Productivity and energy transferEdexcel A-Level Biology A: Revision notes
Section 1
Productivity and how it is measured
Productivity is the rate at which plants (producers) convert light energy into chemical energy stored in organic molecules, per unit area and per unit time. Biomass is the mass of living material, usually measured as dry mass because water content varies.
Productivity is expressed as energy, in kJ m⁻² year⁻¹, or as biomass, in g m⁻² year⁻¹ (dry mass). Energy in biomass can be found by burning a dried sample in a calorimeter and measuring the temperature rise of water.
Productivity is described per unit area and per year so that ecosystems of different size and different time spans can be compared.
Always give the units with the area and the time, such as kJ m⁻² year⁻¹. Productivity is a rate, not a total.
Section 2
Gross and net primary productivity
Gross primary productivity (GPP) is the total chemical energy fixed in organic molecules by photosynthesis in a given area in a given time.
Plants respire to release energy for their own life processes. Some of the GPP is therefore lost as heat in plant respiration (R). The rest is stored in new plant tissue, as biomass, and is the net primary productivity (NPP):
NPP = GPP − R
NPP is the energy available to the next trophic level: primary consumers and decomposers.
Worked example: GPP = 18 000 kJ m⁻² year⁻¹ and R = 11 700 kJ m⁻² year⁻¹, so NPP = 18 000 − 11 700 = 6 300 kJ m⁻² year⁻¹. This is (6 300 ÷ 18 000) × 100 = 35% of the GPP.
NPP is smaller than GPP. Subtract respiration; do not add it.
Section 3
Energy lost between trophic levels
Only part of the energy in one trophic level is passed to the next. Energy is lost because:
- Some parts of the organism are not eaten (roots, bones, fur) or the organism is not eaten at all.
- Some of the food is not digested, so it is lost in faeces (for example cellulose and lignin).
- Energy is released as heat in respiration, which supplies energy for movement, keeping warm and other processes, and this energy is not available to the next level.
- Energy is lost in excretory products such as urine.
For a consumer, net production = energy taken in − (energy lost in faeces + energy lost in respiration and excretion). This is the energy stored as new biomass that can be passed on.
Energy is never destroyed: it is transferred to the surroundings, mostly as heat.
Section 4
Calculating the efficiency of transfer
The efficiency of energy transfer between trophic levels is:
efficiency (%) = (energy transferred to the next level ÷ energy available at the previous level) × 100
The same calculation can be used for biomass, using dry mass instead of energy.
Worked example: NPP of grass = 12 000 kJ m⁻² year⁻¹; energy in the grasshoppers = 1 320. Efficiency = (1 320 ÷ 12 000) × 100 = 11%. If the frogs hold 132, efficiency from grasshoppers to frogs = (132 ÷ 1 320) × 100 = 10%.
Efficiency is typically about 10%, but it varies, from about 5% to 20%. It is higher for carnivores than for herbivores because meat is easier to digest than plant cell walls, but it is lower in warm-blooded (endothermic) animals because they lose more heat in respiration.
Put the energy in the higher trophic level on top of the fraction and the energy in the lower level underneath, then multiply by 100. The answer must be below 100%.
Section 5
Consequences of inefficient transfer
Because so much energy is lost at each step:
- Food chains rarely have more than four or five trophic levels, as too little energy is left to support another level.
- The energy and biomass at each level fall up the chain, which is why pyramids of energy are always narrower at the top.
- Farmers can increase the efficiency of conversion by reducing respiratory losses (for example by keeping animals indoors so they move less and lose less heat) and by giving highly digestible food so less is lost in faeces.
- People gain more energy per hectare by eating plants directly than by eating animals that have fed on the plants.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Productivity and energy transfer
- A forest ecologist studies a plot of tropical forest. Over one year the plants in the plot fix 18 000 kJ m⁻² year⁻¹ of light energy as chemical energy in organic molecules. In the same year plant respiration releases 11 700 kJ m⁻² year⁻¹ of this energy.Calculate the percentage of the gross primary productivity that is stored as net primary productivity, and explain why herbivores can only use the net primary productivity.2 marks
- Energy flow through a grassland food chain was measured over one year. The net primary productivity of the grass was 12 000 kJ m⁻² year⁻¹. The energy stored in the biomass of the grasshoppers (primary consumers) that fed on the grass was 1 320 kJ m⁻² year⁻¹, and the energy stored in the biomass of the frogs (secondary consumers) that fed on the grasshoppers was 132 kJ m⁻² year⁻¹.Calculate the percentage efficiency of energy transfer from the grasshoppers to the frogs.2 marks
- A beef farmer keeps cattle in a grass field. Over one year the grass in the field has a net primary productivity of 60 000 kJ m⁻² year⁻¹. The cattle take in 24 000 kJ m⁻² year⁻¹ of this energy as food. Of the energy taken in, 7 200 kJ m⁻² year⁻¹ is lost in faeces and 12 600 kJ m⁻² year⁻¹ is lost through respiration and excretion. The rest is stored in the cattle as new biomass.Calculate the percentage of the net primary productivity of the grass that is converted into new cattle biomass. Show your working.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).