--- title: "What if the dingo disappeared?" subtitle: "Year 7 Biology · Food webs and ecosystems · Lesson 6" --- # What if the dingo disappeared? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Where dingoes were removed to protect sheep, kangaroos and foxes boomed, and some small native animals nearly vanished. How could removing one predator harm small animals it rarely ate? :::teacher-note **In one sentence:** Students define biotic factors and predict how removing one species cascades through a food web, tracing effects beyond the species directly linked (AC9S7U02, node 4, a hinge node). **Students should be able to say:** I can define a biotic factor and give three examples. I can predict the effect of removing one species from a food web, with reasons. I can trace at least one effect beyond the species directly linked to the change. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **In a web where dingoes eat kangaroos, rabbits and foxes, and foxes eat rabbits and bandicoots, the dingoes are removed. What do you predict happens to the bandicoots?** ☐ They disappear immediately ☐ They increase: one less predator in the area ☐ No change: dingoes didn't eat them ☐ They decrease My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** They decrease. Without dingoes, foxes increase, and foxes eat bandicoots. The effect travels along the web, two links away. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: tracing a cascade :::keep-together ### Biotic factors **Biotic or abiotic?** Groups: Biotic (living), Abiotic (non-living) - A new predator arriving: {{field-15}} - A disease spreading through rabbits: {{field-15}} - A drought: {{field-15}} - Competition between kangaroos and rabbits for grass: {{field-15}} - The temperature dropping: {{field-15}} - A plant species dying out: {{field-15}} ::: :::teacher-note **Answer:** A new predator arriving → Biotic (living); A disease spreading through rabbits → Biotic (living); A drought → Abiotic (non-living); Competition between kangaroos and rabbits for grass → Biotic (living); The temperature dropping → Abiotic (non-living); A plant species dying out → Biotic (living) ::: :::keep-together ### The chain of effects **Put the cascade in order.** A. `Bandicoot numbers fall` B. `More foxes hunt more bandicoots` C. `Dingoes are removed` D. `With fewer bandicoots eating them, insects have one less predator` E. `Foxes are no longer hunted by dingoes, so fox numbers rise` Correct order (letters): {{field-24}} ::: :::teacher-note **Answer:** see the program below Each step follows one link in the web: that's how an effect reaches species that never met the dingo. ```science Dingoes are removed Foxes are no longer hunted by dingoes, so fox numbers rise More foxes hunt more bandicoots Bandicoot numbers fall With fewer bandicoots eating them, insects have one less predator ``` ::: :::keep-together ### Either way **On the board, the insects could go either way. Why?** ☐ Insects aren't in the food web ☐ Two effects pull opposite ways: less grass (their food) but fewer bandicoots (which eat them) ☐ The board can't tell ::: :::teacher-note **Answer:** Two effects pull opposite ways: less grass (their food) but fewer bandicoots (which eat them) Real outcomes depend on which effect is stronger. Scientists would need to measure. ::: ## Mix it up :::keep-together **Looking back at food webs: a fox eats a rabbit. Which arrow is drawn correctly?** ☐ From the rabbit to the fox ☐ Either way, as long as it is labelled ☐ From the fox to the rabbit ::: :::teacher-note **Answer:** From the rabbit to the fox Arrows point from the food to the eater, the way energy flows. Tracing a cascade follows them one link at a time. ::: :::typical ## Elaborate: wolves in Yellowstone :::keep-together **Wolves were returned to Yellowstone National Park (USA) in 1995. Elk numbers fell and elk stopped grazing near rivers. Willow and aspen trees then grew back along the rivers, and beavers returned. Which explains the beavers?** ☐ Wolves eat beavers' predators ☐ Beavers followed the wolves ☐ Fewer elk eating young trees meant more trees, which beavers use for food and dams ::: :::teacher-note **Answer:** Fewer elk eating young trees meant more trees, which beavers use for food and dams Wolves → fewer elk → more trees → more beavers: a three-link cascade, observed in the field. (Scientists still debate how strong each link was.) ::: ::: :::wider ## Elaborate: your own removal Pick any plant or animal to remove. Which removal do you think changes the most populations? Try a producer and a top predator. ::: :::typical ## Elaborate: the dingo fence The **Dingo Fence** runs about 5,600 km across south-eastern Australia to keep dingoes away from sheep. Studies comparing the two sides found more foxes and kangaroos, and fewer small native mammals, where dingoes are kept out: the cascade you traced. So should the fence stay, move, or let dingoes back in some places? :::keep-together ### Four kinds of consideration **Some ecologists propose letting dingoes back into parts of the land inside the fence. What kind of consideration is each point?** Groups: Environmental, Economic, Social, Ethical - Small native mammals could recover as dingoes keep foxes in check: {{field-15}} - Sheep graziers could lose stock, and income, to dingo attacks: {{field-15}} - Keeping thousands of kilometres of fence in repair costs a lot every year: {{field-15}} - Dingoes matter in many First Nations cultures, and Traditional Owners want a say in how they are managed: {{field-15}} - Is it fair to kill a native predator to protect introduced livestock?: {{field-15}} ::: :::teacher-note **Answer:** Small native mammals could recover as dingoes keep foxes in check → Environmental; Sheep graziers could lose stock, and income, to dingo attacks → Economic; Keeping thousands of kilometres of fence in repair costs a lot every year → Economic; Dingoes matter in many First Nations cultures, and Traditional Owners want a say in how they are managed → Social; Is it fair to kill a native predator to protect introduced livestock? → Ethical ::: :::keep-together ### Judging a plan **A report recommends one plan for the fence. What is the best way to judge it?** ☐ Pick whatever costs the least ☐ Weigh its likely effects on native animals, farmers' incomes, communities and fairness, using evidence from places where it has been tried ☐ Pick whatever helps native animals most; the other points don't matter ☐ Pick the plan the biggest farm wants ::: :::teacher-note **Answer:** Weigh its likely effects on native animals, farmers' incomes, communities and fairness, using evidence from places where it has been tried Scientific responses to an issue have impacts on many groups. Good decisions use evidence and weigh environmental, economic, social and ethical considerations together. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - A **biotic factor** is a living part of an ecosystem: predators, prey, competitors, disease. - Changing one population affects the species linked to it, and then the species linked to those: a **cascade**. - Trace it **one link at a time**: fewer predators → more prey → less of what the prey eats… - When two effects pull opposite ways, the result could go either way; scientists measure to find out. ## Exit ticket :::keep-together **Foxes are controlled with baits in a reserve. Predict one effect two links away, and explain.** {{lines-3}} ::: :::teacher-note **Answer:** For example: fewer foxes → more bandicoots → fewer insects (more bandicoots eating them). :::