Wator is the name of a simple computer simulation used to illustrate the principle of emergant behavior. It was (as far as I know) first published in Scientific American (December 1984) by A.K. Dewdney, and it also appears in his book The Armchair Universe: An exploration of computer worlds].
The basic idea is fairly simple. You imagine (or create inside a computer) a toriodal world covered by a gigantic ocean. There are only two types of creatures on this planet, sharks and fish. The sharks eat the fish, the fish magicly subsist on their own (or eat imaginary plankton, or whatever you want to make up). A simple set of rules is applied to each individual each turn.
Sharks:
- Swim in a random direction.
- Increment both hunger and reproduction counters.
- If they have swam onto a square occupied by a fish, eat the fish and reset the hunger counter.
- If the hunger counter has reached the starvation value, the shark dies.
- If the reproduction counter has reached the reproduction threshold, another shark is spawned in an adjacent square.
Fish:
- Swim in a random direction.
- Increment reproduction counter.
- If reproduction counter reaches its threshold, another fish is spawned (no pun intended) in an adjacent square.
If the system parameters (fish reproduction time, shark starvation time, shark reproduction time) and initial state (usually a large random sprinkling of fish and a small random sprinkling of sharks) are chosen well, the system should settle into a very convincing (but slightly noisy) predator-prey cycle with occasional unusual events but for the most part running in a stable manner for an arbitrary amount of time.
It is also possible to produce unstable parameter sets, for instance if you allow sharks to reproduce quicker than they starve, they will take over and drive fish into extinction.
It is interresting to watch, and it doesn't take long to code up a version of this program and play with it. It has many realistic and amazingly complex behaviors that are all coded for in that simple set of rules. One can also add variations like overcrowding rules, or allowing starving sharks to eat other sharks if no fish are around, etc... The possibilitis are endless.
Anybody who is interrested in learning about artificial life or cellular automation should consider this for a fun and easy way to get their feet wet. Compare to Conway's Life.