Productivity and energy transfer

Gross and net primary productivity

• Gross primary productivity (GPP): the rate at which energy is incorporated into organic molecules by photosynthesis in producers, for example in kJ m-2 year-1.
• Producers use some of this in their own respiration (R).
• Net primary productivity (NPP): the energy left for growth, reproduction and to pass on to consumers.

NPP = GPP - R

Efficiency of transfer

Efficiency = (energy or biomass transferred to the next level / energy or biomass at the previous level) x 100

Transfer between trophic levels is inefficient because:

• not all of an organism is eaten (roots, bones, shells)
• not all that is eaten is digested and absorbed: some is lost in faeces
• energy is lost in excretion (urea)
• much energy is released in respiration and lost as heat (especially in endotherms)

So food chains rarely have more than four or five trophic levels.

Producers and light

Producers convert only a small percentage of the light falling on them into chemical energy. Light may miss the chloroplasts, be reflected, be the wrong wavelength, or other factors such as CO2 and temperature may limit photosynthesis.

Worked example

A grassland has GPP of 24 000 kJ m-2 year-1. Plants use 15 000 kJ m-2 year-1 in respiration, and herbivores take in 900 kJ m-2 year-1 of the NPP. Calculate the NPP and the efficiency of transfer from NPP to herbivores.

1. NPP = 24 000 - 15 000 = 9000 kJ m-2 year-1
2. Efficiency = 900 / 9000 x 100 = 10%

Answer: NPP = 9000 kJ m-2 year-1; efficiency = 10%.

Check yourself

1. GPP is 18 000 kJ m-2 year-1 and respiratory loss is 11 500 kJ m-2 year-1. Calculate the NPP in kJ m-2 year-1.
   Show answer

   6500 kJ m-2 year-1. 18 000 - 11 500 = 6500.

2. NPP is 3200 kJ m-2 year-1 and respiration is 4800 kJ m-2 year-1. What is the GPP in kJ m-2 year-1?
   Show answer

   8000 kJ m-2 year-1. GPP = NPP + R = 8000.

3. Primary consumers contain 1600 kJ m-2 year-1 and secondary consumers contain 176 kJ m-2 year-1. Calculate the percentage efficiency of transfer.
   Show answer

   11 %. 176 / 1600 x 100 = 11%.

4. Producers have an NPP of 45 000 kJ m-2 year-1. Transfer to primary consumers is 8% efficient. How much energy reaches the primary consumers, in kJ m-2 year-1?
   Show answer

   3600 kJ m-2 year-1. 45 000 x 0.08 = 3600.

5. Which is a reason energy transfer between trophic levels is inefficient?
   • All parts of the prey are eaten and digested
   • Consumers do not respire
   • Much energy is released in respiration and lost as heat
   • Energy is created at each level
   Show answer

   Much energy is released in respiration and lost as heat. Respiration, faeces, excretion and uneaten parts all cause losses.

6. What does net primary productivity represent?
   • Energy available for producer growth and for consumers after respiration
   • Total energy fixed by photosynthesis
   • Energy lost in faeces
   • Energy absorbed by chlorophyll as light
   Show answer

   Energy available for producer growth and for consumers after respiration. NPP is what is left after producers' respiration.

7. Why do food chains rarely have more than five trophic levels?
   • Top predators cannot reproduce
   • Producers stop growing
   • Too little energy remains at the higher levels to support another population
   • Decomposers remove all the energy at level three
   Show answer

   Too little energy remains at the higher levels to support another population. Losses at each step leave too little energy.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.