What you'll learn
This revision guide covers how energy and biomass move through ecosystems via feeding relationships. You'll learn to interpret food chains and webs, construct pyramids of biomass, and calculate efficiency of biomass transfers between organisms. These concepts are essential for AQA GCSE Biology Paper 2 and appear regularly in exam questions worth 4-6 marks.
Key terms and definitions
Trophic level — the position of an organism in a food chain, food web or pyramid of biomass; represents the number of energy transfers from producers
Producer — an organism (usually a plant or algae) that makes its own food by photosynthesis and forms the first trophic level in all food chains
Primary consumer — a herbivore that feeds directly on producers; occupies the second trophic level
Secondary consumer — a carnivore that feeds on primary consumers; occupies the third trophic level
Biomass — the mass of living material in an organism or at a trophic level, usually measured as dry mass in grams or kilograms
Pyramid of biomass — a diagram that represents the relative biomass at each trophic level in a food chain, drawn to scale with bars proportional to biomass
Decomposer — an organism (bacteria or fungi) that breaks down dead plant and animal material and waste, returning nutrients to the soil
Efficiency of biomass transfer — the percentage of biomass from one trophic level that is incorporated into the biomass of the next level
Core concepts
Food chains and trophic levels
Food chains show the feeding relationships between organisms in an ecosystem. Each stage represents a trophic level.
A typical food chain follows this pattern:
Producer → Primary consumer → Secondary consumer → Tertiary consumer
Example from UK woodland: Oak tree → Caterpillar → Blue tit → Sparrowhawk
Example from Caribbean coral reef: Algae → Parrotfish → Barracuda → Shark
Key points about trophic levels:
- Producers are always at the first trophic level
- Primary consumers are herbivores at the second trophic level
- Secondary consumers are carnivores at the third trophic level
- Tertiary consumers are top carnivores at the fourth trophic level
- Most food chains contain no more than four or five trophic levels
- Apex predators occupy the highest trophic level and have no natural predators
Food webs show multiple interconnected food chains in an ecosystem. They represent feeding relationships more accurately than single food chains because most organisms eat more than one type of food and are eaten by several predators.
Why biomass decreases along food chains
The biomass at each trophic level is less than the level before it. This creates a pyramid shape when biomass is represented visually.
Reasons for biomass loss between trophic levels:
Not all the organism is eaten
- Roots, bones, shells and woody tissue may not be consumed
- These parts contain biomass that doesn't transfer to the next level
Not all eaten material is absorbed
- Large amounts pass through the gut and are egested as faeces
- This undigested material still contains energy and biomass
- Only absorbed material becomes part of the consumer's biomass
Energy and biomass lost in waste products
- Urea and other metabolic waste products contain energy
- These materials are removed from the body through excretion
- The biomass in these substances is lost from the food chain
Energy lost through respiration
- Organisms use glucose from food for cellular respiration
- This releases energy for movement, growth, reproduction and maintaining body temperature
- Energy transferred to the surroundings by heating is no longer available to the next trophic level
- In warm-blooded animals (mammals and birds), large amounts of energy maintain constant body temperature
Typically, only 10% of the biomass from one trophic level is transferred to the next level. This means approximately 90% is lost at each stage.
Pyramids of biomass
A pyramid of biomass is a diagram showing the relative amount of biomass at each trophic level. Unlike pyramids of number, pyramids of biomass are almost always pyramid-shaped.
Rules for drawing pyramids of biomass:
- Each bar represents one trophic level
- Producers are always drawn at the base
- Bars are drawn to scale — width is proportional to biomass
- Bars are stacked centrally above each other
- Label each trophic level clearly with organism names
- Include units (usually g/m² or kg/m²)
The area of each bar represents the total biomass at that trophic level per unit area at a specific time.
Example measurements from a grassland ecosystem (g/m²):
- Grass (producer): 12,000
- Grasshoppers (primary consumer): 1,200
- Frogs (secondary consumer): 120
- Snakes (tertiary consumer): 12
This shows the typical 90% reduction in biomass between levels.
Biomass is measured as dry mass because:
- The water content of organisms varies greatly
- Wet mass would give unreliable measurements
- Dry mass provides a more accurate measure of the organic material present
Calculating efficiency of biomass transfer
The efficiency of biomass transfer between trophic levels can be calculated using this formula:
Efficiency = (biomass transferred to next level ÷ biomass available at previous level) × 100
This calculation is frequently examined at GCSE level and is worth 2-3 marks.
Steps for calculation:
- Identify the biomass at the lower trophic level
- Identify the biomass at the higher trophic level
- Divide higher by lower
- Multiply by 100 to convert to percentage
- Include the % symbol in your answer
Higher efficiency means:
- More biomass is successfully transferred
- Less energy is wasted
- The organism is better adapted to convert food into body mass
Lower efficiency indicates:
- More biomass is lost as waste, through respiration or as uneaten parts
- This is typical in warm-blooded animals that use energy for temperature regulation
Why food chains are limited in length
Food chains rarely exceed four or five trophic levels. The loss of biomass and energy at each stage explains this limitation.
Key reasons:
Insufficient biomass at higher levels
- After three or four transfers, the remaining biomass is too small to support another trophic level
- There isn't enough food energy to sustain viable populations of predators
Energy dissipation
- By the fourth or fifth level, so little energy remains that organisms cannot obtain enough nutrition
- The energy cost of hunting would exceed the energy gained from prey
Practical example: If a grass plant contains 1000 g of biomass, with 10% transfer efficiency:
- Primary consumer receives: 100 g
- Secondary consumer receives: 10 g
- Tertiary consumer receives: 1 g
- Quaternary consumer would receive: 0.1 g (insufficient)
This mathematical reality prevents longer food chains from forming in natural ecosystems.
Human food production and trophic levels
Understanding biomass transfer helps explain strategies for efficient food production.
Reducing the number of trophic levels increases food availability for humans:
Plant-based diets
- Humans eat producers directly (first trophic level)
- More biomass available per unit area of farmland
- More efficient use of land and resources
- Examples: wheat, rice, cassava, plantain, breadfruit
Meat-based diets
- Humans eat primary or secondary consumers
- Significant biomass lost feeding the animals
- Requires more land, water and crop inputs
- Examples: beef, pork, chicken
Farmers can increase biomass transfer efficiency by:
Limiting animal movement
- Reduces energy loss through respiration
- More energy available for growth
- Increases mass gain per unit of food
Controlling temperature
- Keeping animals indoors in warm conditions
- Reduces energy needed for thermoregulation
- Particularly important for cattle, pigs and poultry
Optimising feed
- High-energy, easily digestible food
- Reduces waste and unabsorbed material
- Increases proportion of food converted to biomass
These intensive farming methods raise ethical concerns about animal welfare that you may be asked to discuss in exam questions.
Worked examples
Example 1: Calculating biomass transfer efficiency
Question: A cow eats grass containing 50,000 kJ of energy. The biomass of the cow increases by an amount containing 5,000 kJ of energy. Calculate the efficiency of energy transfer. [2 marks]
Solution:
- Efficiency = (5,000 ÷ 50,000) × 100
- Efficiency = 0.1 × 100
- Efficiency = 10%
Mark scheme:
- 1 mark: correct substitution into formula (5,000/50,000 × 100)
- 1 mark: correct answer with unit (10%)
Example 2: Explaining biomass loss
Question: Explain why the biomass of primary consumers in a food chain is less than the biomass of producers. [4 marks]
Solution: Not all of the producer is eaten / some parts left behind (1 mark)
Not all eaten material is digested / some egested as faeces (1 mark)
Energy lost through respiration / transferred to surroundings by heating (1 mark)
Energy/biomass lost in excretory products / urine (1 mark)
Mark scheme: Any four correct explanations from the list above. Students must explain the mechanism, not just state "energy is lost."
Example 3: Pyramid of biomass interpretation
Question: A pyramid of biomass shows: Grass (8000 g/m²) → Rabbits (600 g/m²) → Foxes (40 g/m²)
Calculate the percentage efficiency of biomass transfer from rabbits to foxes. [2 marks]
Solution:
- Efficiency = (40 ÷ 600) × 100
- Efficiency = 0.0667 × 100
- Efficiency = 6.7% (or 6.67%)
Mark scheme:
- 1 mark: correct working (40/600 × 100)
- 1 mark: correct answer rounded to 2-3 significant figures with % symbol
Common mistakes and how to avoid them
Confusing biomass with number of organisms — pyramids of biomass show mass, not population size. One large tree has more biomass than 1000 aphids even though there are fewer organisms.
Forgetting to multiply by 100 in efficiency calculations — always complete the calculation with × 100 and include the % symbol to gain full marks.
Using the wrong trophic levels in calculations — carefully identify which two consecutive levels you're comparing. Check the question twice before calculating.
Stating "energy is lost" without explanation — examiners want to know how energy is lost. Always specify: respiration, faeces, urine, heat, or uneaten parts.
Drawing pyramids not to scale — in exam questions requiring pyramid construction, use a ruler and make bar widths accurately proportional to the given biomass values.
Confusing decomposers with consumers — decomposers (bacteria and fungi) break down dead material; they're not part of the main food chain but recycle nutrients back to producers.
Exam technique for "Trophic levels and biomass transfer"
"Calculate" questions require showing your working clearly. Write the formula, substitute numbers, then calculate. Even if your final answer is wrong, you can score method marks (usually 1 out of 2 marks).
"Explain" questions about biomass decrease need multiple points. Aim for one developed point per mark available. For 3 marks, give three distinct reasons why biomass decreases, not three ways of saying the same thing.
Extended response questions (4-6 marks) require linking ideas. Start with producers capturing energy through photosynthesis, then systematically work through each trophic level explaining losses. Use connective words like "therefore," "consequently," and "this means that."
Pyramid drawing questions award marks for: correct order (producers at base), bars drawn to scale, neat central alignment, and accurate labels. Use a ruler and pencil, and check the scale carefully.
Quick revision summary
Trophic levels show feeding relationships, starting with producers (plants/algae) that make food through photosynthesis. Primary consumers (herbivores) eat producers; secondary consumers (carnivores) eat primary consumers. Biomass decreases at each level because organisms don't eat everything, can't digest all they eat, and lose energy through respiration, excretion and heat. Only about 10% of biomass transfers between levels. Pyramids of biomass represent this visually with bars proportional to dry mass. Efficiency = (biomass at higher level ÷ biomass at lower level) × 100. Food chains rarely exceed four levels because insufficient biomass remains to support more consumers.