echoesof.science · Ages 15–16
Energy flow, pyramids and biomagnification
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the game and the quiz
echoesof.science/school/food-chains/ages-15-16/
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Producers capture energy by photosynthesis (gross primary productivity, GPP). Part is used in their own respiration and lost as heat; what remains is net primary productivity (NPP), available to consumers. At Silver Springs, GPP was 20,810 kcal/m²/yr, and 13,187 kcal/m²/yr of it (about 63%) was lost to respiration and heat.
Trophic level transfer efficiency: TLTE = (production at one level ÷ production at the level below) × 100%. At Silver Springs it was about 14.5% from producers to primary consumers; across ecosystems the average is often quoted as about 10%.
Energy leaves the food chain because not all sunlight reaches producers, much is released as heat in respiration, some food is not digested and is excreted, and organisms die without being eaten; decomposers then lose most of that energy as heat.
Experiment: Modelling energy loss and biomagnification
- The numbers in this model are made up to show the idea; they are not real data.
- Energy: the producers in a model pond have NPP = 50,000 kJ/m²/yr. Using a transfer efficiency of 10% at each step, calculate the energy at levels 2, 3 and 4.
- Draw the pyramid of energy to scale (or on a log scale) and explain its shape.
- Biomagnification: put 100 counters (10 marked) in the phytoplankton tray. Each counter is a unit of phytoplankton biomass.
- The small fish 'eat' all the phytoplankton but, with 10% transfer, keep only 10 units of biomass: keep 10 counters and ALL 10 marked counters, because the pollutant is not broken down or excreted.
- The big fish eats the small fish and keeps 1 unit of biomass plus all the pollutant. Calculate the fraction of marked counters at each level.
| Level | Energy (kJ/m²/yr) | Biomass units | Marked counters | Fraction marked |
|---|---|---|---|---|
| Phytoplankton | ||||
| Small fish | ||||
| Big fish | ||||
| Fish-eating bird (extension) |
Questions
Producers: 8,000 kJ/m²/yr. Primary consumers: 900 kJ/m²/yr. Calculate the TLTE.
Explain why a pyramid of energy is always upright, but a pyramid of biomass can be inverted.
Use your model results to explain biomagnification. Why are apex predators most at risk from persistent pollutants?
A caterpillar (NPE 18%) and a squirrel (NPE 1.6%) each assimilate 50 J. How much new biomass energy does each make?
Sources for every fact on this sheet: echoesof.science/school/food-chains/sources/