These mechanisms are simply the limiting factors, it may be man-made or the natural one that tries to keep the population of the species balanced. The main aim of the limiting factor is to balance the interaction between the species in order to stop the other species overgrowing. The limiting factor could be lack of food, forest fire etc.
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.
