The ecosystem of our planet is subjected to continuous change with several natural communities being formed and replaced. In biology, the entire process wherein communities come into existence and eventually get replaced is known as 'ecological succession'. It is divided into two types: (i) primary succession, which begins in an area where there is no soil present initially, and (ii) secondary succession, which occurs in an area wherein soil is present. Primary Succession Definition In environmental science, 'primary succession' refers to the development of natural communities in a barren habitat with absolutely little or no soil at all. It is most often seen on a newly exposed surface, like a rock exposed by glacial retreat or formed by volcanic eruption, wherein no biotic factors have ever existed. Basically, it is the beginning of a gradual growth of an ecosystem which takes several years to complete. In primary succession, species like lichen and algae, which are referred to as the pioneer species, and abiotic factors, like wind and water, have a crucial role to play. One of the best examples of primary succession is the growth of pioneering species on soil surface exposed as a result of glacial retreat. When the pioneering species die, they are added to the Earth. This forms the base for the growth of new species and eventually results in the development of an entire ecosystem. Secondary Succession Definition The second type of ecological succession is the secondary succession, wherein the development of natural communities occurs on a piece of land wherein soil was already present, but natural vegetation was removed by either natural or human-induced activities. This type of succession is triggered by an event which results in drastic changes in the biotic factors of the said area. In this process, the initial succession is disrupted by a particular event, and secondary succession begins with some plant species existing in the surroundings. One of the best examples is the process wherein wildfire destroys a forest cover, which is eventually replaced by new species of plants. The high nutrition content of soil which facilitates the growth of new plants is attributed to the remains of plants burnt during the wildfire. Difference Between Primary and Secondary Succession The most prominent difference between the two happens to be the presence of soil. While primary succession is not dependent on soil, secondary succession is highly dependent on it. Though the islands formed by volcanic eruptions or rock exposed as a result of glacial retreat do contain disintegrated material of the Earth's crust, it doesn't contain organic matter required to facilitate plant growth, and therefore, only species like lichen and algae grow on it. It is the modification of existing vegetation (including the pioneer species) which forms the base for the development of new plant species on this piece of land. This modification need not be restricted to natural occurrences alone. It may also include human activities such as land clearance for agriculture. While the entire phenomenon can be a bit difficult to understand, going through some examples of primary and secondary succession can be a great help. When a volcano erupts in mid-ocean, lava cools down and results in formation of an island. This is a barren island which has no organic soil to support plant species. Eventually, pioneer species, such as lichen and algae, start colonizing this island (primary succession). When these species die, they result in development of favorable condition for growth of new species (secondary succession). The growth of new plants attracts various animal species to this region, who come to this island in search of food. The changes made to the surroundings by existing plants and animals, eventually result in conditions favorable for new species of plants and animals. There is no questioning the fact that the two attributes of ecological succession, primary and secondary succession play a crucial role in the development of new ecosystems and contribute in maintaining the biodiversity of the planet. The entire phenomenon takes place over several years, owing to which the said ecosystem keeps on changing. To sum it up, ecological succession is a perfectly normal phenomenon wherein several existing species go extinct from a particular area once in a while, only to be replaced by several new species.