Define ecological succession. Differentiate between primary and secondary succession. Explain various steps involved in the mechanism of ecological succession.

Defining 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 community after a disturbance. This process is characterized by a series of sequential changes in species composition, leading to a more stable and complex ecosystem.

Differentiating Primary and Secondary Succession

Ecological succession can be broadly categorized into primary and secondary succession, differing significantly in their starting conditions and the speed of development.

Feature	Primary Succession	Secondary Succession
Starting Point	Bare rock, newly formed land (e.g., volcanic islands, glacial till)	Previously vegetated land disturbed by events like fire, flood, or abandonment of farmland
Initial Conditions	Absence of soil, no organic matter	Presence of soil, some organic matter, potentially remaining seeds and roots
Pioneer Species	Lichens and mosses (hardy organisms capable of weathering rock and initiating soil formation)	Grasses and herbaceous plants (faster-growing species that can readily colonize disturbed areas)
Time Required	Very long (hundreds to thousands of years)	Relatively shorter (decades to centuries)
Example	Succession on a newly formed volcanic island like Surtsey (Iceland)	Succession in an abandoned agricultural field or after a forest fire

Mechanism of Ecological Succession: Stages and Processes

Ecological succession proceeds through a series of stages, each characterized by specific communities and processes. These stages are broadly categorized as follows:

1. Pioneer Stage

This initial stage involves the colonization of a bare habitat by hardy species known as pioneer species. These species are typically tolerant of harsh conditions and play a crucial role in modifying the environment, making it suitable for other organisms. In primary succession, lichens and mosses break down rock, contributing to soil formation. In secondary succession, grasses and weeds quickly establish themselves.

2. Intermediate Stages (Seres)

As the environment is modified, intermediate stages develop, often referred to as seres. These stages represent transitional communities, with increasing complexity and diversity. Different types of seres are defined by the dominant environmental factor:

• Xerosere: Succession in dry areas (e.g., deserts, sand dunes). Pioneer species are drought-resistant plants like lichens and grasses.
• Hydrosere: Succession in aquatic environments (e.g., ponds, lakes). Pioneer species are phytoplankton and submerged plants.
• Halosere: Succession in saline environments (e.g., salt marshes, mangrove swamps). Pioneer species are salt-tolerant plants like halophytes.

Each sere involves a sequence of communities, with species gradually replacing each other as conditions change. For example, in a hydrosere, submerged plants are replaced by floating plants, then emergent plants, and eventually marsh vegetation and finally woodland.

3. Climax Stage

The final stage of succession is the climax community, a relatively stable and self-perpetuating community that is in equilibrium with the environment. The climax community is characterized by high biodiversity, complex food webs, and efficient nutrient cycling. The type of climax community depends on the climate and soil conditions of the region. For instance, in a temperate region, the climax community might be a deciduous forest, while in a tropical region, it might be a rainforest.

4. Disturbances and Resetting Succession

It’s important to note that succession is not always a linear process. Disturbances, such as fires, floods, or human activities, can disrupt succession and reset it to an earlier stage. The frequency and intensity of disturbances play a significant role in shaping the structure and composition of ecosystems.