NCERT · Class 12 · Biology · EcosystemExplain the concept of ecological succession in detail. Discuss the different types of succession, the process of succession, and the concept of climax community with special reference to hydrarch and xerarch successions.

Introduction to Ecological Succession\nEcological succession is the process by which the structure of a biological community evolves over time. Two main types are primary succession (starting on lifeless areas like newly cooled lava or bare rock) and secondary succession (starting in areas where natural community has been disturbed).

Process of Succession\nSuccession is a directional and progressive process involving several sequential stages:

• Nudation: The development of a bare area without any form of life.
• Invasion: The successful establishment of species in a bare area through migration, ecesis, and aggregation.
• Competition and Co-action: As species increase, they compete for space and resources, leading to modification of the environment.
• Reaction: The living organisms modify the environment, making it unsuitable for existing species and favorable for new, more demanding species (seral communities).
• Stabilization (Climax Community): The final stage where a stable, self-perpetuating community is formed that is in near equilibrium with the environment.

Hydrarch vs. Xerarch Succession

• Hydrarch Succession: Takes place in wetter areas and progresses from hydric (aquatic) to mesic conditions. It starts with phytoplankton, followed by submerged plants, floating plants, reed-swamp, marsh-meadow, woodland, and finally culminates in a forest as the climax community.
• Xerarch Succession: Takes place in dry areas and progresses from xeric (arid) to mesic conditions. It starts on bare rocks with pioneer species like lichens, which secrete acids to dissolve rock and form soil. This is followed by bryophytes, herbs, shrubs, and finally a stable forest climax community.