• 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 20000 kJ of energy. About 10% of the energy passes to each next level. How much energy reaches the primary consumers, in kJ? → 2000 kJPrompt: 10% means divide by 10 at each step.
2. A level of a food chain contains 10000 kJ. The next level gets 1000 kJ. What is the percentage efficiency of the energy transfer? → 10 %Prompt: efficiency = energy passed on ÷ energy available × 100
3. Why do food chains rarely have more than four or five levels? (There is too little energy left to support more levels / Plants stop growing / There are not enough names for them / Top predators do not need energy) → There is too little energy left to support more levelsPrompt: Think about how much is left after several 10% steps.

• 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 40000 kJ/m² of energy per year. Primary consumers gain 2000 kJ/m². What is the efficiency of energy transfer, in %? → 5 %Prompt: efficiency = energy transferred to the next level ÷ energy in the level below × 100
2. Producers capture 200000 kJ. 10 % is passed to primary consumers and 10 % of that is passed to secondary consumers. How much energy reaches the secondary consumers, in kJ? → 2000 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 are food chains rarely longer than 4 or 5 levels? (There are not enough species on Earth / Plants stop growing / Too little energy is left to support another level / Animals refuse to eat higher animals) → Too little energy is left to support another levelPrompt: About 90 % is lost at each step.
2. Producers capture 500000 kJ. 10 % is passed to primary consumers and 5 % of that is passed to secondary consumers. How much energy reaches the secondary consumers, in kJ? → 2500 kJPrompt: Do one step at a time.

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