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by Ed Hazzard (Submitted: 02/07/2005)

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Download sheep-populationC
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(You can also run this model in your browser, but we don't recommend it; details here.)

Evolution: Sheep-populationC
This is a model of a flock of sheep whose survival depends on eating grass. This version focuses on the issue of extinction.

Here are are some explorations:

1. Picture yourself as a rancher with a large field of sheep. You start with an equal number of males (horns) and females (no horns). They live for six years. The sheep move around the field and eat grass, which grows back at a certain rate. The patch is green if there is grass there, and brown if there is no grass. In the model, the sheep move and eat during the year. They use up energy as they move, and gain energy from eating grass. If their energy goes to zero, they die.

2. Once a year, from age 3 to age 6, each female gives birth to a baby.

3. To run the model, always first hit SETUP. Hit GO-ONCE to run the model for one year. Hit GO-FOREVER to run the model continuously. To stop, hit the same button again. The graph shows the amount of grass (green) and the total population (black) as time goes by.

4. In the long history of life on Earth, many species have come and gone. For example, dinosaurs were around for many millions of years, but now they are entirely extinct. What are some events that could cause extinction of a population?

5. Now observe extinction by changing the variables in this model. Gradually reduce GRASS-REGROWTH-RATE. At what value is extinction quite likely?

6. Return GRASS-REGROWTH-RATE to 80. Gradually reduce INITIAL-NUMBER. You must hit SETUP and start again each time you change it. At what value is extinction quite likely?

7. Return INITIAL-NUMBER to 100. Run the model and gradually reduce GAIN-FROM-FOOD. At what value is extinction quite likely?

Make is a summary of your results. Why might the answers differ from run to run?

Variable Value causing extinction

8. Now that you know the effects of the different variables, here’s a challenge. The sheep rancher wants to sell off as many sheep per year as possible. What’s the largest number he can remove without losing his flock? (hint: use the NUMBER-REMOVED slider to control how many the rancher removes each year.) In this model, the sheep flash blue just before they are removed.
You can change all of the variables. Record the highest NUMBER-REMOVED the rancher can maintain without losing the flock, and the strategy you used to get the highest possible yield.

To study adaptation in this flock of sheep, go on to sheep-selection.

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