Ecological Succession: Primary and Secondary Succession (UPSC Prelims + Mains)
Imagine a forest fire in Uttarakhand, a cyclone hitting the Sundarbans, or an abandoned farm in a village. After some months and years, the place does not remain "empty". First small grasses appear, then shrubs, then trees, and slowly the ecosystem becomes complex again. This natural "step-by-step recovery and replacement" of plant and animal communities is called ecological succession.
📘 Ecological Succession
A natural, orderly and gradual process in which one community of organisms is replaced by another over time, leading to a relatively stable ecosystem at the end.
1) Why Ecological Succession is Important for UPSC
- Prelims: Direct questions on definitions, primary vs secondary succession, pioneer species, seral stages, climax community, hydrosere/xerosere.
- Mains (Environment, Geography, Disaster Management): Recovery after disasters (flood, fire, cyclone), ecosystem restoration, climate change impacts, invasive species, afforestation planning, mined-area reclamation.
- Current relevance: Forest fires, glacier retreat exposing new land, coastal changes after cyclones, degraded land restoration, biodiversity conservation.
2) Key Terms You Must Know
📘 Community
All populations of different species living together and interacting in a particular area.
📘 Pioneer Species
The first organisms to colonise a bare or disturbed area. They are hardy and can tolerate harsh conditions (e.g., lichens on rocks, grasses on open soil).
📘 Sere and Seral Stage
Sere is the complete sequence of communities that develop during succession. Each intermediate step is a seral stage (e.g., grass stage, shrub stage).
📘 Climax Community
The final relatively stable community formed at the end of succession in a given climate. It is more complex, diverse and stable than early stages.
📘 Disturbance
An event that disrupts an ecosystem and community structure (e.g., fire, flood, cyclone, landslide, farming, logging).
3) What Actually Happens During Succession?
Succession is not random. It generally shows a pattern:
- Colonisation: seeds/spores arrive by wind, water, animals.
- Establishment: pioneer species survive and start growing.
- Competition and replacement: as conditions improve, new species replace earlier ones.
- Stabilisation: a mature, stable community develops (climax-like stage).
During succession, the ecosystem usually becomes:
- More complex in food webs and niches
- Richer in soil organic matter (humus)
- Better at nutrient cycling and water retention
- More stable and resilient to small disturbances
4) Types of Ecological Succession (Classification)
- Based on starting point: Primary succession, Secondary succession
- Based on cause: Autogenic, Allogenic
- Based on direction: Progressive, Retrogressive
- Based on habitat: Hydrosere (water to land), Xerosere (dry areas), Lithosere (rock), Psammosere (sand dunes), Halosere (salty areas)
📘 Autogenic vs Allogenic Succession
Autogenic: driven by changes created by organisms themselves (e.g., plants adding humus, improving soil). Allogenic: driven by external forces (e.g., climate shift, flooding, silt deposition).
5) Primary Succession
📘 Primary Succession
Succession that begins on a completely new surface where no soil exists initially (e.g., bare rock, new lava, newly exposed glacial deposits).
5.1 Where Does Primary Succession Occur?
- New lava flows and volcanic rocks
- Newly formed islands or newly exposed land
- Retreating glaciers exposing bare rocks and moraines
- Rocky landslide surfaces where soil is removed
- Fresh sand deposits (very low organic matter, near "new substrate" conditions)
Indian examples (easy to write in Mains answers):
- Himalayan glacier retreat: as glaciers retreat, they expose bare moraines where plants slowly colonise.
- New rocky surfaces after landslides in the Himalayas or Western Ghats can show near-primary succession patches.
- Volcanic surfaces (like on Barren Island region) can represent classic primary succession conditions (bare rock, minimal soil).
5.2 Typical Stages of Primary Succession (Lithosere: Bare Rock to Forest)
This is the most commonly taught example for UPSC.
- Nudation: formation of bare rock surface (lava, glacier retreat, landslide rock).
- Invasion (Pioneer stage): lichens are usually the pioneers. They can grow on rock and release acids that break rock into small particles.
- Moss stage: mosses grow when a thin layer of soil forms. They hold water and add organic matter when they die.
- Herb/grass stage: small herbs and grasses appear. Soil depth and nutrients improve.
- Shrub stage: shrubs establish; more shade and humus form.
- Tree stage: fast-growing, light-demanding trees come first, followed by shade-tolerant trees.
- Climax-like community: a stable forest (or stable local ecosystem) forms depending on the climate.
5.3 Why is Primary Succession Very Slow?
- No soil initially → plants cannot easily grow
- Low nutrients and almost no organic matter
- Harsh microclimate (high temperature variation, strong winds)
- Soil formation takes time (weathering + humus build-up)
5.4 Hydrosere (Succession in Water Body: Pond/Lake to Land)
UPSC sometimes asks habitat-based succession. Hydrosere shows how a water body can slowly become marsh, then meadow, then woodland (often due to silt and organic matter filling it).
- Phytoplankton stage: algae and microorganisms dominate
- Submerged plant stage: hydrilla-like submerged plants appear
- Floating plant stage: lotus/water lilies can appear (depending on conditions)
- Reed-swamp stage: reeds and marsh plants dominate near shallow edges
- Sedge-meadow stage: sedges and grasses grow as the area becomes less watery
- Woodland and climax stage: trees establish as land becomes firm
UPSC angle: Hydrosere is closely linked with wetland formation, siltation, and ecosystem services (but excessive siltation can also mean wetland loss).
6) Secondary Succession
📘 Secondary Succession
Succession that occurs in an area where a community existed earlier but got disturbed; soil remains present along with seeds, roots, microbes and organic matter.
6.1 Where Does Secondary Succession Occur?
- After forest fire
- After floods or cyclones that damage vegetation but leave soil
- In abandoned agricultural fields
- After logging or shifting cultivation (jhum) when land is left fallow
- After overgrazing when grazing pressure reduces later
Indian examples:
- Jhum fallows in North-East: when cultivation stops, grasses come first, then shrubs, then secondary forests.
- Forest fire regeneration: many pine-dominated areas show secondary succession patterns after fires.
- Cyclone-affected coastal vegetation: after cyclones in Odisha or Sundarbans, vegetation regrows in stages.
- Abandoned farmlands: in many rural areas, fields left unused become grassland → shrubs → young forest patches.
6.2 Typical Stages of Secondary Succession
- Immediate post-disturbance: soil, ash (if fire), dead biomass, and seed bank remain.
- Annual weeds and grasses: fast-growing plants dominate first.
- Perennial grasses and herbs: stronger root systems stabilise soil.
- Shrubs: provide shade, attract birds (seed dispersal increases).
- Young trees: light-demanding trees establish.
- Mature community: more shade-tolerant, late-successional species come.
6.3 Why is Secondary Succession Faster?
- Soil is already present
- Seed bank and roots survive
- Microbes and decomposers already exist
- Organic matter supports quick plant growth
7) Primary vs Secondary Succession (Most Important Prelims Table)
| Basis | Primary Succession | Secondary Succession |
|---|---|---|
| Starting condition | Starts on bare surface with no soil | Starts after disturbance with soil present |
| Speed | Very slow (soil formation needed) | Comparatively fast |
| Pioneer species | Lichens, mosses (often) | Grasses, herbs, weeds (often) |
| Nutrient availability | Very low initially | Relatively higher due to existing soil and humus |
| Previous community | Absent | Present earlier but removed/damaged |
| Seed bank/root stock | Absent initially | Often present |
| Examples | New lava rock, glacier retreat surfaces | Fire-affected forest, abandoned farmland, cyclone damage |
8) How Do Species Replace Each Other? (Mechanisms/Models)
📘 Facilitation Model
Early species modify the environment in a way that helps later species (e.g., lichens make soil, enabling grasses and shrubs).
📘 Inhibition Model
Early species make it harder for new species to establish (by taking space, nutrients). Replacement happens when early species are damaged or die.
📘 Tolerance Model
Later species can tolerate low resources and eventually outcompete early species. Early species do not necessarily help or harm later species much.
UPSC trick: Do not memorise only one model. In nature, different ecosystems can follow different patterns.
9) Ecological Succession and Ecosystem Development (Odum-style Trends)
UPSC Mains answers become stronger if you show "directional changes" during succession:
| Feature | Early Succession | Late Succession |
|---|---|---|
| Dominant species | Fast-growing, short-lived (r-selected) | Slow-growing, long-lived (K-selected) |
| Biomass | Low | High |
| Soil organic matter | Low | High |
| Food web | Simple | Complex |
| Nutrient cycling | More "open" (losses common) | More "closed" (efficient recycling) |
| Stability | Low (fluctuations high) | Higher stability and resilience |
Exam-friendly line: As succession progresses, ecosystems generally shift from quantity (fast growth) to quality (stability, diversity, efficiency).
10) Factors Affecting the Rate and Direction of Succession
- Climate: temperature and rainfall decide what climax-like stage can form.
- Substrate/soil: rock type, soil depth, nutrients, pH.
- Topography: slope, aspect, drainage. South-facing slopes (hotter/drier) may differ from north-facing slopes.
- Disturbance frequency: repeated fires can keep an area stuck at grass/shrub stage.
- Availability of seeds: distance from seed sources, presence of seed bank, animal dispersers.
- Invasive species: can arrest succession (e.g., aggressive invasives dominating disturbed areas).
- Human use: grazing, fuelwood collection, mining, road building.
11) Ecological Succession in India: High-Value Examples for Mains
11.1 After Forest Fires
After fire, grasses and fire-tolerant species may dominate first. If fires repeat every year, forests may not return fully. This is why fire management matters for long-term restoration.
11.2 After Cyclones in Coastal Areas
In areas like the Sundarbans or Odisha coast, storms can damage vegetation. Recovery often happens in stages, depending on salinity, tidal influence and sediment deposition. Mangrove regeneration is a form of secondary succession if soil remains and propagules/seeds survive.
11.3 After Shifting Cultivation (Jhum)
When land is left fallow, succession can regenerate forests. But if fallow periods become too short, soil fertility drops and recovery becomes weaker. This links succession with sustainable land use.
11.4 Mined and Degraded Lands
Mining often removes topsoil. If topsoil is removed completely, succession becomes closer to primary. Restoration requires soil replacement, native grasses, and staged plantation rather than planting only large trees immediately.
12) Practical Use: Why Planners and Conservationists Need Succession
- Ecological restoration: choose species according to successional stage (pioneers first, then later species).
- Afforestation success: planting climax forest trees on harsh degraded land often fails; pioneers and nurse plants help first.
- Watershed management: early vegetation reduces soil erosion and improves infiltration.
- Disaster recovery: understanding natural recovery helps in post-disaster ecosystem management.
- Conservation decisions: not every grassland "should become forest". Some natural grasslands are stable ecosystems; forcing forest may harm native biodiversity.
13) Prelims Quick Revision Points (Last-Day Notes)
- Succession = orderly community change over time.
- Primary succession starts without soil; pioneers often lichens/moss.
- Secondary succession starts with soil present; faster due to seed bank and nutrients.
- Seral stages are intermediate communities; sere is the whole sequence.
- Climax community is relatively stable and mature for that climate.
- Hydrosere = water to land succession; lithosere = on rock.
- Succession generally increases biomass, diversity, stability and nutrient cycling efficiency.
14) Mains Answer Framework (Ready-to-Write)
14.1 If asked: "Explain ecological succession and differentiate primary and secondary succession."
- Start with definition + 1-line example (fire/cyclone/abandoned field).
- Explain stages: pioneer → seral stages → climax-like stage.
- Add primary vs secondary table (soil, speed, pioneers).
- End with relevance: restoration, disaster recovery, land degradation.
14.2 If asked: "How does succession support ecosystem resilience?"
- Explain recovery path after disturbance.
- Mention soil formation, humus, nutrient cycling, biodiversity increase.
- Give Indian case: forest fire/cyclone/jhum fallow regeneration.
- Conclude: supports long-term stability, but repeated disturbances can arrest succession.
15) PYQ-Style Practice Questions (Not claiming actual UPSC years)
📝 PYQ-Style - Concept
In ecological succession, which of the following is most likely to be a pioneer community on bare rocks? Explain briefly.
📝 PYQ-Style - Application
After a forest fire, the regrowth of vegetation is generally faster than on a new lava flow. What is the key ecological reason?
16) Practice MCQs (With Answers and Explanations)
-
Ecological succession refers to:
- A) Seasonal change in species
- B) Random appearance of organisms
- C) Gradual replacement of communities over time
- D) Sudden extinction of a species
Answer: C
Explanation: Succession is an orderly, gradual change where one community replaces another.
-
Primary succession starts:
- A) Only after forest fire
- B) Only in ponds
- C) On a surface without soil
- D) Only in deserts
Answer: C
Explanation: The key condition is absence of soil at the beginning.
-
Which of the following is most likely a pioneer species on bare rock?
- A) Lichen
- B) Teak tree
- C) Banyan tree
- D) Deer
Answer: A
Explanation: Lichens tolerate harsh conditions and help in rock weathering and soil formation.
-
Secondary succession is faster mainly because:
- A) There is no competition
- B) Soil and seed bank are already present
- C) Only trees grow in secondary succession
- D) It happens only in rainy areas
Answer: B
Explanation: Existing soil, nutrients, microbes, roots and seeds speed up recovery.
-
The complete sequence of communities in succession is called:
- A) Niche
- B) Biome
- C) Sere
- D) Ecotone
Answer: C
Explanation: Sere = entire successional series; seral stage = each step.
-
Hydrosere refers to succession:
- A) On rocks
- B) In water bodies leading towards land community
- C) Only in deserts
- D) Only in salty marshes
Answer: B
Explanation: Hydrosere is succession starting in aquatic habitat.
-
In general, during succession, which trend is most correct?
- A) Biomass decreases continuously
- B) Food web becomes simpler
- C) Nutrient cycling becomes more efficient
- D) Soil organic matter disappears
Answer: C
Explanation: Late succession usually shows better recycling and more stable nutrient cycles.
-
Autogenic succession means succession mainly driven by:
- A) Human policies
- B) Organisms modifying the environment
- C) Volcanic eruptions only
- D) Earthquakes only
Answer: B
Explanation: Plants/organisms change soil, shade, moisture, enabling new communities.
-
Which statement best differentiates primary and secondary succession?
- A) Primary succession has soil; secondary does not
- B) Primary starts without soil; secondary starts with soil
- C) Secondary starts on bare rock; primary starts after fire
- D) Both always take the same time
Answer: B
Explanation: Soil presence at the start is the key difference.
-
Repeated disturbances like frequent fires can lead to:
- A) Immediate climax forest formation
- B) Arrested succession at early stages
- C) No impact on community structure
- D) Permanent absence of microbes
Answer: B
Explanation: Frequent disturbance can prevent the ecosystem from reaching late successional stages.
Conclusion
Ecological succession is nature's way of building ecosystems step by step. Primary succession starts from "zero soil" and is slow, while secondary succession starts after disturbance with soil and is faster. For UPSC, focus on the core definitions, the primary vs secondary comparison table, classic examples (lithosere, hydrosere), and the Mains linkage with restoration and disaster recovery.