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Trial 2 - Population Explosion

1. What would happen to the population if the sheep had as much grass as they wanted?
   
   

2. Try this with the model. Open the model. See Technical Hints to run and save the evolution model.

   [new model: populationB]

3. Change the LIMITED-GRASS? Switch to OFF. Then there is unlimited grass for the sheep to eat. Click on SETUP. Click on GO-ONCE five times. As the number of sheep gets very large, the model will run more slowly! What happens to the population?
   
   
4. Start again by clicking on SETUP. Try using the REAPER button, which reduces the herd at the end of each year. The NUMBER-REMOVED slider sets how many sheep are removed by the REAPER button. Run the model with the GO-FOREVER button. Can you keep the population under control? How?
   
   

5. What you just did with the REAPER button is what must happen in nature to keep a population stable. Many animals die every year!

6. Now try changing the variable BIRTHRATE-%. This is the chance that a female will have a baby once a year. Try to keep the population stable even when the GO-FOREVER button is on. If the population gets out of control, stop, hit SETUP, and start again. What is a value of BIRTHRATE-% that keeps the population roughly steady when the grass growth is unlimited?
   
   
7. Notice that the effect of changing the birthrate is delayed. Why is this?
   
   
8. What is the lowest value of BIRTHRATE-% for which the herd doesn't die off?
   
   

9. Compare your results with the rest of the class.

10. If the sheep only died of old age, the birthrate for a constant population -- called the "replacement rate" -- should be about 50%. Here's why.
    1. For a 100% birthrate, each female has one baby a year for four years -- a total of 4 babies in her lifetime.
    2. For a 50% birthrate, each female has 2 babies in her lifetime.
    3. Since the male doesn't have babies, the 2 babies just replace the mother and the father.
    Is your minimum birthrate greater or less than 50%?
    
    
11. Many animals -- for example, mosquitoes and fish -- may lay thousands of eggs in one year. Why might this be a good survival strategy? Why don't they take over the world?