How energy from the Sun flows through food chains, why energy is lost at each trophic level, pyramids of numbers and biomass, and calculating the efficiency of energy transfer between trophic levels.

What this dot point is asking

CCEA wants you to explain that energy enters ecosystems from the Sun and flows through food chains, explain why energy is lost at each trophic level, interpret pyramids of numbers and biomass, and calculate the efficiency of energy transfer.

Energy enters from the Sun

Why energy is lost at each level

A trophic level is a feeding level in a food chain. Energy is lost between levels because:

• Organisms respire, releasing energy as heat to the surroundings.
• Mammals and birds use a lot of energy to keep warm.
• Energy is used for movement and life processes.
• Not all of an organism is eaten or digested, so energy is lost in uneaten parts, faeces and urine.

Only the energy stored in the body is passed on, so each level has less energy than the one before.

Pyramids of numbers and biomass

Calculating energy transfer

Examples in context

Example 1. Why food chains are short

Because only about 10 percent of energy passes to each level, there is not enough energy left after four or five links to support another level. By the fifth link almost all the original solar energy has been lost as heat. This is why you rarely see food chains longer than grass to insect to bird to hawk, and it is a favourite CCEA explanation question about energy loss.

Example 2. Why eating plants feeds more people

Growing crops to feed people directly is more efficient than feeding the crops to animals and then eating the animals, because energy is lost at the extra trophic level. A field of wheat can feed far more people than the same field used to raise cattle, because the cattle respire, move and keep warm, losing most of the energy. This applied idea, that shorter food chains waste less energy, is a common exam point about feeding a growing population.

Example 3. Reading a pyramid of biomass

A pyramid of biomass for grass, rabbits and foxes is widest at the bottom (grass) and narrows at each level up to the foxes at the top. The shape reflects the energy loss between levels: because only about 10 percent of energy passes on, each level can only support a smaller mass of living material than the one below. This is why there is a huge mass of grass but only a few foxes. A pyramid of numbers can look different (one oak tree supporting thousands of insects gives a strange shape), but a pyramid of biomass almost always narrows steadily towards the top, which is exactly what the energy-loss rule predicts.

Try this

Q1. Where does the energy in a food chain originally come from? [1 mark]

• Cue. The Sun (captured by producers in photosynthesis).

Q2. A level has 5000 kJ; the next level gains 500 kJ. Calculate the percentage transferred. [2 marks]

• Cue. 500 divided by 5000, times 100, which is 10 percent.