Published by Australian Academy of Science
Activity 1 | A plague on the pest – rabbit calicivirus disease and biological control

Graphing and interpreting variation in rabbit numbers

A farmer counted rabbits on his property in early autumn every year using the same method each time. Early in the evening he and his daughter drove 10 kilometres along a track that followed a creek. They counted all the rabbits seen within 50 metres either side of the track. Each year they calculated the average number of rabbits seen per kilometre of track travelled. The following table shows the numbers of rabbits counted for each of 15 successive years.

Year	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
Rabbits per kilometre	40	60	80	20	10	15	25	10	15	30	10	30	60	80	100
Rabbits per hectare	4	6	8	2	1	1.5	2.5	1	1.5	3	1	3	6	8	10

1. Graph the rabbit density per hectare. How do you explain the apparent existence of half rabbits in years 6, 7 and 9?

2. In which year do you think a biological control agent was released?

3. Why did the farmer and his daughter count the rabbits at the same time every year? Suggest reasons why the results might have been different if they had counted rabbits in early summer rather than early autumn.

4. How do you explain the fluctuations in rabbit numbers between years 4 and 11?

5. How do you explain the increase in rabbit numbers in years 13, 14 and 15?

6. A new biological control agent is introduced after year 15 and rabbits become scarce. What do you think the farmer can do to make sure that rabbit numbers stay low?

Teachers notes

1. Some students may not know how the number of rabbits counted per kilometre are converted to rabbits per hectare. The width of the observed strip is about 100 metres. The reported values are rabbits seen per kilometre, so each reported value applies to 100,000 square metres or 10 hectares. Therefore the approximate densities of rabbits per hectare are one-tenth of the number seen per kilometre.

   Although there are many methods of determining population size, this method is useful in this instance because it is relatively easy to do and the object is to compare relative numbers of rabbits between years not to determine the absolute number of rabbits.

2. A biological control agent (such as a strain of myxomatosis) was introduced towards the end of year 3.

3. To make a valid comparison of rabbit numbers, counts must be made at the same time each year. Any counts made in early summer are likely to be high because rabbits in southern Australia usually breed in spring. However, numbers then decrease, because young rabbits are particularly vulnerable to predation, food shortages and disease.

4. The fluctuations could be caused by factors such as:
   • the availability of food and water;
     
   • predation;
     
   • irregular outbreaks of disease;
     
   • irregular outbreaks of the biological control agent.

   Often a combination of these factors is important in causing the fluctuations in numbers. If the biological control agent introduced at the end of year 3 is transmitted by another organism (ie, a vector, such as mosquitoes transmitting myxomatosis), the abundance of the vector will also affect rabbit numbers.

5. Factors that could explain the increase in rabbit numbers:
   • resistance to the biological control agent released in year 3 could build up in the rabbit population;
     
   • changes in the agent could make it less effective in killing the rabbits;
     
   • a very good season with plentiful food produces more breeding than usual, and more rabbits are produced than are eaten by the predators present.

6. Measures to ensure rabbit numbers stay low could include shooting, trapping, poisoning and ripping up warrens.

   When more data on the spread of the calicivirus become available students could compare the rate of the spread and the killing capacity with that of myxomatosis. To prepare for this, students could record the spread of the calicivirus on a map of Australia and collect data from reports in the media as they become available.