Biosphere: Ecological Succession | Zoology Optional Notes for UPSC PDF Download

Ecological Succession: A Dynamic Process

Ecological succession is a fundamental ecological concept that highlights the progressive transformation of an environment over time. When a region is stripped of its original vegetation due to natural events like fire, flood, or glaciation, it doesn't remain barren for long. Ecological SuccessionThe process of ecological succession commences, with the establishment of a variety of plant and animal species that subsequently modify their surroundings, allowing for the establishment of new species. This transformation in the community structure is a directional change in vegetation during ecological time. Several aspects of ecological succession are worth exploring:

Seral Stages and Climax Community

1. Seral Stages: The transitional series of communities that develop in a given area during ecological succession are referred to as seral stages. These stages represent the dynamic progression of the environment from one state to another.

2. Climax Community: The final, stable, and mature community that emerges after the succession process reaches a point of relative stability is known as the climax community. It represents the endpoint of ecological succession for a particular area.

Types of Ecological Succession

Ecological succession can be categorized based on various criteria:

1. Primary Succession: Primary succession begins on a sterile area that has not been previously occupied by a community. It typically occurs in challenging environments like newly exposed rock or sand dunes where initial conditions are unfavorable.
2. Secondary Succession: Secondary succession takes place when an area has been cleared or altered by various factors, such as fire, grazing, deforestation, or abrupt changes in climate. The rate of secondary succession is often faster than primary succession due to improved nutrient and environmental conditions.
3. Autogenic Succession: In most cases, the community itself, through its interactions with the environment, brings about modifications that lead to its replacement by new communities. This process is known as autogenic succession.
4. Allogenic Succession: In some cases, the replacement of one community by another is primarily driven by external forces rather than the influence of the communities themselves. This is called allogenic succession and can occur in highly disturbed or eroded areas.
5. Autotrophic Succession: Autotrophic succession is characterized by the early and continuous dominance of autotrophic organisms like green plants. It starts in predominantly inorganic environments and is sustained by energy flow, leading to an increase in organic matter.
6. Heterotrophic Succession: Heterotrophic succession is characterized by the early dominance of heterotrophic organisms like bacteria, actinomycetes, fungi, and animals. It often begins in environments rich in organic matter, such as areas heavily polluted with sewage or where leaf litter accumulates.
7. Induced Succession: Human activities, such as overgrazing, frequent scraping, shifting cultivation, or industrial pollution, can lead to ecosystem deterioration and a shift back to an earlier, less mature state.
8. Retrogressive Succession: Retrogressive succession represents a return to a simpler or less dense community from an advanced or climax community. Causes are often external (allogenic) and may result from factors such as grazing pressure, excessive wood and litter removal.
9. Cyclic Succession: Cyclic succession refers to the repeated occurrence of specific stages of succession within a larger community whenever there is an open condition created. It is a localized phenomenon within a larger community.

Variations in Ecological Succession

Additionally, ecological succession can be classified based on the nature of the environment in which it occurs. For instance, it may be hydrosere (occurring in water-rich areas like ponds and swamps), mesarch (occurring under conditions of adequate moisture), or xerosere (in arid environments like deserts or rocky areas). Further classifications include lithosere (initiating on rocks), psammosere (initiating on sand), and halosere (in saline water or soil).

In conclusion, ecological succession is a dynamic and universal process that shapes ecosystems over time. It demonstrates the adaptability and resilience of nature, as communities evolve in response to changing environmental conditions. Understanding the different types of succession is crucial for comprehending ecosystem dynamics and conservation efforts.

The Process of Ecological Succession

The process of ecological succession is a complex and dynamic series of stages that lead to the development of stable communities in an area that was once barren. This transformation occurs through several sequential steps, leading to the establishment of a diverse community and eventually a stable climax community.

1. Nudation:

• Nudation marks the initial stage of succession, involving the development of a bare area without any life.
• Nudation can result from topographic factors (like soil erosion), climatic factors (e.g., fires, frost, and drought), or biotic factors, notably human activities, such as deforestation.

2. Invasion:

• Invasion is the successful establishment of a species in the bare area, often through the migration of seeds, spores, or propagules from another location.
• Invasion involves three key steps: migration (dispersal of propagules), ecesis (establishment through adjustment to local conditions), and aggregation (population growth through reproduction).

3. Competition and Coaction:

• As the population of the invading species grows, competition for resources like space and nutrition intensifies.
• Coaction refers to the various ways in which individuals of the same species affect each other's lives, leading to the success or failure of certain species.
• Those species that can effectively compete and coexist with others are more likely to persist.

4. Reaction:

• The reaction stage involves the modification of the environment by living organisms, such as plants, that influence soil composition, water availability, light conditions, and temperature.
• These environmental changes make the existing conditions less suitable for the initial community, eventually leading to its replacement.

5. Stabilization (Climax):

• The climax stage is characterized by the establishment of a final, stable, and long-lasting community that is in equilibrium with the local climate.
• The climax community represents the endpoint of ecological succession and is considered the ultimate expression of the environment's conditions.

Concepts of Climax

There are three theoretical approaches to the concept of climax in ecological succession:
1. Monoclimax Theory:

• Developed by Frederick Clements, this theory recognizes only one climax community, primarily determined by climate.
• All other communities are considered subclimax, disclimax, preclimax, or postclimax, depending on their relation to the climax community.

2. Polyclimax Theory:

• Proposed by Tansley, this theory suggests that a region's climax vegetation consists of a mosaic of different climaxes influenced by various factors such as soil moisture, nutrients, topography, fire, and animal activity.
• Each stable community is considered a climax, with prefixes like edaphic climax, topographic climax, biotic climax, and fire climax.

3. Climax Pattern Hypothesis:

• Developed by Whittaker, Maclntosh, and Sellack, this theory emphasizes that the composition and structure of a climax community are determined by the entire ecosystem's environment, not just climate.
• It considers a pattern of populations that correspond to environmental gradients, with the central community expressing the area's prevailing climate.

4. Information Theory:

• Proposed by Leith, Odum, and Golley, this theory views succession and climax in terms of ecosystem development.
• It suggests that in autotrophic succession (ecosystem development), diversity increases with organic matter and energy content.
• In contrast, heterotrophic succession results in an energy decrease due to higher respiration rates, but in an ecosystem, both successions operate in coordination.

In summary, ecological succession is a process of dynamic change in vegetation and ecosystems, leading to the development of stable climax communities. The concept of climax has sparked debates among ecologists, with different theories emphasizing climate, environmental factors, and ecosystem development as key determinants of climax communities.