Ecological succession is the gradual process by which ecosystems change and develop over time. There are two types: primary succession, which occurs on new habitats that have never been colonized, and secondary succession, which occurs on previously colonized habitats that have been disturbed. Primary succession from bare rock follows steps from pioneer lichens and mosses breaking down rock to develop soil, to larger plants like ferns and shrubs, and eventually forests over hundreds of years. Secondary succession is faster as soil and seeds are already present, such as after a forest fire or deforestation.