Ecosystem Energy Budgets: GPP, NPP and Transfer Efficiency
Gross primary production is what producers capture, net primary production is what remains after their own respiration, and only a small share reaches the next trophic level. The numbers are why food chains stay short.
What a learner can do afterwards
- Calculates net primary production from gross production and respiratory loss.
- Works out percentage energy transfer between two trophic levels from given data.
- Explains where the energy that fails to transfer actually goes.
1 · Read
GPP is the total chemical energy producers fix per area per time. Plants respire too, releasing energy from glucose as other cells do, and that respiration burns some of what they fix just to keep them alive. NPP equals GPP minus respiration, and NPP feeds every herbivore and everything above.
Each feeding step in a food chain is a trophic level. Only about a tenth of the energy at one level becomes new biomass at the next. A rabbit burns most of the energy in its grass in respiration, losing it as heat. More leaves in urine and faeces, and the roots are never eaten. Short chains and narrow pyramid tops follow.
Work the transfer as a percentage: energy in new biomass above, divided by energy available below, times 100. Grass holding 24000 units with 1800 reaching new rabbit tissue gives 1800 divided by 24000, times 100, which is 7.5 percent.
Use a chain for the maths and the web for the full story. One grass plant feeds rabbits, grasshoppers and voles while the fox eats all three, so removing one species ripples in several directions.
Subtract respiration from GPP for NPP, expect about a tenth to move up, and trace the rest to heat, waste, and uneaten parts.
The two rewritten readings were written for this dataset. Readings summarise two Khan Academy articles on energy flow and food webs. Videos are linked from Khan Academy. License terms for these sources are not recorded.
2 · Watch
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Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.