B43: Trophic Levels and Biomass Transfer
How energy is transferred through trophic levels
HIGHER ONLY: This topic is only examined on the Higher tier paper. You do not need this if you are taking the Foundation paper.
Key Definitions
Trophic level: The position of an organism in a food chain, food web or pyramid of biomass/numbers. Trophic levels are numbered starting from producers at level 1.
Biomass: The total mass of living material in an organism or at a particular trophic level. Biomass is measured as the mass of carbon-containing compounds or the dry mass of tissue (water is excluded because its amount varies).
Efficiency of biomass transfer: The percentage of biomass from one trophic level that is transferred to the next trophic level. Typically only about 10% of biomass is transferred between levels.
Trophic Levels Explained
Each trophic level in a food chain has a specific role:
| Trophic Level | Organism Type | Role | Example (Grassland) |
|---|---|---|---|
| 1 | Producer | Makes food by photosynthesis; converts light energy to chemical energy stored in biomass | Grass |
| 2 | Primary consumer | Herbivore – eats producers | Rabbit |
| 3 | Secondary consumer | Carnivore – eats primary consumers | Fox |
| 4 | Tertiary consumer | Carnivore – eats secondary consumers | Eagle owl |
| 5 | Apex predator | Top of the food chain; not eaten by any other organism in that ecosystem | Polar bear |
Apex predators sit at the top of the food chain and have no natural predators within their ecosystem. Not all food chains reach trophic level 5 – many end at level 3 or 4 because there is insufficient energy remaining to support another level.
Calculating Efficiency of Biomass Transfer
The efficiency of biomass transfer tells us what proportion of the biomass at one trophic level is passed on to the next:
Efficiency = (Biomass transferred to next level ÷ Biomass intake at this level) × 100
Or equivalently:
Efficiency = (Biomass at higher trophic level ÷ Biomass at lower trophic level) × 100
The typical efficiency of biomass transfer between trophic levels is approximately 10%. This means about 90% of biomass is lost between each level.
Worked Example 1: Basic Efficiency Calculation
A field of grass has a biomass of 50,000 kJ/m²/year. The rabbits feeding on the grass have a biomass of 5,000 kJ/m²/year. Calculate the efficiency of biomass transfer from grass to rabbits.
Solution:
Efficiency = (5,000 ÷ 50,000) × 100 = 0.1 × 100 = 10%
This means only 10% of the grass biomass is converted into rabbit biomass.
Worked Example 2: Finding Biomass at the Next Level
Phytoplankton in a lake have a biomass of 80,000 kJ/m²/year. The efficiency of transfer to zooplankton is 8%. Calculate the biomass of the zooplankton.
Solution:
Biomass of zooplankton = (Efficiency ÷ 100) × Biomass of phytoplankton
Biomass of zooplankton = (8 ÷ 100) × 80,000 = 0.08 × 80,000 = 6,400 kJ/m²/year
Worked Example 3: Multi-Level Calculation
A food chain has the following biomass values:
- Producers (oak trees): 200,000 kJ/m²/year
- Primary consumers (caterpillars): 24,000 kJ/m²/year
- Secondary consumers (blue tits): 2,400 kJ/m²/year
Calculate the efficiency of transfer at each stage.
Solution:
Producers to primary consumers: (24,000 ÷ 200,000) × 100 = 0.12 × 100 = 12%
Primary to secondary consumers: (2,400 ÷ 24,000) × 100 = 0.10 × 100 = 10%
Worked Example 4: Calculating How Much Producer Biomass Is Needed
A farmer wants to produce 500 kg of beef cattle biomass. If the efficiency of transfer from grass to cattle is 10%, how much grass biomass is needed?
Solution:
Efficiency = (Biomass of cattle ÷ Biomass of grass) × 100
10 = (500 ÷ Biomass of grass) × 100
Biomass of grass = (500 ÷ 10) × 100 = 50 × 100 = 5,000 kg
This shows that 5,000 kg of grass is needed to produce just 500 kg of cattle – a key reason why eating plants directly is more energy-efficient than eating meat.
Why Biomass Decreases at Each Trophic Level
Approximately 90% of biomass (and energy) is lost between each trophic level. This happens because:
| Reason for Loss | Explanation |
|---|---|
| Respiration | Organisms use most of their biomass (glucose) for respiration to release energy for life processes (movement, keeping warm, active transport, cell division). The CO₂ and water produced are waste and not passed to the next trophic level |
| Excretion | Waste products such as urea in urine contain energy that leaves the body and is not transferred to the next trophic level |
| Egestion | Not all food is digested – some passes through the gut as faeces. The undigested material (e.g. cellulose in plant cell walls, bones, fur) contains energy that is not absorbed |
| Parts not eaten | Consumers do not eat every part of their food. Roots, bark, large bones and hooves may be left. Even when eaten, some tissues are indigestible |
| Maintenance of body temperature | Warm-blooded animals (mammals and birds) use a large proportion of their energy simply maintaining a constant body temperature – this energy is "lost" as heat and cannot be passed on |
Why food chains are usually only 4–5 trophic levels long: After each transfer, only about 10% of biomass remains. After 4 transfers (producer → primary → secondary → tertiary), only 0.1% of the original biomass is left (0.1 × 0.1 × 0.1 × 0.1 = 0.0001). There is simply not enough energy remaining at higher levels to sustain a viable population of consumers. Apex predators are rare because the energy available to them is so small.
Pyramids of Biomass
Pyramids of biomass are always pyramid-shaped because biomass decreases at each trophic level. This is because:
- Producers have the most biomass (they capture all the energy from sunlight)
- Each consumer level has less biomass due to energy losses (respiration, excretion, egestion, parts not eaten)
- The narrow top bar represents the small remaining biomass at the highest trophic level
Pyramid of biomass vs pyramid of numbers: A pyramid of biomass is always pyramid-shaped. A pyramid of numbers (counting individual organisms) may not be. For example, one oak tree (producer) may support thousands of caterpillars (primary consumers), giving an inverted pyramid of numbers. But the total biomass of the tree is still greater than the total biomass of all the caterpillars combined.
Comparison: Why Different Trophic Levels Have Different Biomass
| Trophic Level | Relative Biomass | Energy Available | Reason |
|---|---|---|---|
| Producers (1) | Highest | All energy captured from Sun | Convert light energy to chemical energy; no losses from a previous level |
| Primary consumers (2) | ~10% of producers | Limited by what they can eat and digest | 90% of plant biomass lost (respiration, indigestible cellulose, roots not eaten) |
| Secondary consumers (3) | ~10% of primary consumers | Further reduced | 90% of primary consumer biomass lost (respiration, bones/fur not eaten, heat loss) |
| Tertiary consumers (4) | ~10% of secondary consumers | Very little remaining | Mammals/birds lose most energy as heat for thermoregulation |
| Apex predators (5) | Tiny fraction of original | Barely sustainable | So little energy remains that apex predators must be few in number and cover large territories |
Practice Questions
Q1: Higher The biomass of producers in a meadow is 120,000 kJ/m²/year. The biomass of primary consumers (grasshoppers) is 14,400 kJ/m²/year. Calculate the efficiency of biomass transfer from producers to primary consumers.
Q2: Higher In a marine food chain, the biomass of phytoplankton is 250,000 kJ/m²/year. The efficiency of transfer to zooplankton is 10%, and from zooplankton to small fish is 12%. Calculate the biomass of the small fish.
Q3: Higher Explain why biomass pyramids are always pyramid-shaped but pyramids of numbers are not always pyramid-shaped. Use an example in your answer.
Q4: Higher A farmer produces 2,000 kg of wheat biomass and 200 kg of chicken biomass from the same area of land. Calculate the efficiency of biomass transfer from wheat to chicken. Explain why the chicken biomass is much less than the wheat biomass.
Q5: Higher Explain why food chains rarely have more than five trophic levels. Use calculations to support your answer.
Q6: Higher The efficiency of transfer from producers to primary consumers is 10%. From primary to secondary consumers it is 15%. From secondary to tertiary consumers it is 5%. If the producer biomass is 500,000 kJ/m²/year, calculate the biomass at each trophic level and the overall efficiency from producers to tertiary consumers.
Answers
- Efficiency = (Biomass of primary consumers ÷ Biomass of producers) × 100
= (14,400 ÷ 120,000) × 100 = 0.12 × 100 = 12% - Step 1 – Phytoplankton to zooplankton: 250,000 × 0.10 = 25,000 kJ/m²/year
Step 2 – Zooplankton to small fish: 25,000 × 0.12 = 3,000 kJ/m²/year - Pyramids of biomass are always pyramid-shaped because biomass (and therefore energy) is always lost at each trophic level through respiration, excretion, egestion and parts not eaten. Producers always have the greatest total mass, and each subsequent level has less. Pyramids of numbers count individuals, not mass. An inverted pyramid of numbers can occur when one large producer supports many small consumers. For example, one oak tree (1 individual) may support 5,000 caterpillars (5,000 individuals) – the pyramid of numbers is inverted. But the biomass of the single tree is far greater than the combined biomass of all the caterpillars, so the biomass pyramid is still pyramid-shaped.
- Efficiency = (200 ÷ 2,000) × 100 = 0.10 × 100 = 10%. The chicken biomass is much less than the wheat biomass because: (1) Chickens respire – they use most of the energy from the wheat they eat to maintain their body temperature, move and carry out life processes; this energy is lost as heat and CO₂. (2) Not all parts of the wheat are eaten or digested – stalks, roots and cellulose in cell walls pass through as undigested material (egestion). (3) Chickens produce waste (urea in urine, faeces) containing energy that is not converted into chicken biomass. (4) Some wheat is used for maintenance of existing body tissues rather than growth of new tissue.
- Food chains rarely have more than five trophic levels because the efficiency of biomass transfer between each level is only about 10%. This means that at each step, 90% of the biomass is lost. If we start with 100,000 kJ/m²/year of producer biomass: Level 1 (producers) = 100,000; Level 2 = 10,000 (10%); Level 3 = 1,000 (10%); Level 4 = 100 (10%); Level 5 = 10 (10%). By trophic level 5, only 0.01% of the original energy remains – just 10 kJ/m²/year. This is insufficient to support a viable population of organisms at a sixth trophic level. The energy available would be too small for organisms to survive, reproduce and maintain a stable population.
- Level 1 (producers): 500,000 kJ/m²/year
Level 2 (primary consumers): 500,000 × 0.10 = 50,000 kJ/m²/year
Level 3 (secondary consumers): 50,000 × 0.15 = 7,500 kJ/m²/year
Level 4 (tertiary consumers): 7,500 × 0.05 = 375 kJ/m²/year
Overall efficiency from producers to tertiary consumers = (375 ÷ 500,000) × 100 = 0.075 × 100 = 0.075%
Exam Tips
- Always show your working in efficiency calculations – you can gain marks for correct method even if the final answer is wrong
- The efficiency formula is: (biomass transferred ÷ biomass intake) × 100 – don't forget to multiply by 100 to get a percentage
- When explaining why biomass decreases, give at least three specific reasons: respiration (biggest loss), excretion/egestion, and parts not eaten
- For "why food chains are short" questions, use calculations to show how little energy remains after 4–5 transfers – this earns more marks than just saying "energy is lost"
- Remember that respiration is the single largest cause of energy loss – organisms use most energy just staying alive (especially warm-blooded animals maintaining body temperature)
- Don't confuse pyramid of biomass (always pyramid-shaped) with pyramid of numbers (can be inverted) – examiners test this distinction
- This is a Higher-only topic – if you're sitting Foundation, you still need to know basic food chains and pyramids from B40, but not the efficiency calculations
🔢 Maths Skills
Mathematical Skills
Efficiency of biomass transfer = (biomass transferred / biomass available) × 100. For example, if producers have 50,000 kJ/m²/year and primary consumers have 5,000 kJ/m²/year, efficiency = 5,000 / 50,000 × 100 = 10%.
Multi-step calculations: if each transfer is 10% efficient, then after 4 trophic levels only 0.01% of the original biomass remains (0.1 × 0.1 × 0.1 × 0.1 = 0.0001). Always show your working for full marks.
⚠️ Common Misconceptions
Watch Out!
Students often think energy is lost at each trophic level. Wrong: Energy is lost at each trophic level Correct: Energy is transferred to other stores — it is not lost but converted to heat through respiration, stored in uneaten parts, or excreted as waste
Students often think all food energy becomes biomass. Wrong: All food energy becomes biomass in the consumer Correct: Most energy from food is used in respiration to release energy for life processes; only a small fraction is converted into new biomass
✍️ 6-Mark Question
Extended Answer
6 marks: Explain why food chains rarely exceed five trophic levels.
Food chains rarely exceed five trophic levels because only about 10% of biomass (and energy) is transferred between each level. The remaining 90% is lost through: respiration (organisms use most energy for life processes such as movement, keeping warm and cell division); excretion and egestion (waste products and undigested material contain energy not passed on); and parts not eaten (bones, fur, roots). Starting with 100,000 kJ/m²/year of producer biomass: level 2 receives 10,000 (10%); level 3 receives 1,000; level 4 receives 100; level 5 receives just 10. After four transfers, only 0.01% of the original energy remains — this is insufficient to support a viable population at a sixth trophic level. Apex predators are rare because so little energy is available to them.
Mark scheme: 1 mark for 10% transfer rule, 1 mark per reason for energy loss (up to 3), 1 mark for calculation showing diminishing energy, 1 mark for conclusion
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A food chain has: oak trees (200,000 kJ/m²/year) → caterpillars (24,000) → blue tits (2,400) → sparrowhawks (120). Calculate the efficiency at each transfer. The sparrowhawk efficiency is notably lower than the others. Suggest why warm-blooded predators at the top of food chains have particularly low efficiency. Evaluate the implications for conservation of top predators.
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🎬 Video Resources
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