Primary and Secondary Succession

The ecosystem of our planet is subjected to continuous change with several natural communities being formed and replaced over a period. In biology, the entire process in which these new communities come into existence, and eventually get replaced is known as 'ecological succession'. It is further divided into two types - 'primary succession', which begins in an area where there is no soil present initially, and '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 (such as 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 such as 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 these pioneering species die they are added to Earth, and this forms the base for the growth of new plant species, which eventually result in 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 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 still 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 Succession 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. One has to take a note of the fact that even though islands formed by volcanic eruptions or rock exposed by glacial retreat do contain disintegrated material of the Earth's crust, it doesn't contain organic matter required to facilitate plant growth - and that explains why 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 development of new plant species on this piece of land; and this modification need not be restricted to natural occurrences, such as wildfires, alone, but also includes human activities such as land clearance for agricultural purpose.

While the entire phenomenon can be a bit difficult to understand, going through some examples of primary succession 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 plant species (secondary succession.) The growth of new plant species attracts various animal species to this region, who come to this island in search of food. The changes made in the surroundings because of the existing plants and animals, eventually result in conditions favorable for new species of plants as well as animals.

There is no questioning the fact that these two attributes of ecological succession - primary and secondary succession, play a crucial role in development of new ecosystems, and thus contribute in maintaining the biodiversity on our 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 that are introduced to it.