What you'll learn
Energy and biomass are passed along food chains, but a great deal is lost at every step — which is why food chains are short and why some ways of producing food are more efficient than others. For AQA GCSE Biology you need to understand what biomass is, how it is transferred between trophic levels, why so much is lost, how to calculate the efficiency of transfer, and how food production can be made more efficient. This guide covers biomass and trophic levels, the losses between levels, the efficiency calculation, and the methods and ethics of efficient food production. By the end you should be able to explain why biomass decreases up a food chain and evaluate ways of producing food efficiently.
Key terms and definitions
Biomass — The mass of living material, often measured as dry mass.
Trophic level — A stage in a food chain, such as producer or primary consumer.
Producer — An organism, usually a plant, that makes its own food by photosynthesis.
Consumer — An organism that eats other organisms.
Food chain — A sequence showing how energy and biomass pass from one organism to the next.
Efficiency of transfer — The percentage of biomass passed from one trophic level to the next.
Respiration — The process that releases energy from glucose, transferring some energy to the surroundings.
Food security — Having enough food to feed a population.
Core concepts
Biomass and trophic levels
Biomass is the mass of living material in an organism. In a food chain, organisms are arranged into trophic levels: producers (plants) at the first level make biomass by photosynthesis, primary consumers (herbivores) eat the producers, secondary consumers eat the primary consumers, and so on. Biomass and the energy it contains are passed from one trophic level to the next when one organism is eaten by another.
Why biomass decreases up a food chain
At each trophic level, only a small proportion of the biomass is passed on to the next level. This is because a lot of biomass and energy is lost at each stage. As a result, the biomass decreases as you go up the food chain, which is why food chains rarely have more than four or five levels — there is not enough biomass to support higher levels.
How biomass and energy are lost
Biomass and energy are lost between trophic levels in several ways:
- Not all of an organism is eaten — for example, roots, bones and other parts may be left uneaten.
- Not all of what is eaten is absorbed — some material passes out as waste (faeces).
- Energy is used in respiration — organisms respire to release energy for movement and other life processes, and this energy is eventually transferred to the surroundings, often as heat.
- Energy is lost in excretion, for example in urine.
Because of these losses, only about 10% of the biomass is typically transferred from one trophic level to the next.
Calculating the efficiency of biomass transfer
The efficiency of transfer between trophic levels can be calculated using:
efficiency = (biomass transferred to the next level ÷ biomass available at the previous level) × 100
This gives the percentage of biomass passed on. Because so much is lost, this figure is usually low. You may be given the biomass at each level and asked to work out the percentage transferred.
Improving the efficiency of food production
Because so much biomass is lost at each trophic level, food production can be made more efficient by reducing the energy losses in the animals we farm. Two common methods are:
- Restricting movement — keeping animals in smaller spaces so they use less energy moving around, meaning more of their food becomes biomass.
- Controlling temperature — keeping animals warm so they lose less energy keeping their body temperature up, so less food is used in respiration.
These methods mean more of the food eaten by the animal is turned into biomass that we can eat, making the production more efficient. Eating plants directly (rather than feeding them to animals first) is also more efficient, because it avoids the losses at an extra trophic level.
The ethics and drawbacks of efficient food production
While these methods increase efficiency, they raise ethical concerns. Restricting movement and keeping animals indoors in crowded conditions can reduce animal welfare, and intensive farming can increase the risk of disease, sometimes leading to the use of antibiotics. There are also environmental costs. So questions on this topic often ask you to weigh the benefits of efficient, cheaper food production against the welfare and environmental drawbacks.
Where the producers' biomass comes from
The biomass at the start of a food chain is made by the producers (plants) through photosynthesis. Plants absorb only a small percentage of the light energy that reaches them, and use some of the glucose they make in their own respiration, so even at the first level not all the available energy becomes biomass. This is why the total amount of biomass that can be built is limited from the very start, and it explains why increasing the number of producers (for example, growing more crops) is the base on which all food production depends. Understanding that biomass ultimately traces back to photosynthesis links this topic to the wider study of energy in ecosystems.
Food security and feeding a growing population
Because so much biomass is lost between trophic levels, how we produce food has a big effect on food security — having enough food to feed a population. Producing meat is less efficient than growing crops, because feeding plants to animals loses about 90% of the biomass at that extra step. As the world's population grows, this raises important questions about how to use land and resources efficiently. Some argue that eating more plant-based food and less meat would allow the same land to feed more people. Others point to factors such as farming difficult land where only animals can be raised. Being able to discuss food security in terms of biomass transfer shows strong understanding.
Worked examples
Example 1: Why food chains are short
Explain why food chains rarely have more than five trophic levels. At each level, most biomass and energy is lost through uneaten parts, waste, respiration and excretion, so only about 10% passes to the next level. After several levels there is too little biomass left to support another level, so food chains are short.
Example 2: Calculating efficiency
A trophic level contains 20,000 kJ of biomass, and 2,000 kJ is transferred to the next level. Calculate the efficiency of transfer. Efficiency = (2,000 ÷ 20,000) × 100 = 10%.
Example 3: Explaining energy loss
Give two ways energy is lost between trophic levels. Energy is lost through respiration, which transfers energy to the surroundings, and through waste materials such as faeces and urine, which contain biomass that is not passed on. Uneaten parts of organisms also mean biomass is not transferred.
Example 4: Efficient food production
Explain how keeping animals in warm, enclosed conditions makes food production more efficient. Restricting movement means the animals use less energy moving, and keeping them warm means they lose less energy maintaining body temperature. So more of the food they eat becomes biomass, increasing the proportion of food energy converted into meat we can eat.
Common mistakes and how to avoid them
A common error is saying biomass "disappears". It is not destroyed — it is transferred elsewhere: to the surroundings by respiration, or into waste and uneaten material. Explain where it goes.
Students often forget respiration as a cause of energy loss. Respiration releases energy that is used for life processes and eventually transferred to the surroundings, and it is a major reason so little biomass is passed on.
Another mistake is confusing the direction of the efficiency calculation. Efficiency = biomass transferred ÷ biomass at the previous level × 100. Make sure the transferred amount is on top.
When discussing efficient food production, do not give only the benefits. A good answer also considers animal welfare and other drawbacks, because "evaluate" questions expect both sides.
Finally, remember that eating plants directly is more efficient than eating animals, because each extra trophic level loses about 90% of the biomass.
Exam technique for "Biomass transfer and efficiency of food production"
For "explain" questions, list the ways biomass and energy are lost — uneaten parts, waste, respiration, excretion — and link them to why biomass decreases up the chain and why chains are short.
Be ready to calculate the percentage efficiency using biomass values, showing your working. Remember the typical figure is around 10%.
For food production questions, describe methods that reduce energy loss (restricting movement, controlling temperature) and explain why they work. For "evaluate" questions, weigh the greater efficiency and cheaper food against the animal welfare and environmental concerns. Use precise terms — biomass, trophic level, respiration, efficiency — throughout.
Quick revision summary
- Biomass is the mass of living material; it is passed between trophic levels in a food chain.
- Only about 10% of biomass is transferred to the next level; the rest is lost.
- Losses occur through uneaten parts, waste (faeces and urine), respiration (energy to the surroundings), so biomass decreases up the chain and chains are short.
- Efficiency = (biomass transferred ÷ biomass at previous level) × 100.
- Food production is made more efficient by restricting movement and controlling temperature, so more food becomes biomass; eating plants directly avoids losses at an extra level.
- Efficient methods raise animal welfare and environmental concerns, so evaluate both sides.