Year 7 Science · Unit 1 · Lesson 12
Apply Worksheet
Learning Goals
Real-world context
In the early 20th century, grey wolves (Canis lupus) were hunted to local extinction in Yellowstone National Park, USA. Without wolves, elk populations grew rapidly and grazed riverside vegetation down to bare earth. River banks became unstable and eroded. In 1995, the US government reintroduced 14 wolves. Within a decade, elk behaviour changed — they avoided open valleys — allowing willows and aspens to regrow. River banks stabilised and even the courses of some rivers changed. Scientists call this a trophic cascade.
(a) In the box below, draw a simple food chain of 3 or 4 organisms that includes wolves and elk. Add an arrow and label the trophic level of each organism (producer, primary consumer, secondary consumer or tertiary consumer).
(b) Using the idea of a trophic cascade, explain why removing wolves from a food web could change the physical landscape, not just the animal populations. Include the terms trophic level, consumer, and cascade in your answer.
Read the graph
The bar chart below shows the relative population size at each trophic level in a simple Australian grassland food chain: Grass → Grasshoppers → Frogs → Hawks.
Data: adapted from ecological pyramid models (Lindeman, 1942; Odum, 1957)
(a) Describe the pattern in population size as you move from producers up to tertiary consumers.
(b) There are 1 000 grasshoppers and 100 frogs. Estimate roughly how many grasshoppers each frog needs to eat to survive in this ecosystem.
(c) Using the idea of energy flow, explain why the producer level (grass) needs to support so many more organisms than the tertiary consumer level (hawks). In which direction does energy flow in this food chain?
1. A marine food chain reads: seagrass → dugong → tiger shark. If tiger sharks were hunted to extinction in a bay, predict what would happen to the seagrass population over time. Explain your reasoning using the terms trophic level and consumer.
2. A student says: "The arrow in a food chain shows which animal is the hunter." Correct this misconception in 2–3 sentences, and give an example using a real Australian organism to support your correction.
Wrap Up
In one sentence, what was the main idea of this lesson?