Primary succession occurs in areas where no life has existed, whereas secondary succession happens when a disaster has devastated an area but has left some life in place. Succession begins with the colonization and activity of the pioneer species.
In primary succession, bare rock or sand is the substrate upon which the pioneer species act. Primary succession occurs in places that are the result of lava flows or where glaciers have retreated. Soil is not yet present, so no plants or animals can survive in such a region.
Pioneer species are the lichens and moss. These organisms break down rock, turning it into soil for plants to put down roots. Shrubs and grasses then start taking advantage of the nutritive soil. Small trees come into the area, replacing the smaller plants. Leaves that fall from these shrubs and small trees produce leaf litter that decompose to produce nitrogen that acts as fertilizer for subsequent plant life. Bigger, fast-growing trees, such as cottonwoods, replace the smaller trees. Coniferous trees are the final plant life moving into the area. The climax community, or the goal of succession, is a forest of coniferous trees.
In secondary succession, the soil remains in place after the disaster. Remaining in the soil are the unharmed roots of some plants and the seeds of others. The process of rebuilding after a fire or other disaster is similar to primary succession and proceeds in much the same way; however, secondary succession occurs in less time.