Year 8 Science · Unit 1 · Lesson 22
Challenge Worksheet
Learning Goals
Find the errors
This paragraph contains exactly three errors. Underline each one and write the correction below.
A student’s summary
A population is all the different species living in one area. Its size depends on births, deaths, immigration and emigration. If dingo numbers go up, kangaroo numbers will probably go up too, because more dingoes means more animals in the area. Cane toads have done so well in Australia because many native predators can eat them safely.
1. Write the three corrections.
Scenario: wallabies in a fenced reserve
Rangers keep records of a rock-wallaby population inside a fenced reserve. Table 1 shows one year of records.
| Record | Number of wallabies |
|---|---|
| Population at the start of the year | 240 |
| Births | 60 |
| Deaths | 35 |
| Immigration (moved in through a gate) | 12 |
| Emigration (moved out through a gate) | 5 |
Source: invented numbers for this worksheet.
2. Calculate the population at the end of the year, and the percentage change over the year. Show your working.
End population Change %
3. The next year a fox gets through a hole in the fence. Births, immigration and emigration are the same as in Table 1, but deaths rise to 95. Calculate the new end-of-year population, then explain which factor changed and why.
New population wallabies
Sketch a population curve
A new arrival
A few mice reach an island that has plenty of seeds and no predators or diseases that affect mice. After several years the mice have eaten most of the seeds.
4. Sketch the likely shape of the mouse population over time. Label the rapid rise, the peak, and what happens after the seeds run short.
5. Mark an X on your sketch at the time when removing the mice would be easiest. Explain your choice using what makes introduced species hard to remove.
Someone claims
“We should just release a virus to kill all the cane toads.”
6. Use Claim, Evidence and Reasoning to evaluate this proposal. Use facts from the lesson about introduced species, ecosystems and unintended consequences, including what happened when cane toads themselves were released to control a pest.
Find the errors
1. (i) A population is all the organisms of one species in an area, not all the different species. (ii) Dingoes are predators of kangaroos, so if dingo numbers go up, kangaroo numbers are likely to fall. (iii) Cane toads have toxic skin, so native predators that eat them are poisoned; that is why toads have few predators. (The births, deaths, immigration and emigration sentence is correct.) [3 marks: 1 per correction]
Your Turn
2. 240 + 60 − 35 + 12 − 5 = 272. Change = 272 − 240 = 32; 32 ÷ 240 × 100 = 13.3% increase (accept 13%). [3 marks: 1 working, 1 end population, 1 percentage]
3. 272 + 60 − 95 + 12 − 5 = 244 wallabies (a fall of 28). Deaths increased because the fox is a predator that kills wallabies; now deaths (95) plus emigration (5) are greater than births (60) plus immigration (12), so the population shrinks. [3 marks: 1 working, 1 answer, 1 explanation]
Sketch a population curve
4. Axes labelled (population, time). The curve starts low, rises slowly then very steeply (rapid rise), reaches a peak, then crashes or falls to a lower, roughly stable level when the seeds run short. [3 marks: 1 rapid rise, 1 peak, 1 crash or levelling after food runs short]
5. X at the very start, before the rapid rise. There are only a few mice in a small area; later they are widespread and breeding quickly, which makes removal extremely difficult. [2 marks: 1 position, 1 reason]
Show What You Know
6. Marking guide: 1 clear claim (disagree or partly agree is best supported); 1 cane toads were released in 1935 to control cane beetles, failed at that job and became a pest, so releasing something to control a pest can backfire; 1 a virus might also infect native frogs or other animals (an unintended consequence); 1 food-web effect (for example, if toads vanish suddenly, or if the virus harms native frogs, predators and prey that depend on them change); 1 toads are hard to remove because they are widespread across northern Australia, breed quickly and have few predators; 1 reasoned conclusion (only with careful testing on native species first, and alongside other long-term control). [6 marks]
Common error to watch for: assuming a released control species or virus will only affect its target. The cane toad itself was a control that went wrong.