Year 8 Science · Unit 1 · Lesson 22
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Learning Goals
Match each word to its meaning
Write the matching letter in the middle column. Each letter is used once.
| Word | Letter | Meaning |
|---|---|---|
| Population | A. An animal that hunts and eats other animals. | |
| Introduced species | B. All the organisms of one species living in one area. | |
| Native species | C. Two species trying to use the same food or shelter. | |
| Predator | D. Individuals moving into an area from somewhere else. | |
| Competition | E. A species brought by humans to a place where it did not naturally live. | |
| Immigration | F. A species that occurs naturally in an area without human help. |
Odd one out
In each row, circle the one that does not belong. Give the reason.
| Row | Examples | Reason |
|---|---|---|
| 1 | rabbit · cane toad · wombat · red fox | |
| 2 | a new disease · plenty of food · few predators · a mild climate | |
| 3 | aerial counts · trapping · surveys · releasing a virus |
1. When reading population data, scientists look for three things. Name them.
Classify with a reason: how is the native species harmed?
Write predation, competition or poisoning for each example. Each word is used twice.
| Example | Type of harm |
|---|---|
| A quoll dies after biting a cane toad. | |
| Rabbits eat the plants that bilbies also need. | |
| A fox kills and eats a small native mammal. | |
| Cane toads eat insects that native frogs also eat. | |
| A snake dies after swallowing a cane toad. | |
| A feral cat hunts a native bird. |
2. Choose one “competition” example from the table. Explain how it could make the native population smaller even though no native animal is killed directly.
Read the data
Scenario: toads reach a river track
Rangers counted yellow-spotted goannas and small lizards along one river track in northern Australia each dry season. Cane toads were first seen on the track in Year 2.
| Year | Yellow-spotted goannas | Small lizards |
|---|---|---|
| 1 (no toads) | 25 | 110 |
| 2 (toads first seen) | 23 | 115 |
| 3 | 10 | 140 |
| 4 | 6 | 170 |
| 5 | 5 | 185 |
Source: made-up survey numbers that follow the pattern described in the lesson.
3. Calculate the percentage fall in goannas from Year 1 to Year 5.
Answer %
4. Describe the trend in goanna numbers, and state between which two years the rate of change was fastest.
5. A ranger says: “The toads must be good for small lizards.” Use the data to explain what is really happening, and name this kind of effect.
6. Rabbits live in Europe too, but their numbers there did not explode the way they did in Australia. Explain why. Use the ideas of predators, competition and available resources, and say what this tells us about how ecosystems normally keep a population in check.
Match each word to its meaning
Population → B; Introduced species → E; Native species → F; Predator → A; Competition → C; Immigration → D.
Odd one out
Row 1: wombat (native; the others are introduced). Row 2: a new disease (it makes a population smaller; the others help it grow). Row 3: releasing a virus (it is a way to kill a pest; the others are ways to count a population).
1. The trend (up, down or stable), the rate of change (fast or slow), and patterns (such as seasonal cycles or sudden drops). [3 marks]
Classify with a reason: how is the native species harmed?
Quoll bites toad: poisoning. Rabbits and bilbies: competition. Fox and mammal: predation. Toads and frogs: competition. Snake swallows toad: poisoning. Cat and bird: predation.
2. Example: rabbits eat the plants bilbies need, so bilbies have less food and shelter. Fewer bilbies survive and breed, so births fall and the population shrinks without any bilby being killed. (Frog example: native frogs get fewer insects, so fewer survive and breed.) [2 marks: 1 less resource, 1 effect on survival or births]
Read the data
3. Fall = 25 − 5 = 20; 20 ÷ 25 × 100 = 80%. [2 marks]
4. Goanna numbers crashed after toads arrived, then levelled off at a low number. The fastest fall was between Year 2 and Year 3 (23 to 10). [2 marks]
5. The toads did not help the lizards directly. Goannas died from eating the toxic toads; goannas hunt small lizards, so with fewer goannas fewer lizards were eaten and lizard numbers rose (110 to 185). This is an indirect effect. [3 marks: 1 goannas poisoned, 1 fewer predators of lizards, 1 named indirect effect]
Show What You Know
6. Marking guide: 1 in Europe predators and disease kill many rabbits; 1 in Australia almost no predators could control them; 1 mild climate and abundant food meant births far outnumbered deaths; 1 rabbits competed with native herbivores such as bilbies and took their resources; 1 rule: populations are normally kept in check by predators, disease, competition and limited food, and removing these lets numbers explode. [5 marks]
Common error to watch for: saying rabbits exploded because they are “better” animals. The difference is the missing controls in the new ecosystem, not the rabbits themselves.