Ecological succession : Definition, Process & Its Types

Ecological succession refers to the process of gradual and predictable change in the composition and structure of an ecological community over time. It occurs in ecosystems following a disturbance or the establishment of a new habitat, such as a volcanic eruption, a forest fire, or the formation of a new island.

During ecological succession, the community of organisms in an ecosystem undergoes a series of distinct stages, each characterized by different species assemblages. These stages are known as seral stages or seres.

The term “sere” is used to describe different stages or types of succession within an ecosystem.

The process of succession can take place over several years or even centuries, depending on the nature of the ecosystem and the rate of environmental changes.

Climax community

Ecological successional changes lead finally to a community that is in near equilibrium with the environment and is called a climax community.

The pioneer community is followed by a specific orderly sequence of a series of plant communities called transitional communities or seral communities

Types of Ecological Succession

Depending upon the nature of the habitat

The two types of ecological succession are primary succession and secondary succession.

  1. Primary succession
  2. Secondary succession

Primary succession

It occurs in areas that have been completely devoid of life or where the existing ecosystem has been completely destroyed, leaving no organic matter or soil.

The process begins with the colonization of pioneer species, such as lichens and mosses, which gradually modify the physical environment by breaking down rocks and creating soil. As soil forms, more complex plant species, like grasses and shrubs, can establish themselves, attracting a wider array of organisms.

Eventually, a mature, stable community, such as a forest, develops.

Secondary succession

It occurs in areas where an existing ecosystem has been disturbed or partially destroyed, but the soil remains intact. Common causes of secondary succession include forest fires, hurricanes, or human activities like logging or farming.

In this case, the process begins with the growth of herbaceous plants and pioneering species that rapidly colonize the area. Over time, larger plants and trees take hold, gradually restoring the ecosystem to its pre-disturbance state, although the composition may differ somewhat.

Throughout the process of ecological succession, the structure and function of the ecosystem change, with different species dominating at different stages. The overall goal of succession is to re-establish a stable and self-sustaining ecosystem that is adapted to the prevailing environmental conditions.

Depending upon the type of nudity in the area

It can be divided into two parts-

  1. Hydrarch succession
  2. Xerarch succession

Hydrarch succession

It also known as hydrosere is a type of ecological succession that occurs in aquatic or waterlogged environments, such as ponds, lakes, or wetlands. It refers to the sequential colonization and development of plant and animal communities in response to changes in water conditions and the accumulation of organic matter.

Hydrarch succession begins with the colonization of pioneer species in a body of water. These species are usually submerged or floating plants, such as algae, duckweed, and pondweeds, which can tolerate aquatic conditions. These pioneer species modify the environment by capturing sediments, absorbing nutrients, and increasing the amount of organic matter.

As the pioneer plants grow and reproduce, they gradually alter the physical and chemical properties of the water and create a more favorable habitat for other plant species. Over time, emergent plants like reeds, rushes, and cattails start to take root along the shoreline, forming dense vegetation near the water’s edge. These emergent plants further change the habitat by reducing water flow, trapping more sediments, and providing shelter for a variety of organisms.

As the hydrarch succession progresses, the accumulation of organic matter and sediments leads to the gradual filling of the water body. This process, known as siltation or infilling, results in the formation of marshes or swamps.

The rising soil level eventually allows the establishment of trees and shrubs, transforming the area into a forested wetland or swamp.

Throughout the hydrarch succession, the structure and composition of plant and animal communities change. Early successional species are gradually replaced by more specialized and competitively dominant species that are better adapted to the changing environment. This succession process continues until a stable and self-sustaining community, such as a mature forested wetland, is established.

The following stages are found during hydrach succession-

  1. Lichen stage
  2. Moss stage
  3. Annual grass stage
  4. Perennial grass stage
  5. Shrub stage
  6. Climax stage

Xerarch succession

It also known as xerosere is a type of ecological succession that occurs in dry or arid environments, such as deserts or rocky terrains. It refers to the sequential colonization and development of plant and animal communities in response to the unique challenges posed by these water-limited ecosystems.

In xerarch succession, the process typically begins with the colonization of pioneer species that are adapted to tolerate extreme aridity and have mechanisms to conserve water. These can include xerophytic plants like lichens, mosses, and certain types of grasses.

These early colonizers are often able to grow on bare rocks or in shallow soils where water is scarce. As the pioneer species become established, they modify the environment by capturing and retaining moisture, breaking down rocks, and adding organic matter through their decomposition. This creates more favorable conditions for the growth of subsequent plant species.

Over time, as more organic matter accumulates and soil depth increases, larger plants with deeper root systems, such as shrubs and drought-tolerant trees, begin to colonize the area. These plants are better adapted to access limited water resources and can provide shade and shelter for other organisms.

As the xerarch succession progresses, the increasing plant biomass and the accumulation of organic matter in the soil lead to improved water retention and nutrient availability. This allows for the establishment of more diverse plant communities, including species that require relatively higher moisture levels. Eventually, a more stable and diverse community, such as mesic shrubland or woodland, may develop in the xeric environment.

Xerarch succession is a slow and gradual process that occurs over long periods of time in response to changes in environmental conditions.

The following stages are found during xerarch succession-

  1. Plankton stage
  2. Submerged stage
  3. Floating stage
  4. Reed swamp stage
  5. Sedge or marsh meadow stage
  6. Scrub/Woodland stage
  7. Climax Forest

Pioneer species

Pioneer species, are primary colonizers and are the first organisms to colonize and establish in a previously barren or disturbed habitat during ecological succession.

They play a crucial role in initiating and facilitating the process of succession by creating suitable conditions for other species to follow.

Pioneer species are typically characterized by certain adaptations that enable them to thrive in harsh or challenging environments. These adaptations can include-

  1. Rapid growth and reproduction: Pioneer species often have the ability to grow quickly and reproduce rapidly, allowing them to establish themselves and colonize the area efficiently.
  2. Tolerance to harsh conditions: They are adapted to tolerate adverse conditions such as extreme temperatures, low nutrient availability, high salinity, or lack of water. These adaptations allow them to survive in otherwise inhospitable environments.
  3. Ability to fix nitrogen: Some pioneer species, such as certain types of algae, lichens, and leguminous plants, have the ability to convert atmospheric nitrogen into forms that can be used by other organisms, thereby contributing to soil fertility.
  4. Dispersal mechanisms: Pioneer species possess mechanisms for dispersing their seeds or propagules over long distances, which allows them to reach and colonize new or disturbed areas more effectively.

Pioneer species typically modify the physical and chemical characteristics of the habitat, making it more suitable for other, more specialized species to establish. They can do the following-

  • Break down rocks
  • Create organic matter
  • Improve soil conditions
  • Provide shade and shelter for subsequent colonizers.

As ecological succession progresses, the pioneer species are gradually replaced by other, more competitive species that are better adapted to the changing environmental conditions. These later-successional species typically outcompete the pioneers and establish a more diverse and stable community.

Examples of pioneer species

Lichens and mosses are pioneer species in primary successions and fast-growing herbaceous plants like fireweed in secondary successions.

Here are some examples of pioneer species in hydrarch succession-

1. Algae:

Various types of algae, such as diatoms, green algae, and blue-green algae (cyanobacteria), are often the primary colonizers in aquatic environments.

They can grow and multiply rapidly, forming a green or brown scum on the water’s surface or attaching to submerged surfaces. Algae contribute to the process of primary production by photosynthesis and help in nutrient cycling.

2. Duckweed (Lemnaceae family):

It is a small, floating aquatic plant with rounded leaves that float on the water surface. It reproduces rapidly and forms dense mats, providing shelter and habitat for other organisms.

Duckweed plays a role in nutrient uptake, oxygen production, and creating microhabitats in the early stages of hydrarch succession.

3. Pondweeds (Potamogeton spp.):

Pondweeds are submerged aquatic plants that can establish themselves in shallow water areas. They have long, ribbon-like leaves and are capable of rapid growth.

They help stabilize sediments, improve water clarity, and provide hiding places for aquatic organisms.

4. Watermilfoil (Myriophyllum spp.):

Watermilfoil is a group of submerged aquatic plants characterized by feathery or whorled leaves. They can establish in deeper water and contribute to the formation of underwater vegetation.

They provide habitats for fish, invertebrates, and other organisms, and assist in oxygenation and nutrient cycling.

Here are some examples of pioneer species in xerarch succession:

1. Lichens:

They are often among the first organisms to colonize barren or rocky surfaces. Lichens can tolerate desiccation and harsh environmental conditions.

They break down rocks and contribute to soil formation through their growth and decomposition.

2. Mosses:

Mosses are small, non-vascular plants that can survive in dry environments by efficiently conserving water.

They can colonize bare soil, rocks, or tree trunks, and play a role in soil development by trapping organic matter and enhancing moisture retention.

3. Succulent Plants:

Succulents, such as cacti and agaves, are well-adapted to arid conditions. They have fleshy leaves or stems that store water, allowing them to survive in environments with limited water availability.

They are often pioneer species in xerarch succession, creating microhabitats and providing shade and protection for other organisms.

4. Annual Grasses:

Certain species of annual grasses, such as cheatgrass (Bromus tectorum), can establish and dominate in disturbed or recently burned areas.

They have rapid growth and reproduction cycles, enabling them to quickly colonize bare soil and compete with other plants. These grasses contribute to soil stabilization and nutrient cycling.

5. Shrub Species:

Some shrubs, like creosote bush (Larrea tridentata) and sagebrush (Artemisia spp.), are adapted to arid environments and can act as pioneer species in xerarch succession.

They have deep root systems that allow them to access limited water resources. Shrub species provide shelter, and shade, and contribute to organic matter accumulation in the soil.

Major trends during ecological succession

The major structural and functional attributes of ecological succession are:

            (i) Increase in species diversity.

            (ii) Increase in structural complexity.

            (iii) Increase in organic matter.

            (iv) Decrease in net community production.

            (v) Food chain relationship becomes complex.

            (vi) Niche becomes specific and narrower.

            (vii) Stability increases.

            (viii) More immobilization of nutrients (mineral nutrients fixed in biota).

            (ix) Increased energy efficiency.

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