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# What if the dingo disappeared?

Year 7 Biology · Food webs and ecosystems

<!-- Speaker note: 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). About 25 minutes (Engage and Explore 9 min · Explain 9 min · Evaluate 3 min · Typical adds 2 min, Wider about 4 min). -->

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## Today

**We are learning to** a biotic factor is a living part of an ecosystem. When one population changes, the effect ripples along the food web, beyond the species directly linked to it. Tracing the links lets you predict the cascade.

- 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.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it does not replace a real case study with population data. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge risk is stopping at the first link ("dingoes gone, so kangaroos go up"). The board asks for a prediction for every population before it shows the reasons. Model notes: the board's cascade follows each feeding link outward, nearest effects first, and says "could go either way" when effects at the same distance disagree. Real outcomes depend on how much each species relies on each food, competition and time. The dingo, fox and bandicoot story is a real pattern (mesopredator release) studied in Australia; the details vary between regions. Teacher: In your own words: the link-by-link explanation is the evidence. -->

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## Why it matters

> 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?

<!-- Speaker note: You can build a web and follow its energy. This lesson uses the web to predict. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**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

<!-- Speaker note: What happens: They decrease. Without dingoes, foxes increase, and foxes eat bandicoots. The effect travels along the web, two links away. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: most choose 'increase' or 'no change' for bandicoots. Keep the vote for Evaluate. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-7%2Fcascades%2Fstep-2&code=widget%3A+foodweb%0Amode%3A+remove%0Aremove%3A+D%0Aorganism+G%3A+Grass%0Aorganism+I%3A+Insects%0Aorganism+R%3A+Kangaroo%0Aorganism+A%3A+Rabbit%0Aorganism+N%3A+Bandicoot%0Aorganism+X%3A+Fox%0Aorganism+D%3A+Dingo%0Aeats+I%3A+G%0Aeats+R%3A+G%0Aeats+A%3A+G%0Aeats+N%3A+I%0Aeats+X%3A+A%2C+N%0Aeats+D%3A+R%2C+X%2C+A%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-7/cascades/?full=step-2#step-2)

<!-- Speaker note: Press **Remove the dingo**. Then click each organism to choose **↑ more**, **↓ fewer** or **– little change**. When you have predicted all of them, press **Check my predictions** and read the reason for each. -->

<!-- Speaker note: Teacher: students must predict every population before Check unlocks. Ask a student to explain one ✗. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* They decrease. Without dingoes, foxes increase, and foxes eat bandicoots. The effect travels along the web, two links away.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Biotic factors

**Biotic or abiotic?**

Groups: Biotic (living) · Abiotic (non-living)

A new predator arriving · A disease spreading through rabbits · A drought · Competition between kangaroos and rabbits for grass · The temperature dropping · A plant species dying out

* 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)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the parsons puts the cascade in order; the 'either way' question shows the limits of prediction. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## The chain of effects

**Put the cascade in order.**

```science
Bandicoot numbers fall
More foxes hunt more bandicoots
Dingoes are removed
With fewer bandicoots eating them, insects have one less predator
Foxes are no longer hunted by dingoes, so fox numbers rise
```

<!-- Speaker note: Each step follows one link in the web: that's how an effect reaches species that never met the dingo. Correct order: 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 -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: Two effects pull opposite ways: less grass (their food) but fewer bandicoots (which eat them)

<!-- Speaker note: Real outcomes depend on which effect is stronger. Scientists would need to measure. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Mix it up

**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

* Answer: From the rabbit to the fox

<!-- Speaker note: Arrows point from the food to the eater, the way energy flows. Tracing a cascade follows them one link at a time. What this step is for: A cascade is followed along the arrows, so the rule for which way an arrow points has to be secure. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: wolves in Yellowstone

**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

* Answer: Fewer elk eating young trees meant more trees, which beavers use for food and dams

<!-- Speaker note: Wolves → fewer elk → more trees → more beavers: a three-link cascade, observed in the field. (Scientists still debate how strong each link was.) What this step is for: A famous real cascade, with evidence, connects the model to observed ecology. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: your own removal

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-7%2Fcascades%2Fstep-6&code=widget%3A+foodweb%0Amode%3A+remove%0Aorganism+G%3A+Grass%0Aorganism+T%3A+Gum+tree%0Aorganism+H%3A+Grasshopper%0Aorganism+K%3A+Koala%0Aorganism+L%3A+Lizard%0Aorganism+B%3A+Kookaburra%0Aorganism+S%3A+Snake%0Aorganism+E%3A+Wedge-tailed+eagle%0Aeats+H%3A+G%0Aeats+K%3A+T%0Aeats+L%3A+H%0Aeats+B%3A+L%2C+H%0Aeats+S%3A+L%0Aeats+E%3A+S%2C+B%2C+K%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-7/cascades/?full=step-6#step-6)

<!-- Speaker note: Pick any plant or animal to remove. Which removal do you think changes the most populations? Try a producer and a top predator. What this step is for: Choosing which species to remove and predicting the whole web tests transfer to an unfamiliar web. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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 · Sheep graziers could lose stock, and income, to dingo attacks · Keeping thousands of kilometres of fence in repair costs a lot every year · Dingoes matter in many First Nations cultures, and Traditional Owners want a say in how they are managed · Is it fair to kill a native predator to protect introduced livestock?

* 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

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: Weigh its likely effects on native animals, farmers' incomes, communities and fairness, using evidence from places where it has been tried

<!-- Speaker note: Scientific responses to an issue have impacts on many groups. Good decisions use evidence and weigh environmental, economic, social and ethical considerations together. -->

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## 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.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Foxes are controlled with baits in a reserve. Predict one effect two links away, and explain.**

* For example: fewer foxes → more bandicoots → fewer insects (more bandicoots eating them).

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: models, limitations and predictions (node 5). The board's model is one example: what does it leave out? In class: a dingo fence case study (populations on each side of the fence), or local invasive species data. -->