 0.2 kJ m⁻² yr⁻¹ [1 mark]. This is insufficient to support a viable population [1 mark]. Even the smallest organisms need minimum energy for metabolism, movement, growth and reproduction [0.5 marks].
Q3 (6 marks): The 10% rule means cattle (T2) convert only ~3–10% of grass energy into beef [1 mark]. Producing 1 kg of beef requires ~13 m² of pasture, while delivering the same energy from wheat requires only ~0.2 m² — a 50–100 fold difference [1 mark]. Therefore, a plant-based diet dramatically reduces land-use pressure [0.5 marks]. Ecological consequence 1: grazing removes ground cover and reduces habitat for ground-nesting birds, reptiles and small mammals [1 mark]. Ecological consequence 2: hoof compaction reduces soil water infiltration, increases erosion and can cause dryland salinity [1 mark]. Evaluated conclusion: reducing beef consumption is an effective strategy because it addresses the root cause — trophic inefficiency — while reducing habitat destruction, soil degradation and greenhouse gas emissions. However, sustainable grazing management and protected areas are also required [1.5 marks].
Five timed questions on trophic efficiency, the 10% rule, and energy vs matter flow. Beat the boss to bank a tier.
Enter the arenaThe 2018 FAO report found that beef requires 25 kg of grain per kg of meat protein, with 97% of plant energy lost. This matches what the 10% rule predicts: a steer eating 10,000 kJ of grass gains only 300–1,000 kJ as body mass, losing the rest as heat (respiration), faeces (egestion) and urea (excretion). The FAO's calculation that shifting 10% of beef consumption to plant protein would free land the size of India is simply the 10% rule applied at global agricultural scale.
Return to your Think First response. Could you now calculate how much energy remains at trophic level 15 starting from 20,000 kJ at T1? Write the energy vs matter distinction from memory in one sentence each.