Ecological succession is a normal phenomenon which lays the foundation for the formation of an 'ecosystem'. The entire process, which occurs over hundreds of years, is either triggered by formation of a new habitat or by some disturbance in the existing habitat. In biology, there exist two different types of 'ecological succession': primary succession and secondary succession. Primary succession takes place in a newly formed habitat which is typically characterized by lack of organic soil to support new plant species. Secondary succession, on the other hand, occurs in an area wherein soil with organic matter is present and is used along with previously existing vegetation to support new species. What is Secondary Succession? Going by its definition, it is a phenomenon triggered by an event, either natural or human-induced, which disrupts the growth of existing species of plants and replaces them with new species. While natural events with the tendency to trigger the same include wildfires and hurricanes, human-induced events include land clearance for agricultural practice and logging. Simply put, it is revival of the ecosystem post the disrupting event that it was subjected to. The fact that soil fertility and structure has been modified by previously existing species makes the process of plant growth much easier. There are different stages of secondary succession, namely the establishment stage, thinning stage, transition stage, and the steady-state stage. More importantly, it is much more common than primary succession, as the occurrence of a disruptive event, which can erase all the traces of existing species in a particular region, are very rare. How Does it Differ from Primary Succession? While primary succession is growth of new life on the surface of the planet wherein no habitat existed previously, secondary succession is modified-regrowth of a previously existing habit which is partially destroyed by some event. In this case, organic matter in the soil has an important role to play, which is virtually absent in case of primary succession. Unlike primary succession, which is more often observed in new habitats like a newly formed volcanic island or rock exposed as a result of retreating glacier, secondary succession is observed in a previously colonized habitat, which is reviving after being subjected to some disruptive event. The fact that there is an already existing seed-bank makes it much quicker than its primary counterpart. Examples One of the best examples of secondary succession is the process in which numerous plant species colonize an abandoned farmland. Within a year after the said farmland is abandoned, you will see the growth of crabgrass on it, which is eventually followed by other species, including the horseweed and broomsedge. After a decade or so, species of large trees, such as pine and oak, take over this very abandoned farmland. Yet another prominent example is rejuvenation of grasslands that were destroyed by wildfire. Ecological succession is a broad concept and the process can go on for hundreds of years, before it bears fruit. Of these, around 30 - 60 years are attributed to secondary succession. Irrespective of how much time it takes, it is important for the planet's biodiversity, as the end result of the entire process is an ecosystem―not just rich in terms of plants, but also in terms of animal species.