Teacher sheet · Ages 15–16 · about 45–50 minutes
Energy flow, pyramids and biomagnification
Lesson page: echoesof.science/school/food-chains/ages-15-16/ · Pupil worksheet: echoesof.science/school/food-chains/ages-15-16/print/
Learning aims
- Define GPP, NPP and trophic level transfer efficiency and calculate them.
- Explain where energy is lost between trophic levels.
- Compare pyramids of numbers, biomass and energy, including inverted pyramids.
- Explain biomagnification with a model and real examples (DDT, PCBs).
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: why are big fierce animals rare? |
| 10 min | Read the text; Silver Springs calculations. |
| 20 min | Energy pyramid and biomagnification model. |
| 5 min | Discuss the limits of the model and the real data. |
| 5 min | Quiz, or the True or false? game as homework. |
Materials for the experiment
- About 100 dried beans or paper counters per group, 10 of them marked with a pen (the 'pollutant')
- Three cups or trays labelled phytoplankton, small fish, big fish
- Graph paper or a spreadsheet
- Calculator
Safety: Do not eat the beans. Wash hands after handling counters. Clear spilled beans from the floor.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Energy is recycled in an ecosystem. | Nutrients are recycled; energy flows through and is lost as heat. |
| The 10% rule is exact. | It is an average; e.g. Silver Springs producers to primary consumers was about 14.5%. |
| All pyramids must be pyramid-shaped. | Numbers and biomass pyramids can be inverted; only energy pyramids are always upright. |
Worksheet answers
- 900 ÷ 8,000 × 100 = 11.25% (about 11%).
- Energy is lost at every transfer, so each level has less energy than the one below. Biomass is a snapshot of standing mass: fast-reproducing producers such as phytoplankton can have a small biomass but a large production.
- The biomass shrinks at each level but the persistent pollutant is kept, so its concentration rises (0.1 → 0.5 → about 0.9 in the model). Apex predators eat many contaminated prey and sit at the top, so they collect the highest concentrations.
- Caterpillar 0.18 × 50 = 9 J; squirrel 0.016 × 50 = 0.8 J.
Quiz answers
- Producers have 1,000 kJ/m²/yr of production and primary consumers 120 kJ/m²/yr. What is the TLTE? 12%. 120 ÷ 1,000 × 100 = 12%.[4]
- At Silver Springs, about what share of GPP was lost to respiration and heat? About 63%. 13,187 ÷ 20,810 ≈ 0.63.[4]
- Which ecological pyramid is always upright? The pyramid of energy. Energy is lost at every transfer.[4]
- Why can the biomass pyramid of the English Channel be inverted? Phytoplankton turn over (reproduce and are eaten) very quickly. A small standing biomass with a fast turnover can feed a larger biomass of zooplankton.[4]
- Which animal has the higher net production efficiency? A caterpillar. Ectotherms are about 10 times more efficient than endotherms.[4]
- Where is the concentration of a persistent pollutant usually highest? In the top consumers. Biomagnification concentrates it up the food chain.[4]
Going further
Find real data on mercury or PCBs in a food chain from a government or university source and plot concentration against trophic level. Discuss the uncertainty in such measurements.
Sources used in this lesson
- [3] Energy Flow and the 10 Percent Rule (National Geographic Education): reliability Medium-high
- [4] Biology 2e, 46.2 Energy Flow through Ecosystems (OpenStax, Rice University): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/food-chains/sources/
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