GCSE Biology revision

Transfer of biomass

Trophic levels in an ecosystem (biology only)

AQA 4.1.1.1

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AQA student objectives

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ALL 🎯

Identify and describe What Is Biomass?, Trophic Levels Explained and Biomass Transfer Between Trophic Levels in the context of transfer of biomass.

MOST 🎯🎯

Explain the relationships between What Is Biomass?, Trophic Levels Explained and Biomass Transfer Between Trophic Levels and how they contribute to transfer of biomass.

SOME 🎯🎯🎯

Apply and analyse knowledge of What Is Biomass?, Trophic Levels Explained and Biomass Transfer Between Trophic Levels to interpret unfamiliar information about transfer of biomass.

Revision summary

Key knowledge

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What Is Biomass?

Biomass is the amount of dried matter contained in all of the organisms at a particular trophic level.
Producers (plants) transfer an average of 1% of the sunlight incident on their surfaces into biomass by using solar energy to synthesise glucose.
A pyramid of biomass represents the relative amount of biomass at each trophic level in a food chain, with the largest bar always at the base.

1. What Is Biomass?AQA

Notes

Biomass is the amount of dried matter contained in all of the organisms at a particular trophic level.
Producers (plants) transfer an average of 1% of the sunlight incident on their surfaces into biomass by using solar energy to synthesise glucose.
A pyramid of biomass represents the relative amount of biomass at each trophic level in a food chain, with the largest bar always at the base.

Trophic Levels Explained

Trophic levels describe the position an organism occupies in a food chain, starting with producers at the first trophic level.
Producers form the base of the pyramid, primary consumers the second level, secondary consumers the third, and so on up the chain.
Each bar in a pyramid of biomass corresponds to one trophic level, so a four-organism food chain produces a pyramid with four bars.

2. Trophic Levels ExplainedAQA

Notes

Trophic levels describe the position an organism occupies in a food chain, starting with producers at the first trophic level.
Producers form the base of the pyramid, primary consumers the second level, secondary consumers the third, and so on up the chain.
Each bar in a pyramid of biomass corresponds to one trophic level, so a four-organism food chain produces a pyramid with four bars.

Biomass Transfer Between Trophic Levels

Approximately 10% of the biomass at one trophic level is passed on to the next trophic level.
For example, if producers have a biomass of 100 g, primary consumers gain around 10 g, and secondary consumers gain only around 1 g.
The remaining ~90% of biomass is lost at each transfer and does not become part of the next organism's body.

3. Biomass Transfer Between Trophic LevelsAQA

Notes

Approximately 10% of the biomass at one trophic level is passed on to the next trophic level.
For example, if producers have a biomass of 100 g, primary consumers gain around 10 g, and secondary consumers gain only around 1 g.
The remaining ~90% of biomass is lost at each transfer and does not become part of the next organism's body.

How Is Biomass Lost? — Egestion

Egestion is the expulsion of undigested material from the body as faeces, which removes biomass from the food chain.
Some parts of an organism, such as plant roots or shells, may not even be eaten, so they never become biomass at the next trophic level.

How Is Biomass Lost? — Respiration

Respiration is the breakdown of glucose using oxygen to produce water and carbon dioxide, which are removed from the body, so that glucose does not become part of the organism's biomass.
The rate of respiration increases when an animal must chase or fight prey, meaning more biomass is lost as energy in these situations.

5. How Is Biomass Lost? — RespirationAQA

Notes

Respiration is the breakdown of glucose using oxygen to produce water and carbon dioxide, which are removed from the body, so that glucose does not become part of the organism's biomass.
The rate of respiration increases when an animal must chase or fight prey, meaning more biomass is lost as energy in these situations.

How Is Biomass Lost? — Excretion

Excretion is the removal of metabolic waste products, including urine, from the body, meaning those substances are lost and do not become biomass at the next trophic level.
For molecules to become part of the next trophic level's biomass, they must be digested and used within the organism's cells to build or enlarge structures.

6. How Is Biomass Lost? — ExcretionAQA

Notes

Excretion is the removal of metabolic waste products, including urine, from the body, meaning those substances are lost and do not become biomass at the next trophic level.
For molecules to become part of the next trophic level's biomass, they must be digested and used within the organism's cells to build or enlarge structures.

Calculating the Efficiency of Biomass Transfer

The efficiency of biomass transfer is calculated using the formula: Efficiency (%) = (biomass transferred / biomass available) × 100.
For example, if 150 kg is transferred from grass (400 kg) to rabbits, the efficiency is (150 / 400) × 100 = 37.5%.
The efficiency of biomass transfer between rabbits (150 kg) and foxes (15 kg) is (15 / 150) × 100 = 10%.
As you move up the pyramid, the efficiency of biomass transfer tends to decrease with each successive trophic level.

7. Calculating the Efficiency of Biomass TransferAQA

Notes

The efficiency of biomass transfer is calculated using the formula: Efficiency (%) = (biomass transferred / biomass available) × 100.
For example, if 150 kg is transferred from grass (400 kg) to rabbits, the efficiency is (150 / 400) × 100 = 37.5%.
The efficiency of biomass transfer between rabbits (150 kg) and foxes (15 kg) is (15 / 150) × 100 = 10%.
As you move up the pyramid, the efficiency of biomass transfer tends to decrease with each successive trophic level.

Why Food Chains Are Usually Short

Because so much biomass is lost at each trophic level, there are usually no more than three or four trophic levels in a food chain.
At the top of a very long food chain, a predator would expend more energy catching and killing the top carnivore than it would gain from eating it, making additional trophic levels unsustainable.

8. Why Food Chains Are Usually ShortAQA

Notes

Because so much biomass is lost at each trophic level, there are usually no more than three or four trophic levels in a food chain.
At the top of a very long food chain, a predator would expend more energy catching and killing the top carnivore than it would gain from eating it, making additional trophic levels unsustainable.

Constructing a Pyramid of Biomass

To draw a pyramid of biomass, choose a suitable scale so that the width of each bar accurately represents the biomass at that trophic level.
All bars should be centred on the same vertical axis, with the widest bar at the bottom representing the producers and each successive bar narrowing as biomass decreases.
For example, using a scale of 1 cm = 25 kg, 500 kg of phytoplankton would be drawn as a 20 cm bar, 250 kg of krill as 10 cm, 75 kg of penguins as 3 cm, and 25 kg of seals as 1 cm.

9. Constructing a Pyramid of BiomassAQA

Notes

To draw a pyramid of biomass, choose a suitable scale so that the width of each bar accurately represents the biomass at that trophic level.
All bars should be centred on the same vertical axis, with the widest bar at the bottom representing the producers and each successive bar narrowing as biomass decreases.
For example, using a scale of 1 cm = 25 kg, 500 kg of phytoplankton would be drawn as a 20 cm bar, 250 kg of krill as 10 cm, 75 kg of penguins as 3 cm, and 25 kg of seals as 1 cm.

Largest Percentage Loss in a Food Chain

The largest percentage loss of biomass often occurs at the first transfer, for example from an oak tree to a caterpillar, because the caterpillar only eats the leaves and not the trunk or roots.
Significant biomass can also be lost between higher trophic levels, such as a blue tit and a hawk, because the hawk must chase the blue tit, increasing its respiration rate and using up glucose.

10. Largest Percentage Loss in a Food ChainAQA

Notes

The largest percentage loss of biomass often occurs at the first transfer, for example from an oak tree to a caterpillar, because the caterpillar only eats the leaves and not the trunk or roots.
Significant biomass can also be lost between higher trophic levels, such as a blue tit and a hawk, because the hawk must chase the blue tit, increasing its respiration rate and using up glucose.