Population Interactions — Predator/Prey, Competition
In 2015, Parks Australia researchers found that removing just 2,000 feral cats from Macquarie Island caused rabbit numbers to jump from 400 to over 130,000 within 5 years — a real predator-prey crash in action.
Printable Worksheets
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Q1 · If there are suddenly twice as many rabbits in a paddock, what do you predict will happen to the number of foxes over the next year? Why?
Q2 · A clownfish lives inside a sea anemone. The anemone stings most fish — but not the clownfish. What might each species GET from this arrangement?
● Know
- Predator-prey populations rise and fall in linked cycles (with a time delay)
- Competition can happen within a species (intraspecific) or between species (interspecific)
- Three types of symbiosis: mutualism (++), commensalism (+0), parasitism (+−)
● Understand
- Why predator numbers rise AFTER prey numbers rise (not at the same time)
- Why two species competing for the same food usually cannot both win
- How to identify which type of symbiosis a relationship is
● Can do
- Read a predator-prey graph (such as lynx–hare data) and describe the cycle
- Classify a real-world relationship as mutualism, commensalism or parasitism
- Predict what happens to other populations when one population changes
- Population
- Competition
- Mutualism
- Commensalism
- Parasitism
- Both species benefit (++)
- One species benefits, the other is harmed (+−)
- All members of one species in one area
- One benefits, the other is unaffected (+0)
- Two organisms need the same limited resource
Watch a nature documentary about wolves and deer: when deer numbers boom one spring, wolf cubs born that summer have more food and more survive — but by the next spring there are so many wolves that deer numbers crash, and then the wolves start starving. The classic dataset comes from Canadian fur-trapping records of snowshoe hares (prey) and the Canadian lynx (predator) over 90 years. It shows a roughly 10-year cycle:
| Phase | Hare numbers | Lynx numbers | What's happening |
|---|---|---|---|
| 1 | Rising ↑ | Low, steady | Lots of plants, hares breeding fast. |
| 2 | High ↑ | Rising ↑ (after a delay) | Lynx have plenty of hares to eat, breed more, more young survive. |
| 3 | Falling ↓ | High | Too many lynx eating too many hares. |
| 4 | Low | Falling ↓ (after a delay) | Lynx run out of food; many starve. |
| (back to 1) | Rising ↑ | Low, steady | With fewer lynx, hares breed up again. |
The key word is delay. Predator numbers don't change at the same time as prey — they follow a step behind. A new generation of lynx kittens takes a year to be born and survive. When hares crash, it takes another season for lynx numbers to drop.
When hare numbers , lynx have more food, so lynx numbers also rise — but after a . Eventually so many lynx hares that hare numbers fall, and then lynx numbers fall too. The whole cycle repeats every years or so.
Wrong: "When hares rise, lynx instantly rise too." There is always a delay — predators have to breed and their young have to survive, which takes months or years. The lynx graph is shifted to the right compared to the hare graph.
Right: Predator changes always lag behind prey changes because it takes time to breed and raise young.
Wrong: "All close relationships between species are mutualism." That's not true. The clownfish + anemone is mutualism (++), but a tick on a dog is parasitism (+−), and a barnacle on a whale is commensalism (+0).
Right: Always check: who benefits? Who is harmed? Use that to decide if it's mutualism, commensalism or parasitism.
Wrong: "Competition only happens between different species (e.g. rabbits vs kangaroos)." Members of the same species also compete heavily — sometimes the strongest competition is between siblings for the same food.
Right: Competition can be intraspecific (within the same species) or interspecific (between different species).
Whenever two organisms need the same limited resource — food, water, space, a mate — there is competition. Two main types:
- Intraspecific competition (within the same species). Example: two male kangaroos boxing for the right to breed. They want the same mate, but only one can win.
- Interspecific competition (between different species). Example: rabbits and kangaroos both eating grass in the same paddock. If grass runs short, only one species can keep its numbers up.
Competition is a powerful force in ecosystems. Introduced species like the European rabbit have caused huge problems in Australia because they out-compete native marsupials for grass and burrows.
Symbiosis means a close, long-term relationship between two species. There are three main kinds, sorted by who wins and who loses:
| Type | Effect | Example | What each side gets |
|---|---|---|---|
| Mutualism | ++ | Clownfish + sea anemone | Clownfish gets protection inside stinging tentacles; anemone gets cleaned and gets scraps of food. |
| Commensalism | +0 | Barnacles on a whale's skin | Barnacles get a free ride through nutrient-rich water; the whale is unaffected. |
| Parasitism | +− | Tick on a dog | Tick gets blood (a food source); the dog is harmed (itchiness, possible disease). |
Tip: to classify a relationship, ask of EACH species: "Does it benefit, get harmed, or is it unaffected?"
Symbiosis is everywhere in Australian ecosystems:
- Mutualism: Honey-eating birds (like rainbow lorikeets) feed on nectar from gum-tree flowers and pollinate them in return. Both benefit.
- Commensalism: A small marsupial like a bandicoot uses a wombat's old burrow as shelter. Bandicoot gets a home; wombat is not affected.
- Parasitism: The cane toad's tadpoles can be parasitised by tiny lungworms. Lungworm gets food (toad tissue); toad is weakened or killed.
- Parasitism (plant): Mistletoe grows on gum-tree branches and takes water and nutrients from the tree. Mistletoe gains; tree is weakened.
The key skill for Year 7 is sorting these by their effect — not by whether they look "nice".
In a paddock, rabbit numbers suddenly double after good rainfall. Predict (a) what will happen to the fox population over the next year and (b) what will happen to grass cover. Lock in your prediction, then reveal.
How close was your prediction?
Excellent — you spotted the delay AND the effect on grass.
Good — the cascade goes both up (predators) and down (food).
At the start of the lesson you were asked: when lots of hares are born, lynx numbers go up — but only after a delay. Why doesn't it happen instantly?
Now you understand the predator-prey cycle, write your full answer. What has to happen first before lynx numbers can rise, and why does that take time?
Q1. Define mutualism, commensalism and parasitism. Give one example of each. (3 marks)
Q2. Describe what happens in a predator-prey cycle. Use the lynx–hare example and explain WHY there is a delay between changes in the two populations. (4 marks)
Q3. European rabbits were introduced to Australia and now compete with native kangaroos and wombats for grass. Use the ideas of interspecific competition AND predator-prey cycles to predict TWO effects on the native ecosystem. (4 marks)
Answers
▾MCQ 1
C — Predator numbers rise after a delay because it takes time for predators to breed and raise young. A and B are not what the data show; D is wrong (they do change).
MCQ 2
A — Both clownfish and anemone benefit, so this is mutualism (++). The clownfish gets protection inside the stinging tentacles; the anemone gets cleaned and scraps of food.
MCQ 3
D — The tick benefits (gets blood) and the dog is harmed (itchiness, potential disease), so this is parasitism (+−).
MCQ 4
B — Both kangaroos are the same species, so this is intraspecific competition. Interspecific would be two different species (e.g. rabbits vs kangaroos).
MCQ 5
C — One species (barnacles) benefits, the other (whale) is unaffected, so this is commensalism (+0). Not mutualism (whale gets no benefit), not parasitism (whale is not harmed).
Short Answer 1
Model answer: Mutualism is a relationship where both species benefit (++) — e.g. clownfish + sea anemone. Commensalism is where one species benefits and the other is unaffected (+0) — e.g. barnacles on a whale. Parasitism is where one species benefits and the other is harmed (+−) — e.g. a tick on a dog, or mistletoe on a gum tree. 1 mark for each definition with a correct example.
Short Answer 2
Model answer: In a predator-prey cycle, when prey (hare) numbers rise, predators (lynx) have more food, so they breed more successfully and lynx numbers rise too — but after a delay of several months. Eventually so many lynx are eating hares that hare numbers crash; then lynx start to starve and lynx numbers also fall (again after a delay). With fewer lynx, hares can breed up again and the cycle starts over. The delay happens because it takes time for predators to breed and for their young to grow up before they too start hunting. 1 mark for prey rises → predator rises, 1 for prey crash → predator crash, 1 for "after a delay", 1 for clearly explaining the delay (breeding time).
Short Answer 3
Model answer: Effect 1: Rabbits compete with native kangaroos and wombats for the same grass (interspecific competition). Where rabbit numbers are high, kangaroos and wombats may lose food and decline in numbers. Effect 2: Rabbits are also prey for dingoes and wedge-tailed eagles, so predator numbers may rise (predator-prey cycle). With more predators around, native prey species like bandicoots also come under more pressure — even though they aren't the rabbit's direct competitor. So introducing rabbits affects native species both through direct competition AND by inflating the predator population. 1 mark for naming competition correctly, 1 for predicting native decline from it, 1 for naming predator-prey link, 1 for the knock-on effect on other native prey.