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.
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?
You can build a web and follow its energy. This lesson uses the web to predict.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Practise: A few on your own, to check you have it.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: remove the dingo
ExploreTry it on the board and compare what happens with your prediction.
Explore: predict, then check
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.
Try it yourselfYour edits are saved in this browser.
Remove the dingo, predict every other population, then check.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: tracing a cascade
In your book: explain why removing dingoes can lead to fewer bandicoots, one link at a time.
Biotic or abiotic?
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
Put the cascade in order.
Pieces (click to add)
Your program
PractiseA few on your own, to check you have it.
Mix it up
Elaborate TypicalUse the same idea in a new context.
Elaborate: wolves in Yellowstone
Elaborate WiderUse the same idea in a new context.
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.
Try it yourselfYour edits are saved in this browser.
Choose a species to remove, predict every other population, then check.
Use the board. This ticks when you get there.
Elaborate TypicalUse the same idea in a new context.
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?
Some ecologists propose letting dingoes back into parts of the land inside the fence. What kind of consideration is each point?
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 yearMoney spent or saved is an economic consideration, even when the spender is a government.
Dingoes matter in many First Nations cultures, and Traditional Owners want a say in how they are managedWho is heard, and whose culture is affected, is a social consideration.
Is it fair to kill a native predator to protect introduced livestock?A question about what is right or fair is ethical, even when it also has costs.
EvaluateLook back at your first ideas and what changed.
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.
For teachers
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.
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.
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.
Running the lesson
Engage: most choose 'increase' or 'no change' for bandicoots. Keep the vote for Evaluate.
Explore: students must predict every population before Check unlocks. Ask a student to explain one ✗.
Explain: the parsons puts the cascade in order; the 'either way' question shows the limits of prediction.
In your own words: the link-by-link explanation is the evidence.
The hard part (on purpose)
Following the effect past the first link. When a student stops at the dingo's prey, ask what eats that animal, and what it eats.
When students get stuck
A student predicts no change for anything not eaten by the dingo. Ask: What eats bandicoots? What happened to that animal? Fix: Foxes eat them, and foxes went up. Follow the link.
A student thinks removing a predator helps everything. Ask: What happens to the grass when kangaroos and rabbits increase? Fix: It's eaten more: fewer. Effects alternate up and down the web.
A student is frustrated by 'could go either way'. Ask: What would you need to measure to decide? Fix: Which effect is stronger: that's what field studies find out.
Stretch and support
Stretch: Cane toads poison the quolls and goannas that eat them. Draw a small web and predict the cascade.
Support: Give a cascade frame: 'X went … so Y (which eats / is eaten by X) went … because …'.
Where it 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.
What each step is for
Engage: remove the dingo: Students usually predict only the immediate effect (prey increase); the cascade to bandicoots is the reasoning the node targets.
Explore: predict, then check: Committing to a prediction for every population before seeing the model's reasons makes students trace each link themselves.
Explain: tracing a cascade: Naming biotic factors and tracing a cascade step by step turns the board's reasons into the student's own reasoning.
Mix it up: A cascade is followed along the arrows, so the rule for which way an arrow points has to be secure.
Elaborate: wolves in Yellowstone: A famous real cascade, with evidence, connects the model to observed ecology.
Elaborate: your own removal: Choosing which species to remove and predicting the whole web tests transfer to an unfamiliar web.
Elaborate: the dingo fence: Weighing a real Australian choice against several kinds of consideration shows that a scientific response helps some people and costs others.
Now you can
Define a biotic factor and give three examples.
Predict the effect of removing one species from a food web, with reasons.
Trace at least one effect beyond the species directly linked to the change.