Pyramids of number, biomass and energy

Ecology and the environment · Feeding relationships · note 3 of 4

Spec 4.7Download all Feeding relationships notes (PDF)Download all notes (PDF)7 pages

Pyramids of number, biomass and energySpec 4.7

Pyramids use bars to compare the trophic levels in a food chain, with trophic level 1 at the bottom. There are three types.

Types of ecological pyramid
Pyramid	What each bar shows	Shape
Pyramid of number	The number of organisms at each trophic level	Often a pyramid, but can be inverted
Pyramid of biomass	The mass of living material at each trophic level	Almost always narrows at each level
Pyramid of energy transfer	The amount of energy transferred to each trophic level, for example in kJ per m² per year	Always narrows at each level

Pyramids of number

A pyramid of number counts the organisms but ignores their size. Where one large producer supports many small consumers, such as one oak tree supporting thousands of caterpillars, the bottom bar is narrower than the bar above it, so the pyramid is not pyramid-shaped. It is partly or fully inverted. Parasites can do the same: one fox may carry hundreds of fleas. A pyramid of biomass for the same chain is pyramid-shaped, because the oak tree has a very large mass.

For the food chain oak tree, caterpillars and blue tits, the pyramid of number is partly inverted, with a narrow bar for one oak tree under a wide bar for many caterpillars, while the pyramid of biomass narrows upwards from a wide oak tree bar. — The same food chain: the pyramid of number is inverted, but the pyramid of biomass is pyramid-shaped.

Pyramids of biomass

Each bar shows the mass of living material at that trophic level, and it almost always gets narrower at each higher level. To draw one, choose a scale so the length of each bar is proportional to its biomass, put trophic level 1 at the bottom and centre the bars on a vertical line.

Drawing a pyramid to scale

A food chain has biomass of 400 g per m² of grass, 40 g per m² of grasshoppers and 4 g per m² of frogs. Describe how to construct the pyramid using a scale of 1 mm = 4 g.

Grass: 400 ÷ 4 = a bar 100 mm long, at the bottom.
Grasshoppers: 40 ÷ 4 = a bar 10 mm long, above the grass.
Frogs: 4 ÷ 4 = a bar 1 mm long, at the top.

Answer: Three centred bars of 100 mm, 10 mm and 1 mm, with grass at the bottom and frogs at the top.

Pyramid of biomass drawn to scale from the worked example: a wide bar for grass (400 g per m²) at level 1, a much narrower bar for grasshoppers (40 g per m²) at level 2 and a very thin bar for frogs (4 g per m²) at level 3, with a scale bar showing 100 g per m². — The pyramid from the worked example, drawn to scale. Each bar is about 10% of the one below it.

Pyramids of energy transfer

A pyramid of energy transfer shows how much energy is transferred to each trophic level in a given time. It is always pyramid-shaped, because energy is lost at every step, so each level receives less than the one below.

Exam tip:

Pyramids of number are often inverted. Pyramids of energy transfer never are. If a question shows a pyramid that is not pyramid-shaped, it is most likely a pyramid of number.