Teacher sheet · Ages 15–16 · about 45–50 minutes
Energy flow and interactions in ecosystems
Lesson page: echoesof.science/school/ecosystems/ages-15-16/ · Pupil worksheet: echoesof.science/school/ecosystems/ages-15-16/print/
Learning aims
- Describe an ecosystem in terms of biotic and abiotic factors and trophic levels.
- Calculate energy transfer between trophic levels and explain the losses.
- Explain why food chains are short and why energy pyramids are upright.
- Plan a field investigation into abiotic factors and evaluate correlation versus causation.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: why are there far fewer hawks than grasshoppers? |
| 10 min | Read the text; practise 10% rule and Silver Springs calculations. |
| 25 min | Field investigation in two contrasting areas. |
| 5 min | Calculate means and compare. |
| 5 min | Quiz, or the Where does the energy go? game as homework. |
Materials for the experiment
- Frame quadrats (0.5 m × 0.5 m)
- Two long tape measures
- Random number tables or app
- Light meter or a phone light-meter app
- Thermometer
- Soil moisture meter (or a soil sample and a balance)
- Plant identification key
- Recording sheet
Safety: Teacher checks the site and does a risk assessment. Wear long sleeves and trousers (brambles, nettles, ticks) and sturdy shoes, watch for branches at eye level and uneven ground, never work alone, wash hands after handling soil and plants. Place quadrats; never throw them.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Energy is recycled in an ecosystem like nutrients. | Energy flows through and is lost as heat; nutrients are recycled by decomposers. |
| Exactly 10% is passed on at every level. | 10% is an average; Silver Springs had about 14.5% between the first two levels. |
| The Yellowstone story proves wolves control everything. | The first effects matched predictions, but bears, cougars, bison and humans also matter. |
Worksheet answers
- Level 1: 20,000 kJ; level 2: 10% = 2,000 kJ; level 3: 10% of 2,000 = 200 kJ/m²/yr.
- Any three: heat from respiration; indigestible material lost in waste; organisms that die without being eaten; parts not eaten such as bones; energy used for movement.
- Energy is lost at every transfer, so each level has less energy than the one below; energy cannot increase going up.
- A change at one trophic level spreads to others: wolves reduced elk where they hunted, so willows, aspens and cottonwoods recovered and beavers returned. Scientists are careful because later other predators (bears, cougars), bison and humans also affected the system, and field observations cannot control all variables.
Quiz answers
- Producers in a field capture 5,000 kJ. Using the 10% rule, about how much energy reaches the primary consumers? 500 kJ. 10% of 5,000 kJ is 500 kJ.[2]
- Where does most of the 'lost' energy at each trophic level go? Heat released during respiration. Energy is not destroyed; much escapes as heat.[2]
- In Silver Springs, 13,187 of 20,810 kcal/m²/yr were used in respiration or lost as heat. About what percentage is that? 63%. 13,187 ÷ 20,810 × 100 ≈ 63%.[3]
- Why do food chains rarely have more than four to six energy transfers? Too little energy is left to support another level. Energy is lost at every transfer.[3]
- Which statement about the Yellowstone wolves is best supported? Wolves helped plants recover, but other factors became important later. Bears, cougars, bison and humans also matter.[4]
- What is the main limitation of comparing plants in two areas in a field study? Many factors differ between the areas, so it shows correlation, not cause. Field studies cannot control all variables.[6]
Going further
Use the Silver Springs numbers to calculate net primary productivity (gross minus respiration) and discuss what net primary productivity means for the herbivores.
Sources used in this lesson
- [1] Ecosystem (National Geographic Education): reliability Medium-high
- [2] Energy flow and the 10 percent rule (National Geographic Education): reliability Medium-high
- [3] Biology 2e, 46.2 Energy flow through ecosystems (OpenStax (Rice University)): reliability High
- [4] Study documents wolves' role in transforming Yellowstone (The Wildlife Society): reliability Medium-high
- [6] Diversity: terrestrial method (Field Studies Council): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/ecosystems/sources/
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