Pyramids of biomass

Ecology · Trophic levels in an ecosystem · note 2 of 3

Spec 4.7.4.2Triple onlyDownload these notes (PDF)4 pages

Pyramids of biomassSpec 4.7.4.2

Pyramids of biomass can be constructed to represent the relative amount of biomass in each level of a food chain. Trophic level 1 is at the bottom of the pyramid.

Each bar is narrower than the one below it, because the biomass decreases at each higher trophic level. This gives the pyramid shape.

Draw horizontal bars, one for each trophic level, with trophic level 1 at the bottom.
Choose a scale so that the length of each bar is proportional to the biomass of that level.
Centre the bars on a vertical line and label each bar with the organism or level.

Constructing a pyramid

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.