Calculating Productivity & Efficiency (AQA A Level Biology): Revision Note

The wheat crop growing in a farmer’s field has a gross primary productivity of 10 000 kJ m^-2  yr^-1. The wheat crop loses 5500 kJ m^-2 yr^-1 as heat from respiration.

Calculate the net primary productivity of the wheat crop.

Answer:

Step 1: write out the equation and substitute in the known values

NPP = GPP - R

NPP = 10 000 - 5500

Step 2: calculate the net primary productivity and give appropriate units

NPP = 4500 kJ m^-2 yr^-1

In a grassland ecosystem the net productivity of rabbits is 2400 kJ m^-2 yr^-1. These rabbits ingest 18 900 kJ m^-2 of grass per year.

Calculate the total energy loss of the rabbits.

Step 1: rearrange the equation to find the total energy loss (F + R)

N = I - (F + R)

(F + R) = I - N

Step 2: substitute in the known values

(F + R) = I - N

(F + R) = 18 900 - 2400

Step 3: calculate the total energy loss and give appropriate units

total energy loss = 16 500 kJ m^-2 yr^-1

The total solar energy received by an area of heather is 6 × 10⁵ kJ m^-2 yr^-1. The gross primary productivity of the heather is 4.25 × 10³ kJ m^-2 yr^-1.

Calculate the percentage of solar energy converted into chemical energy by photosynthesis.

Step 1: write out the equation and substitute in the known values

% efficiency = x 100

= x 100

Step 2: calculate the efficiency

% efficiency = x 100

= 0.0071 × 100

= 0.71 %

A wheat farmer’s crops were eaten by insect pests. The net primary productivity of the wheat is 112 500 kJ m^-2 yr^-1. The net productivity of the insect pests is 10 000 kJ m^-2 yr^-1.

Calculate the percentage efficiency of energy transfer from the wheat to the insects.

Step 1: write out the equation and substitute in the known values

% efficiency = x 100

= x 100

Step 2: calculate the efficiency

% efficiency = 0.089 × 100

= 8.9 %

A wheat farmer decided to use biological control against insect pests that were eating a wheat crop. The farmer introduced a species of toad. The toads ingested 10 000 kJ m^-2 yr^-1 of energy from the pests but lost 2000 kJ m^-2 yr^-1 of this energy in faeces and urine. They lost a further 7000 kJ m^-2 yr^-1 during respiration.

Calculate the percentage efficiency of energy transfer from the insects to the toads.

Step 1: calculate the net productivity of the toads

N = I - (F + R)

N = 10 000 - (2000 + 7000)

N = 10 000 - 9000

N = 1000 kJ m^-2 yr^-1

Step 2: write out the equation for % efficiency and add known values

% efficiency = x 100

= (1000 ÷ 10 000) × 100

Step 3: calculate the efficiency

% efficiency = 0.1 × 100

= 10 %

When carrying out calculation questions you should:

• be aware of the units being used; is this productivity, or just a measure of production?

• consider which formula, or formulae, you may need to use; remember that some calculations may require multiple steps