By the end of this lesson
- Explain why only about 10% of energy is transferred between trophic levels.
- Calculate the efficiency of energy transfer.
- Explain the consequences for food chain length and food production.
BIG QUESTION
Why are there so few top predators?
Each transfer passes on only a small fraction of energy, which limits how many levels and individuals can be supported.
Where the energy goes
Not all of an organism is eaten: roots, bones, shells and fur may be left. Not all that is eaten is digested and absorbed; some is lost in faecesUndigested food and other material egested from the gut; the energy in faeces is not passed on to the next trophic level. (egestion). Some absorbed material is lost in excretion, such as urea.
Much of the energy that is assimilated is released by respirationA series of enzyme-controlled chemical reactions in cells that release energy from nutrient molecules, such as glucose; it is not the same as breathing. for movement and maintaining body temperature, and is lost to the surroundings as heat. Only the energy stored in new biomass is available to the next trophic level.
About 10% is a guide, not a constant
On average, around 10% of the energy at one level becomes biomass at the next. Actual values vary, often from about 1% to 20%.
Mammals and birds usually transfer a smaller percentage than reptiles or fish, because they use more energy maintaining a constant body temperature.
Calculating efficiency
efficiency of energy transferThe percentage of energy at one trophic level that passes to the next: energy transferred to the next level ÷ energy at the previous level × 100. = (energy transferred to the next level ÷ energy available at the previous level) × 100.
Illustrative example: grass receives 20 000 kJ m⁻² year⁻¹ in its biomass; rabbits gain 2 000 kJ m⁻² year⁻¹. Efficiency = 2 000 ÷ 20 000 × 100 = 10%.
Consequences
Because so little energy passes on, food chains rarely have more than four or five trophic levels, and there are fewer top predators.
Feeding people on crops directly supports more people per hectare than feeding the crops to livestock first, although land quality, nutrition and culture also matter.
Energy losses between trophic levels
Name the loss and explain it.
| Loss | Cause | Example |
|---|---|---|
| Not eaten | Parts of the organism are not consumed. | Roots, bones, wood. |
| Not digested | Some food is egested in faeces. | Cellulose in herbivore faeces. |
| Excretion | Waste products contain energy. | Urea in urine. |
| Respiration | Energy transferred to surroundings as heat. | Movement, maintaining body temperature. |
Language that earns precision
- energy transfer efficiency
- The percentage of energy at one trophic level that is transferred to the next.
- egestion
- Removal of undigested food as faeces.
- respiration
- Chemical reactions in cells that release energy from glucose.
Common misconceptions
Claim: Exactly 10% is transferred every time.
Correction: About 10% is an average; real efficiencies vary widely.
Claim: The lost energy is destroyed.
Correction: It is transferred to the surroundings, mainly as heat, or to decomposers.
Claim: Long food chains are inefficient because predators are lazy.
Correction: Energy is lost at every transfer, so less is available at each higher level.
Match the answer to the command word
Give at least two separate losses: not eaten, not digested (faeces), excretion and heat from respiration.
Divide the later level by the earlier level and multiply by 100. Give units where energy values are quoted.
Recall, apply, then justify
Explain why only about 10% of the energy in grass is transferred to rabbits.
Hint
Three different losses.
Suggested marking points
- Not all the grass is eaten (for example roots).
- Some is not digested and is lost in faeces.
- Energy is lost as heat from respiration (for movement or keeping warm).
Primary consumers receive 1500 kJ and secondary consumers receive 90 kJ (illustrative data). Calculate the efficiency of energy transfer.
Hint
Later ÷ earlier × 100.
Suggested marking points
- 90 ÷ 1500 × 100 shown.
- 6%.
Evaluate the claim that eating less meat would allow more people to be fed from the same area of farmland.
Planning hint
Use the 10% idea, then consider limitations.
What a strong design includes
- Energy is lost at each trophic level transfer.
- Feeding crops directly to humans removes one transfer.
- So more energy per hectare is available to people.
- Some land is unsuitable for crops but can be grazed.
- Animals can eat crop residues that humans cannot digest.
- Diets must still supply enough protein and micronutrients.
- So the claim is generally supported but depends on land type and diet.