A bare volcanic island emerges from the ocean. Within years, the first organisms — often cyanobacteria and lichens — begin to colonise. Within decades, mosses appear. Within centuries, a forest may stand where bare rock once existed.
This process — the gradual change in species composition of a community over time — is called ecological succession. It’s not random. It follows a broadly predictable sequence, driven by how each species modifies the environment in ways that favour the next.
CBSE Class 12 and NEET both treat succession as a key ecosystem concept. The distinctions between primary and secondary succession, seral stages, and pioneer and climax communities are standard exam material.
Key Terms & Definitions
Ecological Succession: The process of change in the species structure of an ecological community over time, moving toward a more stable state.
Sere: The entire sequence of communities from pioneer to climax. Each stage is called a seral community or seral stage.
Pioneer Community: The first organisms to colonise a bare, uninhabited area. They modify the environment, making it more habitable for subsequent species.
Climax Community: The final, stable community of a succession. Relatively self-sustaining and in equilibrium with the environment. Species composition remains constant unless disturbed.
Primary Succession: Succession that begins in an area where no soil exists and where no community previously lived (bare rock, sand dune, lava flow).
Secondary Succession: Succession in an area where a community previously existed but was removed by disturbance (fire, flood, deforestation, farming). Soil and seed bank are present.
Hydrarch Succession: Succession that begins in water (pond, lake) and progresses toward land.
Xerarch Succession: Succession that begins on bare rock (xeric = dry conditions) and progresses toward mesic conditions.
Primary Succession
Primary succession begins where life has never existed — or where all soil and biological material has been completely removed (volcanic lava, exposed rock after glacial retreat).
Typical Sequence on Bare Rock (Xerarch)
Stage 1: Pioneer community — Lichens and Cyanobacteria
Lichens are a symbiosis of fungi and algae/cyanobacteria. They secrete acids that slowly weather the rock, creating thin mineral particles. When lichens die, they add organic matter — the very first hint of soil.
Stage 2: Moss stage
Thin soil from lichen decomposition allows mosses to establish. Mosses trap more particles, accelerate weathering, add more organic matter. Soil depth increases.
Stage 3: Herb stage
Enough soil now exists for small herbaceous plants (grasses, small flowering plants). Their root systems further stabilise and enrich the soil with nitrogen and organic matter.
Stage 4: Shrub stage
Shrubs colonise, shading out sun-requiring herbs. Birds begin visiting — they bring seeds in their droppings, accelerating the arrival of new species.
Stage 5: Tree/Forest stage — Climax community
Larger trees establish, dominating the canopy. The climax community for most temperate zones is a mixed or deciduous forest. In tropical India, it’s often a tropical rainforest or dry deciduous forest depending on rainfall.
Typical Sequence in a Pond (Hydrarch)
Starting from open water: Phytoplankton → Submerged plants → Floating plants → Reed swamp → Marsh meadow → Scrub → Woodland → Climax forest
The pond gradually fills with sediment and organic matter, eventually becoming land.
Secondary Succession
Secondary succession is faster than primary succession because soil already exists. The seed bank in the soil (dormant seeds and spores) immediately activates when disturbance is removed.
Examples of Secondary Succession
After a forest fire: Within weeks, pioneer grasses emerge from soil seeds. Within years, shrubs return. Within decades, the original forest species reestablish.
Abandoned agricultural field (old-field succession): In India, a fallow field first shows weeds (parthenium, Lantana), then native grasses, then shrubs, then forest trees over 50–100 years.
After a flood: Riparian (river bank) communities reestablish rapidly because soil and seeds are preserved.
Why Secondary Succession is Faster
- Soil is already developed — no need to weather rock
- Soil seed bank is intact
- Residual plant fragments can regenerate vegetatively
- Mycorrhizal fungi in soil accelerate plant establishment
Comparison: Primary vs Secondary Succession
| Feature | Primary Succession | Secondary Succession |
|---|---|---|
| Starting substrate | Bare rock/sand/lava | Disturbed ecosystem (soil present) |
| Soil | Absent initially | Present |
| Seed bank | Absent | Present |
| Pioneer species | Lichens, cyanobacteria | Annual herbs, grasses |
| Duration | Hundreds to thousands of years | Decades to hundreds of years |
| Example | Volcanic island, glacial moraine | Burnt forest, abandoned field |
Solved Examples
Example 1 — CBSE Class 12 (3 marks)
Q: Differentiate between primary and secondary succession with one example each.
Solution:
Primary succession: Occurs on a lifeless substrate where soil has never formed. Pioneer organisms (lichens, cyanobacteria) begin the process of soil formation. Very slow — may take hundreds of years. Example: Succession on bare lava rock of a newly emerged volcanic island.
Secondary succession: Occurs in an area where a community existed but was destroyed by disturbance. Soil and seed bank are retained, so the process is faster. Example: Abandoned agricultural land gradually reverting to forest.
Example 2 — NEET Level
Q: Which of the following is the CORRECT sequence in hydrarch succession?
(a) Phytoplankton → Rooted hydrophytes → Free-floating plants → Reed swamp → Sedge meadow → Scrub → Forest
(b) Lichens → Mosses → Ferns → Shrubs → Trees
Answer: (a) is hydrarch succession (begins in water). (b) is xerarch succession (begins on bare rock).
Exam-Specific Tips
CBSE/NEET Favourite: “Name the pioneer organism in bare rock succession” → Lichens (sometimes also cyanobacteria are accepted). Never say “bacteria” without qualification — it’s specifically photosynthetic cyanobacteria.
NEET 2022 asked about which stage of hydrarch succession has maximum biomass. Answer: the climax forest stage has maximum biomass. Biomass increases progressively through succession.
Remember: succession always moves from simpler to more complex communities. Species diversity and biomass generally increase. Net Primary Productivity (NPP) increases, but Net Community Productivity (gross − respiration) decreases as the ecosystem becomes more mature and respiration increases.
Common Mistakes to Avoid
Mistake 1: Saying secondary succession is faster because it is “better.” It’s faster because it retains soil and seed bank — not due to any inherent property of the plants. In primary succession, the same later stages occur; they just take longer to reach.
Mistake 2: Confusing hydrarch and xerarch. Hydrarch = starts in water (hydro = water). Xerarch = starts on dry, bare rock (xero = dry). Both end at the same climax — a mesic (moderate moisture) forest in most cases.
Mistake 3: Saying climax community “never changes.” Climax is relatively stable but can be disrupted by catastrophic events (fire, floods, human activity). After disturbance, secondary succession begins again. True climax is theoretical; real ecosystems fluctuate around this equilibrium.
Mistake 4: Saying the pioneer community “kills” later species. In reality, the pioneer community modifies the environment to be suitable for subsequent species. By creating soil, adding organic matter, and altering microclimate, pioneers make space for the next seral stage. This facilitation is key to understanding succession.
Practice Questions
Q1. Name the climax community in a hydrarch succession in a tropical region.
Tropical rainforest (or tropical deciduous forest, depending on the specific rainfall pattern of the region). The general climax of both hydrarch and xerarch succession in a region is a forest type determined by regional climate.
Q2. A grassland catches fire. What type of succession will occur after the fire? Why?
Secondary succession, because the soil and seed bank remain intact. The soil was not destroyed — only the above-ground biomass was burnt. Pioneer grasses will germinate from the seed bank within weeks. The process will be much faster than primary succession.
Q3. What are pioneer species? Give two examples.
Pioneer species are the first organisms to colonise a bare, uninhabited area. They can tolerate harsh, nutrient-poor conditions and modify the environment to allow other species to follow. Examples: Lichens (on bare rock) and Phytoplankton (in a new water body like a newly formed pond).
Q4. Why does biomass increase during succession?
At each seral stage, the community accumulates more organic matter (from net primary productivity). Decomposition is not complete enough to break down all accumulated organic matter, so biomass increases net. The community becomes structurally more complex (more layers of vegetation), storing more carbon in wood, roots, and soil organic matter. By climax, biomass is at maximum.
Additional Concepts
Productivity Changes During Succession
As succession progresses, several ecological parameters change in predictable ways:
| Parameter | Early Succession | Late Succession (Climax) |
|---|---|---|
| Species diversity | Low | High |
| Total biomass | Low | Maximum |
| Gross Primary Productivity (GPP) | Increases | High |
| Net Primary Productivity (NPP) | High | Low (respiration increases) |
| Net Community Productivity | High (growth phase) | Near zero (steady state) |
| Food chains | Simple, linear | Complex, web-like |
| Nutrient cycling | Open (nutrients leach) | Closed (efficient recycling) |
| Niche diversity | Few niches | Many specialised niches |
A common NEET trap: “In which stage of succession is NPP highest?” Students often say the climax stage because it has the most biomass. Wrong. NPP is highest during intermediate stages when the community is actively growing and accumulating biomass. At climax, GPP is high but respiration nearly equals GPP, so NPP approaches zero. This is a frequently tested distinction.
Inhibition and Tolerance Models
Not all succession follows the simple facilitation model (where pioneers make conditions better for the next stage). Three models describe how species interact during succession:
Facilitation model (Connell and Slatyer): Each seral stage modifies the environment, making it more favourable for the next species and less favourable for itself. This is the classic NCERT model — lichens create soil for mosses, mosses create conditions for herbs, etc.
Inhibition model: Early colonisers actually resist invasion by later species. Succession proceeds only when early species die or are damaged. Some grasses and shrubs inhibit tree seedling growth by shading or allelopathy.
Tolerance model: Later species are simply more tolerant of resource limitations. They don’t need facilitation — they can grow regardless of who came before, and they gradually out-compete the pioneers.
Solved Example 3 — Analytical (NEET Level)
Q: A pond in a tropical region undergoes hydrarch succession. After 200 years, the pond is completely filled with sediment and supports a forest. What was the sequence of plant communities, and why did each one change?
Solution:
The sequence was:
- Phytoplankton — first colonisers in open water, photosynthesise, die, and add organic matter to the bottom sediment
- Submerged plants (like Hydrilla) — root in shallow sediment, trap more particles, raise the bottom level
- Floating plants (like Nelumbo, Nymphaea) — shade out submerged plants, further reduce water depth through organic matter deposition
- Reed swamp (Typha, Scirpus) — emergent plants that tolerate shallow water, their dense root systems consolidate the substrate
- Marsh/sedge meadow — as water table drops, grasses and sedges replace reeds
- Scrub/woodland — shrubs and small trees colonise the now-dry substrate
- Climax forest — tropical deciduous or evergreen forest, depending on rainfall
Each stage changed because the current community modified the environment — primarily by filling the pond with sediment and organic matter — making it progressively drier. Each transition created conditions that favoured the next community more than the current one.
Q5. Name two Indian examples of secondary succession and explain why each was faster than primary succession.
(1) Western Ghats after shifting cultivation (jhum): When tribal farmers abandon their cleared plots, secondary succession begins rapidly. The laterite soil retains nutrients and the seed bank from surrounding forest ensures rapid colonisation by pioneer herbs, then shrubs, then forest trees. Full forest recovery takes 30-50 years.
(2) Mangrove regeneration in Sundarbans after cyclone damage: After cyclones destroy mangrove canopy, the existing root systems and sediment seed banks allow rapid regrowth. Pioneer mangrove species like Avicennia recolonise within 5-10 years. Full mangrove recovery takes 20-30 years, much faster than primary colonisation of bare mudflats.
Both cases are faster because soil, seed banks, and mycorrhizal networks are intact — the key limiting factors of primary succession are already resolved.
FAQs
Can succession go backwards (retrogressive succession)?
Yes. When environmental conditions worsen — due to human activity, climate change, or catastrophic disturbance — succession can move backward to an earlier seral stage. This is called retrogressive succession or regression. For example, overgrazing can push a grassland back toward bare soil conditions.
Is the climax community the same everywhere?
No. The climax community depends on the regional climate (temperature, rainfall). In India, tropical wet regions have rainforest climax; dry regions have dry deciduous forest or scrub climax. In the Arctic, climax may be tundra. Climate determines the “ceiling” of succession in a region.
How long does primary succession take?
It depends on the substrate and climate. On bare volcanic rock, it may take 500–1,000 years to reach forest climax. On a glacial moraine, it may take 100–300 years. Secondary succession typically takes 50–200 years.
What is the difference between succession and evolution?
Succession involves changes in community composition within an ecosystem over decades to centuries. Evolution involves changes in allele frequencies within a population over generations (thousands to millions of years). Succession is an ecological process; evolution is a genetic one.