An ecosystem is never static. It undergoes a number of changes with the passing time. Some changes are easily noticeable while some require very close observation. Sometimes, there may be a sudden change in the ecosystem due to natural or man-made disturbances whereas sometimes these changes may occur at a very slower rate. But in most of the cases, a change in the ecosystem, whether rapid or a slow rate, result in the adverse effects.
There are two major reasons of ecological successions. They are:
1. Natural disturbances
2. Man made disturbances
Natural disturbances include events like earthquakes, volcanic eruption, cyclones, floods, etc. Man made disturbances include deforestation, pollution, mass devastations like bomb blasting, nuclear pollution, etc. There may be a series of changes that may result due to these events. In the course of succession, older inhabitants may die out and disappear and ultimately get replaced by the new ones. These series of changes that may occur in an ecosystem due to natural or human disturbances is called ecological succession.