Transfer of biomass — AQA GCSE Biology
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Transfer of biomass explained
A pyramid of biomass is a quantitative diagram in which each trophic level is drawn as a horizontal bar whose area is proportional to the dry mass of living material present, usually measured in g m⁻² or kg m⁻².
Read the full explanation
Producers form the wide base, primary consumers the next bar, and so on, so the shape shows how much energy-containing material is available at each stage. Biomass is lost between levels because much ingested material is not absorbed and is egested as faeces, absorbed material is used in respiration and lost as carbon dioxide and water, and some is excreted as urea or other waste. Energy also leaves as heat, and dead organisms and waste become detritus for decomposers. Because only about 10 % of biomass passes to the next level, pyramids usually narrow sharply, and food chains rarely have more than four or five trophic levels.
Producers are mostly plants and algae which transfer about 1 % of the incident energy from light for photosynthesis.
Producers occupy the first trophic level and are mainly green plants on land and algae in water. They absorb light energy using chlorophyll and convert carbon dioxide and water into glucose and oxygen during photosynthesis, storing the glucose as starch, cellulose, lipids and other biomass. Only about 1 % of the incident light energy falling on a producer is transferred into biomass; the rest is reflected, transmitted through leaves, or absorbed by non-photosynthetic parts, or has wavelengths not used by chlorophyll. Some absorbed energy is also lost as heat during respiration. This low efficiency explains why producers need a large surface area of leaves or algal cells, and why the total biomass of producers in an ecosystem is much greater than that of consumers.
Only approximately 10 % of the biomass from each trophic level is transferred to the level above it.
When a consumer eats a producer or another consumer, only about 10 % of the biomass of the eaten organism becomes biomass of the eater. The remaining biomass is accounted for by material that is not eaten, material that is ingested but not absorbed and is egested as faeces, and absorbed material that is used in respiration and lost as carbon dioxide, water and heat, or excreted as urea. Because each transfer is so inefficient, the biomass available at each successive trophic level falls sharply, producing a pyramid with much smaller upper bars. This also limits food chains to about four or five trophic levels, since too little biomass remains to support another level. The figure is an approximation and varies with the organisms and environment.
Losses of biomass are due to: • not all the ingested material is absorbed, some is egested as faeces • some absorbed material is lost as waste, such as carbon dioxide and water in respiration and water and urea in urine.
When a consumer eats food, not all of the biomass in that food becomes part of the consumer. Some ingested material is not absorbed and leaves the body as faeces, so it is lost from the food chain at that point. Of the material that is absorbed, some is used in respiration, where carbon dioxide and water are produced and released as waste. Other absorbed material is converted to waste products such as water and urea in urine. These losses mean that only a fraction of the biomass eaten is available to the next trophic level, which explains why food chains rarely have many levels and why biomass pyramids narrow.
Large amounts of glucose are used in respiration.
Glucose is the main respiratory substrate. In aerobic respiration, glucose is broken down in cells using oxygen to release energy for processes such as muscle contraction, active transport and building large molecules. Because so much energy is needed, a large proportion of the glucose produced by photosynthesis or absorbed from food is used in respiration rather than stored as biomass. This helps explain why biomass decreases along a food chain: much of the glucose taken in by an organism is respired, releasing carbon dioxide and water, and only some becomes new biomass. For example, a cow eats grass containing glucose; most of that glucose is respired to power the cow's life processes, so only a fraction becomes cow biomass available to the next trophic level.
Students should be able to calculate the efficiency of biomass transfers between trophic levels by percentages or fractions of mass.
Biomass transfer efficiency compares the biomass gained by one trophic level with the biomass available from the previous level. To calculate it, divide the biomass of the higher trophic level by the biomass of the lower trophic level, then multiply by 100 for a percentage. For example, if grass has 20 000 g of biomass and the rabbits eating it gain 2 000 g, efficiency = (2 000 ÷ 20 000) × 100 = 10%. As a fraction this is 2 000 ÷ 20 000 = 0.1 or 1/10. Efficiency is never 100% because energy is lost in respiration, movement, excretion and as heat, and not all biomass is eaten or digested. The same method applies to energy values if the question gives energy rather than mass.
Students should be able to explain how this affects the number of organisms at each trophic level.
Because biomass transfer between trophic levels is inefficient, less biomass and energy are available at each successive level. This limits how many organisms each level can support. If only about 10% of biomass passes on, the next trophic level has far less material and energy for growth, reproduction and survival, so it usually contains fewer organisms. For example, a field may support many thousands of grass plants, fewer rabbits and only a few foxes. The effect is not always a simple decrease in numbers because some organisms are larger or smaller than others, but the total biomass and energy generally decrease. This is why food chains are usually short, with only four or five trophic levels.
Your focus
- Draw and interpret a pyramid of biomass with correctly scaled bars for named trophic levels.
- Identify and sequence the processes by which biomass is lost between trophic levels.
- Use the approximate 10 % transfer figure to explain the shape of pyramids and the length of food chains.
Show all 21 objectives
- Describe the role of plants and algae as producers in a food chain.
- State the approximate proportion of incident light energy transferred into producer biomass.
- Explain why most incident light energy is not converted into biomass.
- State the approximate percentage of biomass transferred between trophic levels.
- Calculate the biomass available at a higher trophic level using the 10 % approximation.
- Explain how inefficient transfer limits the number of trophic levels in a food chain.
- Describe the two main ways biomass is lost from a consumer: unabsorbed material egested as faeces and absorbed material lost as waste.
- Explain how respiration and excretion remove carbon dioxide, water and urea, reducing the biomass available to the next trophic level.
- Apply these ideas to explain why biomass pyramids narrow and food chains rarely contain more than four or five trophic levels.
- State that glucose is used in respiration to release energy.
- Explain why a large amount of glucose is respired rather than stored as biomass.
- Relate glucose use in respiration to the loss of biomass between trophic levels.
- Calculate biomass transfer efficiency as a percentage or fraction.
- Interpret an efficiency value in terms of biomass passed between trophic levels.
- Explain why biomass transfer efficiency is always less than 100%.
- Explain why fewer organisms are usually supported at higher trophic levels.
- Relate inefficient biomass transfer to the length of food chains.
- Describe how pyramids of biomass or numbers represent changes between trophic levels.
Transfer of biomass exam tips
Marking Points
- Defines a pyramid of biomass as a diagram whose bar areas represent the dry mass of organisms at each trophic level, with producers at the base.
- States that biomass is measured as dry mass per unit area, for example g m⁻², and that the bars are drawn to scale.
- Explains that not all ingested material is absorbed, so some is egested as faeces and is not available to the next trophic level.
- Explains that absorbed material is used in respiration, releasing carbon dioxide and water, so biomass is lost as these waste products.
- Explains that some absorbed material is excreted as waste such as urea, and that heat from respiration is also lost to the environment.
- Uses the idea that only about 10 % of biomass is transferred to the next trophic level to explain why pyramids narrow and food chains are short.
- Identifies producers as mostly plants and algae that carry out photosynthesis.
- States that producers transfer approximately 1 % of the incident light energy into biomass.
- Names the inputs of photosynthesis as carbon dioxide and water and the products as glucose and oxygen.
- Explains that most incident light is reflected, transmitted or absorbed by non-photosynthetic material, or is of unsuitable wavelength.
- Links the stored products of photosynthesis to the biomass available to primary consumers.
- Recognises that energy absorbed by producers may also be lost as heat during respiration.
- States that approximately 10 % of biomass is transferred from one trophic level to the next.
- Applies the 10 % figure to calculate the biomass available at a higher trophic level from a given starting biomass.
- Explains that the remaining biomass is lost through uneaten material, egestion as faeces, respiration and excretion.
- Links the low transfer efficiency to the narrowing shape of a pyramid of biomass.
- Uses the 10 % figure to explain why food chains rarely contain more than four or five trophic levels.
- Recognises that 10 % is an approximation that varies between ecosystems and organisms.
- Not all ingested material is absorbed; some passes through the gut and is egested as faeces, so it is not available to the consumer.
- Some absorbed material is broken down in respiration, producing carbon dioxide and water that are lost as waste.
- Water and urea are lost in urine after absorbed material is processed, removing more biomass from the consumer.
- These losses reduce the biomass transferred to the next trophic level, which limits food chain length and pyramid shape.
- The losses can be grouped into unabsorbed material and absorbed material lost through respiration and excretion.
- Glucose is broken down in respiration to release energy for the organism.
- Aerobic respiration uses oxygen and releases carbon dioxide and water.
- Energy released is used for life processes such as movement, active transport, cell division and maintaining body temperature.
- Because much glucose is respired, less of the ingested or produced glucose becomes new biomass.
- This loss of glucose as carbon dioxide and water helps explain why biomass and energy decrease between trophic levels.
- Respiration occurs in all living cells, including producers, consumers and decomposers.
- Identify the biomass of the lower trophic level as the starting amount and the biomass of the higher trophic level as the transferred amount.
- Use the relationship: efficiency = (biomass transferred ÷ biomass available) × 100 for a percentage.
- Use the relationship: efficiency = biomass transferred ÷ biomass available for a fraction or decimal.
- Substitute values correctly and carry out the division before multiplying by 100.
- Interpret the result, for example 10% means one tenth of the biomass is passed on.
- Explain that efficiency is less than 100% because energy is lost in respiration, movement, excretion and heat, and some biomass is not eaten or digested.
- Biomass and energy are lost at each transfer, mainly through respiration, movement, excretion and heat.
- Less biomass and energy are available to organisms at higher trophic levels.
- Fewer organisms can be supported at higher trophic levels because there is less material and energy for growth and reproduction.
- Numbers of organisms usually decrease along a food chain, although body size can affect the exact numbers.
- Food chains are usually short because too little energy remains after several transfers to support another level.
- A pyramid of biomass or numbers can represent the decrease, with the producer level usually the largest.
Examiner Tips
- 💡Label each bar with the trophic level and the organism, and state the unit of biomass on the axis.
- 💡When explaining losses, work through the sequence ingestion, absorption, egestion, respiration and excretion so no stage is omitted.
- 💡Use the 10 % figure as a quantitative anchor when asked why a food chain has few trophic levels.
- 💡Quote the 1 % value only when the question asks about light energy captured by producers.
- 💡Use the word incident when describing light falling on a leaf, and distinguish it from light absorbed by chlorophyll.
- 💡Link the low percentage to named fates of light such as reflection, transmission and absorption by non-photosynthetic tissue.
- 💡Show each multiplication step when calculating biomass at successive trophic levels.
- 💡Use the phrase approximately 10 % to signal that the value is not exact.
- 💡When explaining food chain length, refer both to the small proportion transferred and to losses by respiration and egestion.
- 💡Separate your answer into two parts: losses before absorption and losses after absorption, so both bullet points are covered.
- 💡Use the correct terms egestion and excretion, and name the waste products carbon dioxide, water and urea.
- 💡Link the losses to the idea that less biomass is available at each trophic level, which explains why food chains are short.
- 💡Link the idea of glucose use in respiration to the decrease in biomass along a food chain.
- 💡Use the phrase releases energy from glucose rather than makes energy.
- 💡When calculating biomass transfer, remember that respired glucose leaves the organism as carbon dioxide and water, so it is not available to the next trophic level.
- 💡Write the equation before substituting numbers so the examiner can see your method.
- 💡Check whether the question asks for a percentage or a fraction and give the answer in that form.
- 💡Show the division and any conversion clearly, and include the correct unit or symbol, such as %.
- 💡Link the idea of inefficient transfer to the limited number of trophic levels in a food chain.
- 💡Use the phrase less energy available at each trophic level rather than no energy.
- 💡If a question gives biomass values, calculate the transfer efficiency first, then use it to explain the effect on numbers.
Common Mistakes
- Drawing bars of equal width or using numbers of organisms instead of biomass; correction: make each bar's area proportional to the dry mass at that trophic level.
- Claiming that energy is destroyed or simply disappears between levels; correction: state that energy is transferred to the environment by respiration, heat and waste, and is not destroyed.
- Confusing biomass with the total mass of water-containing tissue; correction: use dry mass, since water content varies and is not useful for comparing trophic levels.
- Stating that producers absorb all the light that falls on them; correction: only about 1 % of incident energy is transferred into biomass.
- Writing that plants take in energy from the soil; correction: plants obtain light energy from the Sun and mineral ions, not energy, from the soil.
- Confusing the 1 % figure with the 10 % transfer between trophic levels; correction: 1 % refers to light energy captured by producers, while 10 % refers to biomass passed between consumer levels.
- Treating 10 % as an exact value in every ecosystem; correction: describe it as approximate and note that it varies.
- Calculating 10 % of the energy at the previous level but labelling the answer as energy at the current level without units; correction: keep the unit consistent, such as kJ m⁻² year⁻¹ or g m⁻².
- Assuming the missing 90 % is destroyed; correction: it is transferred to the environment or remains in uneaten and egested material.
- Saying that all eaten food is absorbed; the correction is that some is egested as faeces and never enters the body's cells.
- Confusing egestion with excretion; the correction is that egestion removes unabsorbed food as faeces, while excretion removes waste made by the body, such as urea and carbon dioxide.
- Thinking respiration destroys biomass completely; the correction is that respiration transfers energy and releases carbon dioxide and water as waste products.
- Saying glucose is used to make energy; correct this by stating respiration releases energy from glucose, and energy is not made.
- Thinking plants do not respire; correct this by stating plants respire continuously as well as photosynthesising in light.
- Assuming all glucose eaten becomes biomass; correct this by stating a large amount is respired, so only some becomes new biomass.
- Dividing the lower trophic level biomass by the higher trophic level biomass; correct this by dividing the biomass transferred by the biomass available.
- Forgetting to multiply by 100 when a percentage is required; correct this by converting the decimal to a percentage.
- Mixing units, such as dividing grams by kilograms; correct this by converting all masses to the same unit before calculating.
- Saying energy is destroyed; correct this by stating energy is transferred to the surroundings as heat and is not destroyed.
- Assuming numbers always decrease in exactly the same way; correct this by stating biomass and energy decrease, but numbers also depend on organism size.
- Ignoring the loss of biomass in excretion and uneaten parts; correct this by including respiration, movement, excretion and heat as reasons for loss.