Ecological succession is the gradual replacement of one community by another over time. It is initiated by disturbances that remove existing organisms or alter resources and habitat. There are two types of succession: primary succession occurs in areas without soil, while secondary succession occurs in areas with existing soil following a disturbance. In both cases, pioneer species are the first to colonize, and succession progresses gradually until an equilibrium climax community dominated by less tolerant species is reached if undisturbed.