I would add that maintaining balance is often difficult. Maintaining balance in a fishery is often referred to in Fisheries Science studies and literature as like balancing on a knife edge. The normal state is unbalanced one direction or the other.
There is more than one way to look at balance, I suppose. If Fisheries Science, I guess it means that growth of predator and prey are uninhibited. But I don't see balance this way and I can't think of any biologist other than a fisheries biologist that would. If we view it from an ecosystem perspective, then it becomes clear that there exists a natural balance between predator and prey that is most stable for the ecosystem and its environmental factors. It is about this balance that the pendulum swings so to speak. We can view this as semi-circle that looks like a smile. The opposing effects of predator and prey move in trends about the balance but imbalance is generally reversed by events that turn the system towards natural balance again. Eventually, after successive generations, community members adapt and the swings become more muted and the balance more stable. To achieve stability, the ecosystem generally must support predator reproduction.
If it doesn't support predator reproduction, then the semicircle is upside down and eventually there is nothing but prey and no balance is achieved.
Fisheries Management is, at least regarding small impoundments, trying to work out way to avoid the natural balance achieved when predators become more numerous. This is indeed a powerful trend in small impoundments that takes a great deal of effort to overcome.
