Ecological succession leads to the sustainability of life in an ecosystem. Succession is the observed process of change in the species structure of an ecosystem. The community begins with relatively few pioneering plants and animals and develops through stages of increasing complexity until it becomes stable. Succession may be initiated either by formation of new, unoccupied habitat such as the formation of an island or by some form of disturbance such as fire in an existing ecosystem. Succession that begins in new habitats, uninfluenced by pre-existing communities is called primary succession, whereas succession that follows disruption of a pre-existing community is called secondary succession.