What mechanisms are in place in a stream ecosystem to make sure that the populations of biotic elements are controlled and interactions are balanced?

In any ecosystem, there are factors to control populations.
In a stream, the predators of a certain organism will help keep populations checked. Moreover, the limited resources, such as space and food, will keep the population from growing out of control.

F(x) = -3 * 2 - 6x
So f(-6) = -6 - 6 * -6
= -6 + 36 = 30