IGCSE Biology 0610: Energy Flow and the Pyramid of Energy in a Savanna Food Chain

In an area of savanna grassland, ecologists measured the energy stored in the tissues of the organisms in one food chain, in kilojoules per square metre per year (kJ/m²/year):

Elephant grass (producer): 250 000
Grasshoppers that eat the grass (primary consumers): 20 000
Fork-tailed drongos that eat the grasshoppers (secondary consumers): 1600
Pale chanting goshawks that eat the drongos (tertiary consumers): 80

(a) Describe the shape of a pyramid of energy drawn using these four values, and state whether a pyramid of energy can ever be the opposite shape (wider at a higher trophic level than at the level below it). [2]

(b) Calculate the percentage of the energy in the elephant grass that is transferred to the grasshoppers. Show your working. [2]

(c) Explain, in terms of energy losses, why less than all of the energy stored in the grasshoppers is transferred to the drongos that eat them. [3]

(d) Explain, using the energy values given, why this food chain is unlikely to support a fifth trophic level, a predator that eats the goshawks. [2]

Part (a): The shape of a pyramid of energy

Plotting the four values as bars, one above the other, gives a pyramid that steadily narrows from the elephant grass (250 000) at the base, through the grasshoppers (20 000) and drongos (1600), up to the goshawks (80) at the top.

A pyramid of energy can never be the opposite shape, with a higher level wider than the level below it. This is because energy is always lost at every transfer between trophic levels (in respiration, as heat, and in uneaten or undigested material), so the amount of energy available must always decrease going up the food chain, unlike a pyramid of numbers, which can sometimes be distorted by the size of individual organisms.

Part (b): Percentage energy transfer

The percentage of energy transferred is found by dividing the energy received by the energy available, then multiplying by 100:

20 000 ÷ 250 000 × 100 = 8%

So 8% of the energy in the elephant grass is transferred to the grasshoppers.

Part (c): Why energy transfer is not 100% efficient

Several factors mean that not all of the energy in the grasshoppers ends up in the drongos:

Not every grasshopper is caught and eaten by a drongo.
Some parts of the grasshoppers that are eaten (such as the hard exoskeleton) are not digested and pass out of the drongo as faeces, so their energy is not absorbed.
Of the energy that is absorbed, much is used in respiration to release energy for the drongo’s movement, growth and other life processes, and this energy is ultimately lost to the surroundings as heat.

Only the energy that remains after all of these losses is built into new drongo tissue, which is the only energy available to be passed on to the next trophic level.

Part (d): Why there is no fifth trophic level

Each transfer between trophic levels loses a large proportion of the available energy (only around 8% of the grass’s energy reaches the grasshoppers, and even less reaches the drongos and goshawks). By the time energy reaches the goshawks, only 80 kJ/m²/year remains. This is such a small amount of energy that it could not support a viable population of a further predator feeding on the goshawks, which is why food chains such as this one usually have no more than four trophic levels.

Final answers

(a) A regular, upright pyramid; a pyramid of energy can never be inverted, because energy is always lost at each transfer.
(b) 8%
(c) Energy is lost because not all grasshoppers are eaten, some eaten material is undigested and egested, and much absorbed energy is used in respiration and lost as heat.
(d) Only 80 kJ/m²/year remains at the goshawk level after repeated energy losses, which is too little to support a further trophic level.