Ecological succession is the process by which ecosystems change and develop over time. It occurs in stages, starting with pioneer species like lichens and mosses that colonize bare or disturbed land. As these organisms grow and die, they enrich the soil, allowing more complex plants, such as grasses and shrubs, to take root. There are two main types of succession: primary and secondary. Primary succession happens in lifeless areas, such as after a volcanic eruption, while secondary succession occurs in areas where an ecosystem has been disturbed but soil and some organisms still exist, like after a forest fire.