Ecosystems are dynamic entities in which a number of events take place. Associated with the biotic communities of the ecosystem are some changes, which may be either small-scale changes or large-scale changes. Small-scale changes may be brought about by natural causes or by the activities of man. One of the examples of small-scale changes is a stream, in which some sewage is accidentally dumped. In such a case there would be an Increase in the organic and inorganic chemicals in the water. The organic molecules are consumed by bacteria. With the increased availability of organic matter, the number of bacteria would increase.
Bacteria use up oxygen as they consume organic materials, thus the level of oxygen in the stream usually drops. This can kill the fish and other organisms or cause them to migrate new areas. In due course of time, the stream will return to normal. The bacteria will die if the level of organic matter falls off, and dissolved oxygen will return to normal, thus allowing fish to return. This example, clearly shows that the biotic community of an ecosystem may be temporarily affected by the small scale changes. On the other hand, there are certain long-term changes in the ecosystem which can permanently change the organisation and composition of biotic communities. These long-term changes may be caused by factors like volcanic eruptions, landslides, earthquakes, floods, hurricanes, and of course human interventions such as mining and deforestation. All these disturbances, change the habitat considerably. A variety of species invade the changed or disturbed site, and eventually over a period of time, a new community develops there. This process continues - one community replacing another community, until a stable, mature community develops.