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Rabbits on the Wheat Farm: How a Virus Lost Its Grip

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Rabbits on the Wheat Farm: How a Virus Lost Its Grip

Use test results, farm counts and a model to explain a change in a rabbit population.

Part 1: Testing the rabbits

Read the passage and Table 1, then answer questions 1–5.

The Notebooks in the Farm Office

1Eighth grader Imogen Tsakalos lives on her family's wheat and sheep farm in New South Wales, Australia. Wild European rabbits were brought to Australia in 1859, and within about 50 years they had spread across much of the country. Rabbits graze the same pasture the sheep need, nibble young wheat and dig burrows that can collapse under farm machines. Every winter, Imogen's great-grandfather drove the same 10 km farm track at night with a spotlight and counted the rabbits he saw. His notebooks are still in the farm office. Imogen wanted to know why his counts dropped so suddenly after 1950 and then slowly climbed again, so she began reading about the scientists who studied Australia's rabbits.

2In 1950, scientists released the myxoma virus to control the rabbits. Mosquitoes and fleas carry the virus from rabbit to rabbit, so outbreaks happen mostly in summer, when mosquitoes are common. In the first outbreaks, about 99 out of every 100 infected rabbits died. Rabbits grow up fast: a female can have young when she is about 5 months old and can raise several litters a year.

3Scientists soon noticed that the virus in the wild was changing too, into milder strains that killed fewer rabbits. To measure only the rabbits' side of the story, they kept one sample strain of the virus and tested with it every time. This test strain killed about 9 out of 10 rabbits that came from places the virus had never reached. Each year they caught young wild rabbits from one district, infected them with the test strain and recorded how many died. They also tested young rabbits each year from an area the virus had not reached, using the same test strain.

4In a separate test, they raised young rabbits whose parents had survived an outbreak and young rabbits whose parents came from the virus-free area. Both groups of young lived in the same pens on the same food and had never been infected. When they were infected with the test strain, 55% of the young of survivors lived, but only 10% of the young from the virus-free area lived. Both groups had about the same number of young rabbits.

Table 1: Young Wild Rabbits Infected With the Test Strain
Outbreak summers so farRabbits testedRabbits that diedVirus-free area: % died
0504590
2504492
3403490
4503091
5402090
7501392
The first column counts the outbreak summers that the tested rabbits' district had been through. Table 1 shows results like the ones the scientists reported, with simplified numbers.
1.
Use Table 1. Watch the number of rabbits tested in each row. Write the percent of the tested rabbits that survived.

After 0 outbreak summers: % survived. After 7 outbreak summers: % survived.

2.
Use Table 1. Write the percent of the tested rabbits that survived.

After 3 outbreak summers: % survived. After 4 outbreak summers: % survived.

3.
Use paragraph 3. Why did the scientists test every rabbit with the same test strain, not with the virus they found in the wild that year?
Question 3 options
4.
Look at the last column of Table 1. Why are the rabbits from the virus-free area useful to the scientists?
Question 4 options
5.
Use paragraph 4. Which conclusion does the test of the young rabbits support best?
Question 5 options

Part 2: Spotlight counts and a survival model

Use the bar graph, Table 1 and the model to answer questions 6–10.

Rabbits Seen on the 10 km Farm Track02040608010012019491951195419571960Winter countRabbits seen
6.
Use the bar graph from the great-grandfather's notebooks.

In 1949 he saw times as many rabbits as in 1951. By 1960, the count was back to % of the 1949 count.

7.
Which explanation of the rise in the spotlight counts from 1951 to 1960 fits Table 1 and paragraph 4 best?
Question 7 options
Survival model: One summer outbreak kills 95% of the rabbits that are not resistant and 40% of the rabbits that are resistant. Every survivor then has young, and the young have the same trait as their parents.
8.
Use the model. A district starts the summer with 1,000 rabbits, and 100 of them are resistant. Round the percent to the nearest whole number.

Rabbits that survive the outbreak: rabbits. Percent of the survivors that are resistant: %

9.
Use the model again. A few years later, the district starts a summer with 1,000 rabbits, and 600 of them are resistant. Round the percent to the nearest whole number.

Rabbits that survive the outbreak: rabbits. Percent of the survivors that are resistant: %

10.
Compare your answers to questions 8 and 9. What does the model show about the outbreaks over time?
Question 10 options

Part 3: Putting it together

Use everything on this page to answer questions 11 and 12.

11.
Sort each statement by whether the data on this page support it.
12.
A neighbor tells Imogen, "The rabbits just got used to the virus. Each rabbit's body learned to fight it after being bitten by enough mosquitoes." Make a claim about what caused the change in the rabbit population. Use at least two calculations or numbers from this page as evidence, and explain your reasoning about survival, reproduction and inheritance.