Energy transfer between trophic levelsAQA A-Level Biology: Revision notes

Section 1

Why energy transfer is not 100 % efficient

The biological molecules made by photosynthesis are consumed by other organisms, including animals, bacteria and fungi. Some are used as respiratory substrates, releasing energy, and some form new biomass.

The transfer of biomass and stored chemical energy from one organism to a consumer is not 100 % efficient. Reasons include:

not all of an organism is eaten (roots, bones, wood)
not all of what is eaten is digested and absorbed; some is lost as faeces and urine
much of the energy absorbed is released in respiration, and lost to the environment as heat

Key termstrophic levelrespiratory substrateefficiency

Section 2

Net production of consumers

The net production of a consumer is the chemical energy stored in its new biomass.

$N = I - (F + R)$

I: chemical energy store in the ingested food
F: chemical energy lost to the environment in faeces and urine
R: energy lost in respiration

Worked example. A cow ingests 250 MJ, loses 90 MJ in F and 110 MJ in R. $N = 250 - (90 + 110) = 50$ MJ, which is 20 % of I.

Key termsnet productioningested energyfaeces and urine

Common mistake

Forgetting that N is what is left after both F and R. Subtracting only R gives the energy absorbed, not the net production.

Section 3

Calculating efficiency

The efficiency of energy transfer between trophic levels is the net production of one level as a percentage of the net production (or energy available) at the level below.

$\text{efficiency} = \frac{\text{net production at the higher level}}{\text{net production at the lower level}} \times 100$

Worked example. Phytoplankton 20 000, zooplankton 2 400, fish 360 kJ m⁻² year⁻¹. Phytoplankton to zooplankton: 2 400 ÷ 20 000 × 100 = 12 %. Zooplankton to fish: 360 ÷ 2 400 × 100 = 15 %.

Key termsefficiency of transfernet productivity

Exam tip

Put the higher trophic level on top of the fraction, and always give the unit (%) and the working.

Section 4

Where the energy goes

Energy that does not pass on to the next trophic level is not 'destroyed'. It is lost to the environment as heat from respiration, or remains in faeces, urine and dead material, where it is used by bacteria and fungi (decomposers). Their respiration of this material also releases heat, so all the energy eventually leaves the ecosystem.

The pattern is that only a small percentage passes between trophic levels, so food chains are short and there is less energy and biomass at higher levels.

Key termsdecomposersheat loss

Section 5

Comparing consumers

Net production as a percentage of intake varies between consumers. Endotherms, such as mammals and birds, use much of their energy in respiration to release heat and maintain a constant body temperature, so they convert a small percentage of what they eat to biomass. Ectotherms, such as insects, lose less in respiration and convert a larger percentage.

When evaluating data, comment on the sample: figures for one species in one year cannot be generalised to a whole group.

Key termsendothermectotherm

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Energy transfer between trophic levels

A dairy cow grazes on a pasture. During one day, the chemical energy store in the grass she ingests is 250 MJ. She loses 90 MJ in faeces and urine and releases 110 MJ in respiration.
Calculate the net production of the cow in MJ per day and express it as a percentage of the energy she ingests.2 marks
An ecologist studies a food chain in an estuary. The net primary production of the phytoplankton is 20 000 kJ m⁻² year⁻¹. The net production of the zooplankton that feed on them is 2 400 kJ m⁻² year⁻¹, and the net production of the small fish that feed on the zooplankton is 360 kJ m⁻² year⁻¹.
Calculate the efficiency of energy transfer from the zooplankton to the small fish.2 marks
In an oak woodland the net primary production of the trees is 14 000 kJ m⁻² year⁻¹. Herbivorous insects consume 1 400 kJ m⁻² year⁻¹ of this. Most of the rest falls to the ground as leaf litter and dead wood, and a small amount is stored as new wood in the living trees.
Calculate the percentage of the trees' net primary production that is consumed by herbivorous insects, and suggest two reasons why most of the net primary production is not consumed by them.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).