Energy Transfers

Not all of the energy available at each stage of a food chain is transferred to the next trophic level
- Only around 10% of the chemical energy consumed at each trophic level is passed on to the next
It can be said that energy is lost from the food chain at each stage

Energy transfer in food chains diagram

energy-losses

Not all of the energy at each trophic level of a food chain is passed on to the next level

Energy is lost from food chains because only the energy stored in biomass will be passed to the next trophic level; energy used for other processes, or transferred to the environment, is not available to consumers
Energy may be used for:
- Repairing damaged cells
- Cell maintenance and growth
Energy may be transferred to the environment in the form of:
- Heat generated during cellular respiration
- Waste materials, e.g. faeces and urine
Energy losses from food chains explain why food chains are usually relatively short; the energy available at higher trophic levels eventually becomes too small

Energy losses from food chains diagram

Energy Losses between Trophic Levels

Only energy that is converted into new biomass will be passed on to consumers in a food chain; energy that is used in cellular processes or lost as heat or in waste will not be available to higher trophic levels

Note that you do not need to know the term 'net production'

Calculating the Efficiency of Energy Transfers

Higher Tier Only

Percentage calculations can be used to calculate the efficiency of energy transfer at each stage of a food chain:

T4ewiR2N_energy-transfer-efficiency-calculation

- Remember that when calculating a percentage you divide a part by the whole; here the 'part' is the energy remaining for the next level, while the 'whole' is the energy received from the previous level

Worked example

Figure 1 shows a food chain with three trophic levels.

Figure 1

food-chain-for-plant-disease-q-1

The energy stored in biomass at each trophic level is as follows:

Tomato plants: 50 000 kJ
Aphids: 4 500 kJ
Ladybirds: 570 kJ

Calculate the efficiency of energy transfer from the first to the second trophic level.

Step 1: identify the first and second trophic levels

First trophic level = tomato plants

Second trophic level = aphids

Step 2: identify the energy that is available to be passed on at the second trophic level

The stored energy in the aphids that is available to the next trophic level is 4 500 kJ

Step 3: identify the energy intake at the second trophic level from the first trophic level

The aphids will gain 50 000 kJ from the tomato

Step 4: enter numbers into the equation

$Energy efficiency = \frac{4 500}{50 000} x 100$

= 9 %

How does energy affect the number of organisms at each trophic level?

The low efficiency of energy transfer at each trophic level means that there is less energy available to support organisms at higher trophic levels of a food chain
This means that fewer organisms are present at higher trophic levels than at lower levels, i.e. there are more producers than top predators in a food chain
- There are some exceptions to this, e.g. a tree is a single individual but can support many consumers

Exam Tip

The challenges with calculating energy transfer efficiency lie in remembering the equation, and then working out which information corresponds to each part of the equation. The following may be helpful to bear in mind:

The energy efficiency equation is just a percentage calculation; you are dividing a smaller number by a larger number and then multiplying by 100
The energy stored in biomass at a particular trophic level is the energy available to the next level
The energy stored in biomass at a particular level will be the energy intake for the level above

Energy Transfers (WJEC GCSE Biology: Combined Science)

Revision Note