How to Calculate NPP and Energy Transfer Efficiency
Learn how to calculate net primary productivity from GPP and respiration, then work out energy transfer efficiency between trophic levels with a worked example.
To calculate net primary productivity (NPP), subtract the energy used in respiration by producers from gross primary productivity (GPP):
NPP=GPP−R \mathrm{NPP}=\mathrm{GPP}-R NPP=GPP−RTo calculate energy transfer efficiency between two trophic levels, divide the energy stored in the biomass of the next level by the energy available in the biomass of the previous level, then multiply by 100100100:
Energy transfer efficiency=energy in biomass at the next trophic levelenergy in biomass at the previous trophic level×100% \text{Energy transfer efficiency}=\frac{\text{energy in biomass at the next trophic level}}{\text{energy in biomass at the previous trophic level}}\times 100\% Energy transfer efficiency=energy in biomass at the previous trophic levelenergy in biomass at the next trophic level×100%What do the terms mean? GPP is the total chemical energy fixed by producers through photosynthesis. RRR is the energy they use in respiration. NPP is the energy remaining in new producer biomass and available to the next trophic level. A trophic level is a feeding position in a food chain, such as producers or primary consumers. Productivity is a rate, so its units include an area and a time period.
The denominator depends on the transfer you are measuring. For producers to primary consumers, use producer NPP; for primary to secondary consumers, use the primary consumers' net production. In each case, compare energy stored as new biomass over the same area and time period. The percentage itself has no units.
Worked example: Suppose producers have a GPP of 20 000 kJ m−2 year−120\,000\ \mathrm{kJ\,m^{-2}\,year^{-1}}20000 kJm−2year−1 and use 8 000 kJ m−2 year−18\,000\ \mathrm{kJ\,m^{-2}\,year^{-1}}8000 kJm−2year−1 in respiration. Their NPP is:
20 000−8 000=12 000 kJ m−2 year−1 20\,000-8\,000=12\,000\ \mathrm{kJ\,m^{-2}\,year^{-1}} 20000−8000=12000 kJm−2year−1If primary consumers store 1 200 kJ m−2 year−11\,200\ \mathrm{kJ\,m^{-2}\,year^{-1}}1200 kJm−2year−1 in new biomass, the transfer efficiency from producers to primary consumers is:
1 20012 000×100%=10% \frac{1\,200}{12\,000}\times100\%=10\% 120001200×100%=10%This means one tenth of the energy stored in new producer biomass becomes new primary-consumer biomass. Not all producer material is eaten, and consumers lose energy through respiration and in material they cannot digest. Those losses help explain why the energy passed on is smaller than the energy available at the previous level.
Exam mistake to avoid: Do not use GPP as the denominator for transfer from producer biomass to consumers. Producers have already used some of the energy fixed in photosynthesis for respiration; NPP is what remains. Check that the two values you divide cover the same area and time period and use compatible energy units.
