The point of living is to survive, not to balance anything out.
To survive, you have to find a niche: food to eat which is either plentiful or is something that nobody else is eating, a way to deter predators from eating you, and a way to raise your offspring without too many of them dying before they have offspring of their own.
A species that cannot sucessfully compete in all of these arenas will die out. A species that competes too sucessfully may eat all its food too quickly (usually through over-population) and die. I’ve heard, but cannot cite, that that is what happened to the sabre-toothed cat–it was TOO good at hunting, and most of 'em eventually starved to death.
Those that die become food for something else that eats dead things.
So, yeah, it looks like there is a “natural balance” but the mechanics imply that every individual species is just getting along the best way it can.