Year 8 Science · Unit 1 · Lesson 20
Challenge Worksheet
Learning Goals
Find the errors
This paragraph has exactly three errors. Underline each one and correct it below.
A student’s notes
A food chain starts with a producer that captures energy from sunlight. In a food chain, each arrow points from the predator to the prey it eats. At each trophic level, only about 10 percent of the energy passes on to the next level. The other 90 percent is recycled back to the producers, so no energy is ever lost. An energy pyramid is widest at the top, because top predators are the biggest and strongest animals.
1. Write the three corrections.
Scenario: a creek food web
A Year 8 class studied a freshwater creek in the NSW tablelands. They used field guides to record what the main organisms eat.
| Organism | What it eats |
|---|---|
| Algae | makes its own food |
| Water plants | make their own food |
| Mayfly nymphs | algae |
| Snails | algae, water plants |
| Yabbies | water plants, snails |
| Small fish | mayfly nymphs |
| Platypus | mayfly nymphs, yabbies |
| Kingfisher | small fish, yabbies |
Source: simplified class study; a real creek has many more links.
2. A chemical spill kills all the mayfly nymphs. Which animal is most at risk of dying out in the creek? The platypus also loses one of its foods. Use Table 1 to explain why it is less at risk than the animal you named.
3. In the chain algae → mayfly nymph → small fish → kingfisher, one kingfisher gets 300 kJ a week from fish (a sample figure). Use the 10 percent rule to find how much energy the algae must capture for one kingfisher, and for a pair. Show each step.
One kingfisher kJ A pair kJ
Someone claims
“Eating plants gives you ten times more energy than eating animals that ate the plants. So the cow wastes 90 percent of the energy in the grass it eats.”
4. Which part of this claim is supported by the 10 percent rule? Then explain why “wastes” is misleading, by saying what actually happens to the other 90 percent.
5. A national park wants to protect its wedge-tailed eagles. A ranger says: “We only need to protect the eagles’ nesting trees. The grasses and shrubs don’t matter to eagles, because eagles don’t eat plants.” Evaluate the ranger’s claim.
Find the errors
1. (i) The arrow points from the prey (the organism eaten) to the predator, showing energy flow. (ii) The other 90 percent is lost as heat, movement and waste; energy is not recycled. (iii) An energy pyramid is widest at the bottom, where the producers are; top predators are at the narrow top because they receive the least energy. [3 marks: 1 per correction]
Your Turn
2. Most at risk: the small fish, because mayfly nymphs are their only food, so they have nothing left to eat. The platypus eats both mayfly nymphs and yabbies, so it can switch to yabbies; it has a second food source in the web. [3 marks: 1 small fish, 1 only food source, 1 platypus still has yabbies]
3. Fish level: 300 ÷ 0.1 = 3 000 kJ. Mayfly level: 3 000 ÷ 0.1 = 30 000 kJ. Algae: 30 000 ÷ 0.1 = 300 000 kJ. A pair: 600 000 kJ. [3 marks: 1 method (× 10 at each step), 1 300 000 kJ, 1 600 000 kJ]
Evaluate the claim
4. Supported: only about 10 percent of the energy passes from one trophic level to the next, so eating plants directly gives access to roughly ten times more of the producers’ energy. Misleading: the other 90 percent is not wasted by the cow; it is used for the cow’s movement and life processes, lost as heat, or passes out as waste. It is not available to the next level. [3 marks: 1 supported part, 1 where the energy goes, 1 why “wastes” is misleading]
Show What You Know
5. Marking guide (points expected): 1 right: eagles need nest trees and do not eat plants; 1 food chain, for example grass → wallaby → eagle, or grass → grasshopper → lizard → eagle, with producer, primary consumer and higher levels named; 1 all the eagle’s energy starts in the producers; 1 only about 10 percent passes up each level, so a large base of producers is needed to support a few eagles; 1 the eagles eat several prey that depend on many plants, so losing grasses and shrubs cuts their food across the web; 1 judgement: the claim is wrong; protect the whole habitat, producers included. [6 marks]
Common error to watch for: thinking an animal depends only on what it eats directly. Every consumer’s energy traces back to producers.