Here's my favorite project from A. K. Dewdney's book "The Armchair Universe: An Exploration of Computer Worlds"

The planet Wa-Tor is entirely covered with water. The only two occupants of this watery world are sharks and fish. The sharks eat the fish and the fish exist on a never-ending supply of plankton.

As you will see, this simple ecology is highly dynamic with both sides walking a thin line between continuing life and species extinction.

• The fish rely on their superior breeding skills to supplant their numbers.
  Of the many new born, few make it to adulthood.
• The sharks rely on their formidibile hunting skills to fulfill their never-ending
  quest for food and survival. Gluttony is a sin, and should the sharks eat
  all the available fish, judgement day will surely come a day sooner.

With this simulation you can modify the biological variables of the shark and fish populations. Can you create a self-sustaining ecology? Or is Wa-Tor destined to be a dead life-less planet?