Year 8 Science · Unit 1 · Lesson 22
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Learning Goals
Read the graph
The bar chart below shows the amount of energy (in kilojoules) available at each trophic level in a grassland ecosystem. Study it carefully, then answer the questions.
Data: adapted from Odum (1959), Silver Springs ecosystem energy budget, peer-reviewed ecology literature.
(a) How much energy is lost between the producers (T1) and the primary consumers (T2)? Show your working and express it as a percentage.
(b) A tertiary consumer (T4) has 100 kJ of energy. How many kilojoules of energy were originally captured by the producers that eventually became this 100 kJ? What does this tell you about why apex predators are rare?
(c) A person who eats only plants (a vegetarian) obtains energy at trophic level 2. A person who eats beef obtains energy at trophic level 3. Using the 10% rule, explain why a vegetarian diet uses less land to produce the same amount of food energy.
Real-world context
The kelp forests off the southern Australian coast (including Victoria and South Australia) are critical ecosystems. Kelp is eaten by sea urchins. Sea urchins are normally kept in check by their predators, rock lobsters and large fish. In recent decades, overfishing has removed many of these predators from some areas, causing sea urchin populations to explode. Scientists call the knock-on effects of removing a predator a trophic cascade.
(a) If sea urchin predators (rock lobsters) are removed by overfishing, predict what will happen to: (i) the sea urchin population, and (ii) the kelp. Use the concept of trophic level to explain your answer.
(b) If kelp is destroyed, predict TWO other species that would likely decline. Explain the food web connections that link them to kelp.
1. A food web is described as more realistic than a food chain. Using evidence from the kelp forest scenario above, explain why.
2. A biomass pyramid and an energy pyramid both get narrower toward the top. Explain what this shape tells us about the number of organisms needed at each level to support the level above.
Wrap Up
In one sentence, what was the main idea of this lesson?