Difference Between Primary Succession and Secondary Succession

 Difference Between Primary Succession and Secondary Succession

Primary succession is a type of ecological succession that takes place in an environment that is recently formed and lacks habitable soil but then is colonized for the first time by living organisms.

  • Primary succession is a series of events occurring in an order to develop a stable ecosystem.
  • The new environments are usually formed due to natural events like volcano or glacier eruption that causes the lack of soil or the absence of living organisms.
  • The groups of organisms that colonize these environments are termed ‘pioneer organisms’ and are usually composed of lichens, algae, and fungi.
  • Primary succession begins with the weathering of rocks to form soil to inhabit the pioneer species.
  • The initiation of primary succession usually occurs either due to a biological factor or an external factor.
  • Once some amount of soil is formed, organisms like lichen that have few soil requirements, begin to grow in these areas. These organisms enter the new environment from a different environment.
  • These species further assist the breakdown of mineral-based rocks formed of lava or glaciers into the soil to inhabit other species.
  • The pioneer species continue to grow, reproduce, die, and decompose, creating pockets of soil where other species might grow and flourish.
  • The decomposition of these organisms adds to the organic content of the soil, thus, contributing to soil formation.
  • This process is repeated multiple times throughout the succession. In each new stage, new species move into the environment created by the preceding species, and some may even replace the previous species.
  • Over time, fast-growing vegetation inhabits these areas covering most of the land. Eventually, seeds of large trees reach the new environment via wind or by birds, which further attracts other groups of animals.
  • The ecosystem during primary succession changes continuously with the arrival of new species which eventually leads to a more stable state.
  • The community that ends up during the stable state of the ecosystem is termed as ‘climax community’.
  • The composition and ecosystem at this point are much less volatile than the preceding ecosystems.
  • Primary succession is a very long process that takes years to complete.
  • Some examples of primary succession include the formation of a new ecosystem after a volcano, glacier outbursts, or a nuclear explosion.

Secondary succession is a type of ecological succession that occurs in an environment with an already established ecosystem that gets disruption due to some events like fire or hurricane and is then re-colonized by other organisms.

  • Unlike primary succession, secondary succession begins in an environment with pre-existing soil.
  • Secondary succession thus takes place in an environment where the initial succession has been disrupted, but some plants and animals might still exist.
  • This type of succession usually follows disasters like a forest fire, hurricane, or harvesting, causing the existing ecosystem to be destroyed.
  • The initiation of secondary succession always occurs due to some external factors.
  • After a forest fire, the tall trees become destroyed, and the first plants that grow in such areas are usually annual plants.
  • The following annual plants are grasses and low-lying plants and other pioneer species.
  • The early colonizers in secondary succession are also termed ‘pioneer species’ like in primary succession.
  • These pioneer species, unlike those in primary succession, mostly arise from the pre-existing groups of organisms of the community.
  • Over time, changes in the environment caused due to the growth of grasses result in the growth of new groups of plants like shrubs and herbs.
  • These organisms are called intermediate species which further bring changes to the environment facilitating the growth of taller plants.
  • Finally, over a long period of time, the composition of the environment changes back to its original or pre-fire state.
  • There are many factors that affect the course of succession, including seed dispersal and seed production.
  • Additionally, other factors like landscape structure, climate, pH, bulk density, and soil texture are also responsible for the composition and interactions within the community.
  • The nature of the community formed after secondary succession depends on the trophic interaction, the initial composition of the environment, and the competition-colonization processes.
  • The time required for secondary succession is often less compared to primary succession, the secondary succession of an environment after forest fire takes about 150 years to complete.
  • Some examples of secondary succession include succession after fire, harvesting, logging, or abandonment of land or the renewal after a disease outbreak.

Primary succession after a nuclear explosion
  • In areas tested for nuclear bombs, it was assumed that no life would appear in such areas for centuries.
  • However, it was observed that primary succession begins even in such areas within 30 years.
  • These are considered dead areas as no life would possibly exist in such areas for years.
  • After years, all the harmful radiation in the area was caused due to a nuclear explosion clear out of the area.
  • Like in all other conditions, primary succession after nuclear explosion also begins with the weathering of existing rocks.
  • As rocks begin to break down, pioneer species like lichens and algae enter the environment, resulting in the first groups of living beings appearing.
  • As the process of succession continues, a new group of organisms replaces the existing groups causing an increase in the organic matter in such areas.
  • After 100 years or more, a stable climax community or ecosystem is formed in the area.

Primary succession in sand dunes

  • Seashores are areas with a harsh environment because of high wind speeds, moving sand, and minimum nutrients and organic matter.
  • The environment is thus devoid of stable ecosystems and the pioneer organisms in such an environment include bacteria existing in a symbiotic relationship with pioneer plants.
  • The root systems of these plants allow the plant to anchor onto the shifting sand and also have multiple modifications to prevent water loss lack of organic matter.
  • These grasses are then followed by lichens utilize organic matter deposited over large stones present on the shore.
  • As the environment continues to grow, new species enter and replace the existing living organism until a stable climax community is formed.
  • Eventually, organisms capable of surviving in high salt concentration eventually exist in dunes and seashores.

Harvesting, logging, and abandonment of land

  • Abandonment of cropland is an important example of human-induced secondary succession.
  • Continuously cultivated land is usually getting devoid of nutrients as available nutrients are repeatedly removed due to cultivation.
  • The lack of organic matter creates an unfavorable condition for the growth of vegetation or other organisms.
  • After the abandonment of such lands, secondary succession begins with the establishment of pioneer vegetation.
  • The early vegetation creates organic matter and nutrient density enough for newer organisms to inhabit such areas.
  • The vegetation is then replaced by shrubs and herbs that also act as a protective barrier against natural disasters like soil erosion.
  • Eventually, the cropland is restored with enough nutrients as new communities are formed.
  • However, the secondary succession in human-affected landscapes is different from that in the natural environment because of the soil type and the artificial fertilizers applied to such land.
  • This causes the colonization of the area by specialist plants, which in turn decreases biodiversity.

Renewal after diseases

  • Secondary succession also occurs in areas where the existing ecosystem is wiped out by disease outbreaks.
  • Even though a disease is catastrophic for a single species, it eventually affects the existing plants and other species within that ecosystem.
  • However, the roots or seeds of some plants might still exist in the existing soil which might initiate a secondary succession.
  • Similarly, the removal of one group of species might allow colonization by another group that might have been in competition with the previous colonizers.
  • Secondary succession under this condition allows biodiversity within an environment as new and different groups of organisms might inhabit the environment.

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