The RED dots are foxes. The WHITE dots are rabbits. The GREEN dots are grass.
The foxes hunt the rabbits. The rabbits graze on the grass and run from the foxes. Both animals will reproduce if they have eatten enough, and starve to death (disappear) if they don't eat enough. The grass regrows from nearby grasses over time.
I learned two interesting things from this excersize.
Second, I spent hours and hours trying to tune all the variables along the way so the ecosystem would be balanced (i.e. no species would die out) and totally failed. I learned that it is really really hard to get even a trivial, controlled ecosystem to sustain itself; we live on an amazing planet.