Communities change over time as a result of several factors, including physical disturbances, climatic change, and invasion of new species. Following a major disturbance, a community undergoes a somewhat predictable change in species composition over many years. If the disturbance removes soil and all living things, such as when a lava flow occurs, the community change is termed primary succession. If the disturbance removes only some species and the soil is still present, such as when forest trees are cut down, the community change process is called secondary succession. In primary succession, the first species to become established are called pioneer species. These are often lichens and mosses that are capable of living on rocky surfaces.