18.9 Biomass Transfers

A free GCSE Biology lesson for AQA (8461) from OBA School: how much biomass passes between trophic levels, where the rest goes, and calculating efficiency. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Recall that producers transfer about 1% of the light energy falling on them for photosynthesis
• Recall that only about 10% of the biomass at one trophic level is passed to the level above
• Explain the ways biomass is lost between trophic levels
• Calculate the efficiency of biomass transfers between trophic levels
• Explain how losses limit the length of food chains

Key words

• Ingest: Take food into the body by eating.
• Egest: Get rid of undigested food from the gut as faeces.
• Excrete: Remove waste made by the body’s cells, e.g. urea in urine and CO₂ from respiration.
• Urea: A waste substance made in the liver from excess amino acids and lost in urine.
• Efficiency of transfer: The percentage of the biomass at one trophic level that becomes biomass in the next level.

Only a small fraction is passed on

Producers transfer only about 1% of the light energy falling on them into new biomass by photosynthesis. Much of the light misses the leaves, is reflected, or is the wrong wavelength.

Between trophic levels, only about 10% of the biomass at one level ends up as biomass in the next level.

Where the biomass goes

• Not all eaten; Explanation Roots, bones, shells, wood and fur are often not eaten, and some organisms die without being eaten.
• Egested as faeces; Explanation Not all ingested material can be digested and absorbed, e.g. cellulose in plants. It leaves the gut as faeces.
• Lost as waste (excretion); Explanation Absorbed material is lost as CO₂ and water from respiration, and as water and urea in urine.
• Used in respiration; Explanation Large amounts of glucose are respired to release energy for movement and keeping warm, so they never become new body tissue.

Key fact: Mammals and birds keep their body temperature constant, so they respire a lot of glucose to stay warm. Their efficiency of transfer is lower than that of fish or insects.

Calculating efficiency

Equation: efficiency = (biomass transferred to the next level ÷ biomass available at the previous level) × 100

Worked example: efficiency between two levels

In a field: grass 3600 g/m², grasshoppers 290 g/m², shrews 26 g/m².
Grass → grasshoppers: 290 ÷ 3600 × 100 = 8.1% (2 s.f.)
Grasshoppers → shrews: 26 ÷ 290 × 100 = 9.0% (2 s.f.)

Answer: 8.1% and 9.0%

Worked example: what is left for growth?

A rabbit eats 600 g (dry mass) of grass. 390 g is egested, 180 g is used in respiration and 12 g is lost in urine.
Biomass for growth = 600 − 390 − 180 − 12 = 18 g
Efficiency = 18 ÷ 600 × 100 = 3%

Answer: 18 g of new tissue; 3% efficiency

Why food chains are short

Because so much biomass is lost at each step, there is not enough biomass at the top to support another level. Most food chains have no more than four or five trophic levels, and there are far fewer animals at the top than at the bottom.

Common mistakes

• Saying energy or biomass is “used up” or “destroyed”. Biomass is lost to the environment (as faeces, urine, CO₂ and water); energy is transferred, e.g. as heat.
• Dividing by the wrong level in an efficiency calculation. Divide by the biomass at the lower (earlier) level.
• Forgetting that respiration is the biggest loss for animals that move a lot and keep warm.
• Mixing up egestion (undigested food in faeces) with excretion (waste made by cells, e.g. urea, CO₂).

Revision questions

Try these, then check your answers below each one.

Question 1. About what percentage of the biomass at one trophic level is passed on to the next level? (Choose from: 50%, 90%, 10%, 1%.)

Answer: 10%

Working: Only about 10% of biomass is transferred to the level above.

Question 2. True or false? Producers transfer about 1% of the light energy falling on them for photosynthesis.

Answer: True

Working: True. About 1% of incident light energy is transferred to new biomass.

Question 3. The biomass of producers is 3200 g/m² and the biomass of primary consumers is 256 g/m². Calculate the efficiency of biomass transfer.

Answer: 8 %

Working: 256 ÷ 3200 × 100 = 8%.

Related lessons

• Previous lesson: 18.8 Trophic Levels and Biomass
• Next lesson: 18.10 Factors Affecting Food Security
• In this chapter: 18.1 The Human Population Explosion
• In this chapter: 18.2 Land and Water Pollution
• In this chapter: 18.3 Air Pollution

Continue in the full course

This lesson is section 18.9 (AGCBH1809) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.