Pyramids of Biomass & Biomass Transfers (OCR GCSE Biology A (Gateway))

Pyramids of Biomass

Pyramids

• There are two forms of 'food pyramids' to be aware of

  • Pyramids of number

  • Pyramids of biomass

Pyramids of number

• A pyramid of numbers shows how many organisms are at each trophic level of a food chain

• The width of the box indicates the number of organisms at that trophic level

• For example, consider the following food chain

Grass → Vole → Owl

• Is it likely that there would be more voles in an area than grass plants?

• How many voles might one barn owl need to eat per day?

• If it’s more than one, is it likely that there are more barn owls in an area than voles?

• So, a pyramid of numbers for this food chain would look like this:

A pyramid of numbers

• Despite the name (and the example above), a pyramid of numbers doesn’t always have to be pyramid-shaped, for example:

Pyramids of numbers are not always pyramid-shaped

• This is because the size of the organism is also important

  • One large organism, like the oak tree in the pyramid above, contains enough energy to support many smaller organisms (the insects)

• There are some rules to remember when drawing a pyramid of numbers:

  • You cannot change the trophic level of the organisms - they must stay in the same order as in the food chain with producers on the bottom, followed by primary consumers, then secondary consumers, then tertiary consumers

  • Generally, the larger an individual organism is, the fewer of them there are

Pyramids of biomass

• A pyramid of biomass shows how much mass the organisms at each level would have without including all the water content (their ‘dry mass’)

  • Biomass is a store of energy

• Pyramids of biomass are always pyramid-shaped, regardless of what the pyramid of numbers for that food chain looks like

• This is because the mass of organisms has to decrease as you go up a food chain – if we take our first food chain as an example, it would be impossible to have 10kg of grass feeding 50kg of voles feeding 100kg of barn owls

A pyramid of biomass

• Pyramids of biomass provide a much better idea of the quantity of the plant or animal material at each level of a food chain and therefore are a better way of representing interdependence within the food chain

Biomass Transfers

Loss of biomass at each trophic level

• Pyramids of biomass are always pyramid-shaped, this indicates that at each increasing trophic level, biomass is lost from the food chain somehow

• This 'lost' biomass is transferred to the environment, rather than to the organism in the next trophic level

• In a plant, not all the glucose that it makes from photosynthesis is passed on to animals that feed on that plant

  • Some glucose is used for the plant's own respiration

  • Some glucose is converted to other biological molecules

• Animals respire too, dissipating some energy as heat, which causes some loss of biomass

  • Movement increases the energy/biomass loss

  • Warm-blooded animals that keep a set body temperature, like humans, lose more biomass through keeping warm than cold-blooded animals

• Animals release biomass as urine and faeces

• A secondary consumer may not eat or digest the whole animal it eats

• For this reason, food chains are almost never more than 5 trophic levels high

  • Because of the inefficient transfer of biomass (and hence energy) to the next level

Efficiency of Biomass Transfer

Calculating efficiency of biomass transfer

• You may be asked to calculate the efficiency of biomass transfers between trophic levels using percentages

• Percentage efficiency of biomass transfer is the proportion (expressed as a percentage) of the biomass at a lower trophic level that gets transferred up to the next trophic level