Based on an article by A. K. Dewdney ("Sharks and Fish on the Planet Wa-Tor"), this is a simple ecology simulation.
In this simulation, fish swim and reproduce while sharks swim, reproduce, and eat the fish.
About the parameters...
Breed Time: How old a fish or shark needs to be before it will reproduce. (On Wa-Tor, fish and sharks all reproduce by parthenogenesis, like amoebae.)
Starve: How long a shark can go without eating before it will starve to death.
Initial Population: The total number of each species with which to start the simulation.
World Size: The world wraps around so that, when a fish or shark swims off an edge, they come back on the opposing edge. The world can be anywhere from 2x2 to 20x15.
What parameters will provide a self-sustaining eco-system in which the sharks never hunt the fish to extinction but never starve to extinction themselves?
The original program Dewdney described in the article, back in the mid-1980's, used a simple text display in a grid with 0 . and * to indicate sharks, water, and fish.
Update: The simulation has been updated to remove the upper limit on world size. Very large worlds may cause the simulation to slow down but think about experimenting with the effect using larger worlds or long, thin worlds.