• Quick retrieval from earlier learning (Energy transfer and pyramids). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. Producers in a field capture 10000 kJ of energy. About 10% of the energy passes to each next level. How much energy reaches the primary consumers, in kJ? → 1000 kJPrompt: 10% means divide by 10 at each step.
2. A level of a food chain contains 10000 kJ. The next level gets 2000 kJ. What is the percentage efficiency of the energy transfer? → 20 %Prompt: efficiency = energy passed on ÷ energy available × 100
3. Why is it more energy-efficient for people to eat plants than to eat animals fed on plants? (Plants have more protein than meat / Energy is lost at each extra level of the food chain / Meat is always unhealthy / Animals contain no energy) → Energy is lost at each extra level of the food chainPrompt: How many levels does the energy pass through in each case?

• Show a food web and ask what would happen if one animal disappeared.
• Share the objective: Calculate energy transfer efficiency between trophic levels. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: Producers in a field store 50000 kJ/m² of energy per year. Primary consumers gain 2500 kJ/m². What is the efficiency of energy transfer, in %?
• Step 1: 2500 ÷ 50000 × 100 = 5
• Step 2: So the answer is: 5 %
• Answer: 5 %
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. Producers in a field store 10000 kJ/m² of energy per year. Primary consumers gain 1500 kJ/m². What is the efficiency of energy transfer, in %? → 15 %Prompt: efficiency = energy transferred to the next level ÷ energy in the level below × 100
2. Producers capture 200000 kJ. 20 % is passed to primary consumers and 10 % of that is passed to secondary consumers. How much energy reaches the secondary consumers, in kJ? → 4000 kJPrompt: Do one step at a time.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 6 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. Why is energy transfer between trophic levels inefficient? (Much is used in respiration and lost as heat, and some material is not eaten or digested / Energy is destroyed at each level / Consumers photosynthesise / Predators eat too much) → Much is used in respiration and lost as heat, and some material is not eaten or digestedPrompt: Energy cannot be destroyed, so where does it go?
2. Producers in a field store 10000 kJ/m² of energy per year. Primary consumers gain 2000 kJ/m². What is the efficiency of energy transfer, in %? → 20 %Prompt: efficiency = energy transferred to the next level ÷ energy in the level below × 100

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-12.