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Further Investigations

• Move GRASS-REGROWTH-RATE to a high value and let the population settle down. Try moving it to a low value gradually and identify the lowest value where the population can still survive. Then start again at a high value, but this time move to the same low value quickly. Does the population survive? What's the difference in how the population responds to each situation?
  
• Think of an example of a wild animal (or animals) in your local area. Write an account of how you think its population level is kept roughly constant. What changes might cause it to expand a great deal, or disappear?
  
• You may have heard of "invasive species" -- plants or animals that are not native to an area but arrive there and sometimes spread very fast. Here are several examples that you can investigate. As you do, think about these questions:
  • How was the invasive species introduced?
  • What kept it under control in the habitat from which it came?
  • Why did it spread so fast?
  • How have people tried to stop it?
  • Which measures have been successful, if any?

  1. A famous example is the introduction of wild rabbits in Australia. There was adequate food but no natural predator. You can read about it at:
     http://rubens.anu.edu.au/student.projects/rabbits/home.html

  2. Hawaii has several introduced species, such as mongooses and pigs. Look under mongoose at:
     http://www.birdinghawaii.co.uk/nonavianground2.htm

  3. Kudzu has spread throughout the southeastern US. See:
     http://www.cptr.ua.edu/kudzu/
     http://www.jjanthony.com/kudzu/

  4. For zebra mussels in the Great Lakes, see:
     http://www.great-lakes.net/envt/flora-fauna/invasive/invasive.html