Answer:
Fire. Fire is one of the most common causes of secondary succession and is an important component for the renewal and vitality of many types of ecosystem.
Harvesting, Logging and Abandonment of Crop Land.
Explanation:
1- After a fire, species start to recolonize an area, beginning the secondary succession process. The first species to colonize are usually fast growing herbaceous plants, such as conifers or ferns, which require high levels of light. These species are often already present in the form of seeds within the soil, or are able to rapidly disperse from nearby areas. In time, slow growing, shade-tolerant, woody species begin to suppress the early successional species, which are in-turn replaced or shaded by large trees, eventually leading to the generation of forests and a climax community.
2- The abandonment of land previously utilized for crops is a common cause of human-induced secondary succession. Land which has been intensively cultivated is often nutrient poor, with the nutrients having been repeatedly removed through harvest or logging. Agricultural processes also often leave the soil vulnerable to high levels of erosion. The abandonment of such land allows plants and animals that were previously unable to inhabit the area to colonize. Early succession of vegetation following the abandonment of farmland is responsible for increases in soil organic content, nutrient density and soil porosity. The addition of shrubs and of root systems within the soils, which follow in later succession, acts as a natural barrier against erosion, thereby allowing for restoration of degraded habitats.
