Year 8 Science · Unit 1 · Lesson 20
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Learning Goals
Match each word to its meaning
Write the matching letter in the middle column. Each letter is used once.
| Word | Letter | Meaning |
|---|---|---|
| Producer | A. Many food chains joined together, showing who eats whom in one place. | |
| Primary consumer | B. Eats primary consumers, for example a lizard eating a grasshopper. | |
| Secondary consumer | C. The feeding step an organism is on in a food chain. | |
| Trophic level | D. Captures energy from sunlight to make its own food; the first link of every food chain. | |
| Food web | E. Eats producers, for example a grasshopper eating grass. |
Real-world context: kelp forests
Kelp is a large brown seaweed that forms underwater forests off south-eastern Australia. Sea urchins eat kelp, and large rock lobsters eat sea urchins. In parts of eastern Tasmania, fishing has removed many large rock lobsters, and in some places urchins have eaten the kelp down to bare rock.
1. Write the food chain for kelp, sea urchins and rock lobsters with arrows. Label each organism as producer, primary consumer or secondary consumer.
Cause and effect
Complete the chain. Each box is caused by the box before it.
2. Abalone (a large sea snail) also feed on kelp. Predict what happens to the abalone in an area where the large rock lobsters have been removed. Explain the links in the food web that cause it.
Energy through a grassland food chain
| Trophic level | Organisms | Energy (kJ) |
|---|---|---|
| 1 | grasses | 24 000 |
| 2 | crickets | 2 600 |
| 3 | frogs | 230 |
| 4 | snakes | 25 |
Source: sample values for this exercise, not measured data.
3. Calculate the percentage of energy passed on from level 1 to level 2, from level 2 to level 3, and from level 3 to level 4. Give each to one decimal place.
1 to 2 % 2 to 3 % 3 to 4 %
4. Compare your three answers above with the 10 percent rule. What happened to the rest of the energy at each level, and what does this mean for the number of snakes the grassland can support?
5. A student says: “If there are more kangaroos, there will automatically be more dingoes.” Evaluate this claim using Claim, Evidence and Reasoning.
Match each word to its meaning
Producer → D; Primary consumer → E; Secondary consumer → B; Trophic level → C; Food web → A.
Your Turn
1. kelp (producer) → sea urchin (primary consumer) → rock lobster (secondary consumer). [2 marks: 1 arrows correct, 1 labels]
Cause and effect: Box 2: fewer lobsters eat urchins, so the urchin population grows. Box 3: the many urchins eat more kelp than can grow back.
2. Abalone numbers fall. Fewer lobsters means more urchins, the urchins eat the kelp, and the abalone lose the food they share with the urchins. A change at one trophic level spreads through the web to organisms that share the same producer. [2 marks: 1 prediction, 1 chain of links]
Energy through a grassland food chain
3. 2 600 ÷ 24 000 × 100 = 10.8%. 230 ÷ 2 600 × 100 = 8.8%. 25 ÷ 230 × 100 = 10.9%. [3 marks: 1 each]
4. All three are close to 10 percent. The other 90 percent or so at each level is lost as heat, movement and waste. Only 25 of the 24 000 kJ (about 0.1%) reaches the snakes, so there is only enough energy for a few snakes. [3 marks: 1 close to 10%, 1 where the rest goes, 1 few snakes linked to energy]
Show What You Know
5. Marking guide (points expected): 1 claim: partly correct but oversimplified; 1 evidence: dingoes eat kangaroos, so more kangaroos means more food energy for dingoes; 1 only about 10% of the kangaroos’ energy passes to the dingoes, so the dingo increase is much smaller and slower; 1 food web: dingoes also eat other prey, and more kangaroos eat more grass, so if the grass runs out the kangaroos will decline too; 1 reasoning: other factors (grass, water, disease, other predators) also matter, so the rise in dingoes is not automatic. [5 marks]
Common error to watch for: in Q3, dividing the bigger number by the smaller one (24 000 ÷ 2 600) and giving about 9 instead of a percentage.