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Pyramids of biomass

Definition

Biomass

The mass of living material, usually measured as dry mass, present in an organism or at a trophic level.

Definition

Pyramid of biomass

A diagram in which the width of each bar, drawn to scale, shows the biomass at each trophic level in a food chain.

  1. A pyramid of biomass shows the mass of living material at each trophic level in a food chain.
  2. Each bar is drawn to scale, so its width shows how much biomass is at that level.
  3. The producers form the wide bar at the bottom, and each level above is narrower.

A pyramid of biomass with a wide producer bar at the bottom and narrower bars for each consumer level above, drawn to scale.

Key Idea

Biomass gets smaller at each level up, so a pyramid of biomass is almost always widest at the bottom.

Drawing and reading a pyramid

  1. To draw one, put the producers at the bottom and stack each trophic level above in order.
  2. Make the width of each bar match the biomass at that level, using the same scale for every bar.
  3. To read one, compare the widths to see how the biomass falls from one level to the next.
Common Mistake
  • Do not draw the bars by the number of organisms.
  • A pyramid of biomass uses the mass of living material at each level, drawn to scale.

Why the pyramid narrows

  1. Biomass decreases at each level up, which is why the bars get narrower towards the top.
  2. The reasons for this loss of biomass are covered in the next article, on the transfer of biomass.
Self review
  • What does a pyramid of biomass show?
  • Why is each bar drawn to scale?
  • Which trophic level forms the widest bar, and why?
  • What happens to biomass as you move up the trophic levels?
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