Trophic Levels in an Ecosystem » Transfer of Biomass
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.7.4.3 (Biology only)
- Why pyramids of biomass get narrower towards the top
- How little energy from light producers capture, and how little biomass is passed on
- How biomass is lost between trophic levels
- How to calculate the efficiency of a biomass transfer
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Unlock This CourseDescribing a pyramid of biomass
A pyramid of biomass shows the biomass at each trophic level of a food chain. Producers are at the bottom and the top consumer is at the top. When you describe one, say which level has the most biomass and how the amount changes as you go up.
In nearly every food chain, the biomass gets smaller at each step up. The bottom bar is the longest and the top bar is the shortest. The rest of this lesson explains why.
How to describe a pyramid
“The producers have the most biomass. The biomass decreases at each trophic level, so the top consumer has the least biomass.”
The start of the chain: light energy
Producers are mostly plants and algae. They use light energy for photosynthesis. But they do not capture very much of it.
- Producers transfer only about 1% of the incident energy from light for photosynthesis.
- “Incident” simply means the light that lands on the plant.
- The other 99% is not used to make biomass.
So even the producers start with only a tiny share of the Sun's energy locked up as biomass.
Only about 10% is passed on
Between each trophic level, a lot more biomass is lost. Only approximately 10% of the biomass from each trophic level is transferred to the level above it. About 90% is lost on the way.
The 10% rule in action
Imagine 1000 g of grass biomass. About 100 g of it ends up as biomass in the animals that eat it. If a predator eats those animals, only about 10 g of biomass ends up in the predator.
Where does the biomass go?
An animal does not turn everything it eats into new body mass. Biomass is lost in three main ways.
💩 Egestion
Not all the ingested material is absorbed. Some of it is egested as faeces.
💧 Urine
Some absorbed material is lost as waste: water and urea in urine.
🍃 Respiration
Some absorbed material is lost as carbon dioxide and water in respiration.
Large amounts of glucose are used in respiration. Glucose that is used in respiration is not built into new body tissue, so it is no longer biomass for the next level to eat.
Key terms:
- Egested: passed out of the body as faeces without being absorbed.
- Efficiency of biomass transfer: the percentage of the biomass at one trophic level that ends up in the next level.
Calculating efficiency
Efficiency tells you how much biomass is passed on, compared with how much was available. Use this equation:
Equation
efficiency (%) = (biomass transferred to the next level ÷ biomass available at the first level) × 100
You can also give the answer as a fraction of the mass. For example, 100 g passed on from 1000 g is 100/1000, which is 1/10, or 10%.
Worked example
A meadow has 2000 kg of grass. Rabbits eating it have a biomass of 240 kg. Foxes eating the rabbits have a biomass of 18 kg.
Grass to rabbits: 240 ÷ 2000 = 0.12, and 0.12 × 100 = 12%.
Rabbits to foxes: 18 ÷ 240 = 0.075, and 0.075 × 100 = 7.5%.
Notice both answers are close to the 10% in the spec. Real food chains give values close to, but not exactly, 10%.
Another way round
If you are told the efficiency, you can find the biomass passed on. A level has 800 g of biomass and the efficiency is 10%. The next level gets 10% of 800 g = 0.10 × 800 = 80 g.
How this affects the number of organisms
Each trophic level gets only about 10% of the biomass of the level below. So there is less biomass to support the organisms at each step up. This means:
- there are usually fewer organisms at each higher trophic level;
- the organisms at the top have the least biomass available to them.
This is why you see many more plants than herbivores, and many more herbivores than carnivores, in a habitat.
Common mistakes
Saying the biomass “disappears” or “is used up”. Say where it goes: egested as faeces, lost as carbon dioxide and water in respiration, or lost as water and urea in urine. Another mistake is dividing the wrong way round in the efficiency calculation. The biomass available at the lower level always goes on the bottom.
Exam-style question
A food chain in a garden is: lettuce → snail → thrush. The lettuce has a biomass of 500 g. The snails that eat it have a biomass of 45 g.
(a) Calculate the efficiency of biomass transfer from the lettuce to the snails. [2 marks]
(b) Give two ways that biomass is lost between the lettuce and the snails. [2 marks]
(c) Explain why there are fewer thrushes than snails in the garden. [2 marks]
Model answer
(a) 45 ÷ 500 (1) × 100 = 9% (1)
(b) Any two from: some material is not absorbed and is egested as faeces (1); carbon dioxide and water are lost in respiration (1); water and urea are lost in urine (1).
(c) Only about 10% of the biomass is passed on to each higher level (1), so there is less biomass to support organisms at the thrush level, so fewer thrushes (1).
Exam tip
In the calculation, always write the sum before the answer, such as 45 ÷ 500 × 100. If you slip on the arithmetic, you can still earn a mark for the correct method.