Question
What is ecological succession? Differentiate between primary and secondary succession with suitable examples. Describe the stages of primary succession on a bare rock (xerosere).
(NCERT Class 12, commonly asked in NEET)
Solution — Step by Step
Ecological succession is the gradual, predictable change in the species composition of a community over time. It begins with the colonisation of a barren or disturbed area and progresses through a series of stages (seral stages) until a stable, mature community is established — the climax community.
The entire sequence from pioneer to climax is called a sere.
Primary succession occurs on completely new, lifeless surfaces where no soil or organisms previously existed — bare rock, cooled lava, newly formed sand dunes, or newly exposed land from a retreating glacier.
It is extremely slow (can take hundreds to thousands of years) because soil must be formed from scratch.
Stages of primary succession on bare rock (Lithosere/Xerosere):
- Crustose lichen stage: Lichens are the pioneer species — they can grow on bare rock, secreting acids that break down rock into small particles, beginning soil formation.
- Foliose lichen stage: Larger lichens colonise as thin soil accumulates.
- Moss stage: Mosses grow on the thin soil, further adding organic matter as they die and decompose.
- Herb stage: Grasses and herbs take over as sufficient soil develops.
- Shrub stage: Bushes and shrubs establish themselves.
- Tree/Climax stage: Finally, a forest community develops — this is the climax community, which remains stable unless disturbed.
Secondary succession occurs on areas that previously had a community but were disturbed — after a forest fire, flood, abandoned farmland, or logging. Soil and seeds/roots are already present.
Secondary succession is much faster than primary succession (decades vs centuries) because:
- Soil is already available
- Seeds, roots, and underground organs survive in the soil
- Soil organisms (decomposers, earthworms) are already present
- Nutrient cycling can resume quickly
Example: An abandoned agricultural field first gets colonised by weeds, then grasses, then shrubs, and eventually forest.
Why This Works
Succession occurs because each community of organisms modifies the environment in ways that make it more suitable for the next set of species and less suitable for itself. Lichens create soil, which allows mosses to grow, which enriches the soil further, allowing herbs to establish — each stage prepares the ground for its successor.
The climax community is the endpoint where the community is in equilibrium with the environment. In a given climate, the climax community is predictable — tropical climates lead to rainforest climax, temperate climates to deciduous forest climax.
For NEET, know the pioneer species for different types of succession: bare rock → lichens, bare sand → grasses, water bodies → phytoplankton (hydrosere). Also remember: succession in a water body toward land is called hydrarch succession, and in a dry area toward mesic conditions is called xerarch succession. Both converge toward a mesic climax.
Common Mistake
Students often confuse primary and secondary succession. The key difference is the starting point: primary succession starts on lifeless, soil-free surfaces (bare rock, new lava), while secondary succession starts on disturbed but soil-containing surfaces (burnt forest, abandoned farm). If soil exists, it is secondary succession — even if all visible vegetation is gone.
Another error: writing that succession always leads to a forest. The climax community depends on the climate. In grassland biomes (like the Indian Deccan Plateau), the climax community is grassland, not forest. In tundra, it is lichens and mosses. The climax is the community best suited to the prevailing climate.