What is ecological succession? Give an account of the causes, types and general process of ecological succession in nature.

What is Ecological Succession?

Ecological succession is the process of directional change in the composition of a community over time. It involves the colonization of a new habitat or the re-establishment of a disturbed habitat by a series of communities, each altering the environment and making it more suitable for the next. This process continues until a stable, self-perpetuating community, known as the climax community, is reached.

Causes of Ecological Succession

Several factors initiate and drive ecological succession:

• Abiotic Factors: Changes in physical conditions like temperature, light, soil composition, and water availability.
• Biotic Factors: Interactions between organisms, such as competition, predation, and symbiosis.
• Disturbances: Natural events like fires, floods, volcanic eruptions, or human activities like deforestation and agriculture.
• Evolutionary Changes: Gradual genetic changes within populations that allow them to better adapt to changing environmental conditions.

Types of Ecological Succession

1. Primary Succession

Primary succession occurs in essentially lifeless areas – regions where the soil is unable to sustain life, or where the soil has not been formed yet. This happens in areas like newly formed volcanic islands, bare rock exposed after glacial retreat, or sand dunes. The process is slow and takes centuries to establish a climax community.

• Pioneer Stage: Lichens and mosses are the first colonizers, breaking down rock and initiating soil formation.
• Intermediate Stage: Small plants like grasses and ferns establish themselves as soil develops.
• Climax Stage: Eventually, shrubs and trees colonize, leading to a forest or grassland, depending on the climate.

Example: The colonization of volcanic islands like Surtsey (Iceland) provides a classic example of primary succession.

2. Secondary Succession

Secondary succession occurs in areas where a pre-existing community has been disturbed, but the soil remains intact. This is faster than primary succession because the soil already contains nutrients and seeds. Common disturbances include forest fires, abandoned farmland, and floods.

• Initial Stage: Annual plants and grasses quickly colonize the disturbed area.
• Intermediate Stage: Perennial plants, shrubs, and fast-growing trees establish themselves.
• Climax Stage: Slow-growing, shade-tolerant trees eventually dominate, leading to a stable climax community.

Example: The regrowth of forests after a forest fire in Yellowstone National Park (USA) demonstrates secondary succession.

General Process of Ecological Succession

The general process of ecological succession can be summarized as follows:

1. Initiation: A disturbance or the availability of a new habitat initiates the process.
2. Colonization: Pioneer species colonize the area.
3. Establishment: Pioneer species establish themselves and modify the environment.
4. Competition: Competition between species increases as the community develops.
5. Climax: A stable climax community is reached, characterized by high biodiversity and complex interactions.

Successional Stages and Characteristics

Stage	Characteristics	Dominant Species
Pioneer	Harsh conditions, low biodiversity, rapid growth rates	Lichens, mosses, annual plants
Intermediate	Improved conditions, increasing biodiversity, moderate growth rates	Grasses, shrubs, fast-growing trees
Climax	Stable conditions, high biodiversity, slow growth rates	Slow-growing, shade-tolerant trees

It's important to note that the climax community is not always a forest. In arid regions, the climax community may be a grassland or desert shrubland. The specific climax community depends on the climate, soil type, and other environmental factors.