Realistic aquatic and terrestrial ecosystems constantly adapt to various anthropogenic and natural disturbances. According to the “Alternative Steady State Theory” the ecosystem has various states and can transition from one state to another when ecosystem conditions change (Holling, 1973; Scheffer, 2001). When the magnitude of such disturbance is negligible, change in the structure and functioning of the ecosystem does not occur. In this case the resilience of the ecosystem allows it to return to its original state (REF). Population densities are changing rapidly in response to a small disturbance. This quantitative change does not necessarily lead to a structural and functional change in the ecosystem. Conversely, when the disturbance is large enough, the ecosystem can shift to a different state. When the critical point is reached the change is not reversible and the return to the original state is not possible. The threshold at which the shift occurs depends on the extent of the disturbance and the degree of ecosystem resilience (REF). This ability of the ecosystem to resist changes in conditions...