Biology

3.15 - Biomass Transfer & Pyramids

6 min

Pyramids of biomassOutside your course

Why biomass is lostOutside your course

Efficiency of transfer

note 1 of 1

Efficiency of biomass transfer between trophic levels

The efficiency of transfer is the percentage of the biomass (or the energy it holds) at one trophic level that is passed on to the next, rather than lost.
It's worked out by dividing the amount transferred to the next level by the amount available at the level below, then multiplying by 100 to turn the fraction into a percentage.

Formula for efficiency of transfer

$Efficiency (%) = \frac{biomass transferred to the next level}{biomass available at the previous level} \times 100$

The typical efficiency is about 10% - this is the value usually used when no measured masses are given.

Why food chains are short

Because only about a tenth of the biomass passes on at every step, the amount of biomass and energy available shrinks rapidly up the chain.
This limits both the number of organisms a food chain can support and how long the chain can be:
- The number of organisms tends to fall at each higher level.
- After about four or five levels there's too little biomass left to support any more, so the chain runs out.
Shorter food chains lose less overall, so more of the original energy reaches the top.

Worked example - Efficiency from rabbits to snakes

A population of rabbits holds 144 kg of biomass. The snakes that feed on them gain 15 kg. Calculate the efficiency of transfer from rabbits to snakes.

Step 1 - Identify the values

Biomass transferred to the next level (snakes) = 15 kg
Biomass available at the previous level (rabbits) = 144 kg

Step 2 - Apply the formula

$Efficiency (%) = \frac{15}{144} \times 100$

Step 3 - Calculate

Efficiency = 10.4%

So just over a tenth of the rabbits' biomass became snake biomass - close to the typical figure of about 10%.

The next step up works out much the same way: if the snakes (15 kg) pass on 1.6 kg to the hawks above them, the efficiency of that transfer is $\frac{1.6}{15} \times 100 = 10.7 %$ .

end of notes

Key points to remember

Efficiency of biomass transfer between trophic levels

The efficiency of transfer is the percentage of the biomass (or the energy it holds) at one trophic level that is passed on to the next, rather than lost.

It's worked out by dividing the amount transferred to the next level by the amount available at the level below, then multiplying by 100 to turn the fraction into a percentage.

Formula for efficiency of transfer

$Efficiency (%) = \frac{biomass transferred to the next level}{biomass available at the previous level} \times 100$

The typical efficiency is about 10% - this is the value usually used when no measured masses are given.

Why food chains are short

Because only about a tenth of the biomass passes on at every step, the amount of biomass and energy available shrinks rapidly up the chain.

This limits both the number of organisms a food chain can support and how long the chain can be:

The number of organisms tends to fall at each higher level.
After about four or five levels there's too little biomass left to support any more, so the chain runs out.

Shorter food chains lose less overall, so more of the original energy reaches the top.

Worked example - Efficiency from rabbits to snakes

A population of rabbits holds 144 kg of biomass. The snakes that feed on them gain 15 kg. Calculate the efficiency of transfer from rabbits to snakes.

Step 1 - Identify the values

Biomass transferred to the next level (snakes) = 15 kg
Biomass available at the previous level (rabbits) = 144 kg

Step 2 - Apply the formula

$Efficiency (%) = \frac{15}{144} \times 100$

Step 3 - Calculate

Efficiency = 10.4%

So just over a tenth of the rabbits' biomass became snake biomass - close to the typical figure of about 10%.

The next step up works out much the same way: if the snakes (15 kg) pass on 1.6 kg to the hawks above them, the efficiency of that transfer is

\frac{1.6}{15} \times 100 = 10.7 %