Ecological imbalance is when a natural or human caused disturbance disrupts the natural balance of the ecosystem. a disturbance is any change which causes a damage to the ecosystem.examples of natural disturbances are volcanic eruptions, thunderstorms, floods, droughts, forest fires etc....whereas man made disturbances are  deforestation, pollution etc. 
                      a disruption in the natural balance of the ecosystem leads to the extinction of life. for example, in a pond ecosystem, a disturbance like water pollution can affect the lives of the tiny microorganisms living in the pond which will indirectly affect the lives of the fishes and other organisms which depend on the microorganisms for their survival. thus the fish and other organisms may get poisoned or unhealthy and the consumption of such poisoned organisms by man may endanger the life of man,  leading to his death.