Free Pearson Edexcel International A-level Biology · Unit 4

Productivity and energy transfer through ecosystems

What you’ll learn

  • Calculate net primary productivity and transfer efficiency.
  • Explain why energy available falls between trophic levels.
  • Distinguish energy flow from nutrient cycling.

Understand the idea

Gross primary productivity is the rate at which producers incorporate energy into organic material. Producers use some of this material in respiration. Net primary productivity is the remainder available for growth, reproduction and transfer to consumers or decomposers. Express productivity per area and time so habitats or sampling periods can be compared fairly.

Only a fraction of the energy at one trophic level becomes new biomass at the next. Some material is never eaten; some ingested material is not digested and leaves as faeces. Respiration transfers energy to the surroundings as heat, and excretion also removes organic material. Uneaten material can enter a decomposer pathway, so it is not simply absent from the whole ecosystem.

Energy flows through an ecosystem and ultimately dissipates as heat; it is not recycled into the original light energy. Carbon and other elements cycle through organisms and the physical environment. A biomass pyramid is a snapshot, whereas productivity includes time: rapid replacement of a small producer biomass can support a larger standing consumer biomass in some aquatic systems.

Key formulae

NPP=GPP−R\mathrm{NPP}=\mathrm{GPP}-R

transfer efficiency=energy incorporated at the next levelenergy available at the previous level×100%\text{transfer efficiency}=\frac{\text{energy incorporated at the next level}}{\text{energy available at the previous level}}\times100\%

Respiration separates gross and net productivity

Conditions. Illustrative family of ecosystems with respiration fixed at 60% of GPP. R = 0.6GPP and NPP = 0.4GPP; the fraction is chosen for teaching and is not a universal ecological constant.

Gross primary productivity / kJ m⁻² year⁻¹; Productivity component / kJ m⁻² year⁻¹.

At GPP = 10000, respiration uses 6000 and NPP is 4000. The two plotted component values add to GPP; the graph compares rates, not standing biomass.

Open the interactive lesson to view plotted curves and coordinate tables.

Worked example

A grassland has GPP 24000 kJ m−2 year−124000\,\mathrm{kJ\,m^{-2}\,year^{-1}} and plant respiration 15000 kJ m−2 year−115000\,\mathrm{kJ\,m^{-2}\,year^{-1}}. Herbivore production is 900 kJ m−2 year−1900\,\mathrm{kJ\,m^{-2}\,year^{-1}}. Find NPP and producer-to-herbivore efficiency.

  1. Subtract producer respiration: 24000−15000=900024000-15000=9000.
  2. Use producer NPP as the energy available for transfer in this question.
  3. Calculate 900/9000×100=10%900/9000\times100=10\%.

Answer: NPP is 9000 kJ m−2 year−19000\,\mathrm{kJ\,m^{-2}\,year^{-1}}; transfer efficiency is 10%10\%.

Open the interactive lesson to try original questions and recall. Build a revision plan.