📋 What’s Inside
Succession
What & Why
Stages
Step-by-step
Primary
Bare rock start
Secondary
After disturbance
Autogenic
Self-driven
In Plants
Xerosere
In Water
Hydrosere
What is Ecological Succession?
Ecological succession is the gradual, directional, and predictable process by which the species composition of a community changes over time — from a simpler, pioneer community to a more complex, stable community called the climax community.
🔑 Key Characteristics
- Directional: Change always moves in one direction — from simple to complex.
- Predictable: The sequence of communities that appear is largely predictable for a given region.
- Gradual: It occurs slowly over decades, centuries, or even millennia.
- Ends in Climax Community: A stable, self-sustaining community that does not change unless disturbed.
- Each intermediate community in the sequence is called a seral community or sere.
- The entire sequence from pioneer to climax is called a seral series or prisere.
Imagine a bare rock after a volcanic eruption. Over hundreds of years: lichens grow first → mosses appear → grasses follow → shrubs come in → trees establish → a stable forest develops. This entire journey from bare rock to forest is ecological succession. The forest at the end is the climax community.
Think of ecological succession like the growth of a city. A barren land first gets small shops, then apartments, then schools and hospitals — slowly becoming a fully functioning city. Nature does the same with living communities — starting small and building up to a complex, stable system.
🔑 Key Terms to Know
- Pioneer species: The first organisms to colonise a barren area (e.g., lichens on bare rock, phytoplankton in a new pond).
- Seral community (Sere): Each intermediate stage between pioneer and climax community.
- Climax community: The final, stable community at the end of succession — in equilibrium with the local climate.
- Nudation: The process of formation of a bare area (by fire, landslide, glaciation) which triggers succession.
- Facilitation: When early pioneer species modify the environment and make it suitable for the next community to come in.
Ecological succession has been directly tested in UPSC 2013 (grassland succession PYQ). Understand it as a concept that explains how ecosystems recover from damage — relevant to topics like forest fires, volcanic eruptions, dam construction, and climate change. The concepts of pioneer species, climax community, and seral stages frequently appear in statement-based questions.
Stages in Ecological Succession
Ecological succession proceeds through a series of well-defined stages. Each stage (seral community) modifies the environment — changing soil, microclimate, and available resources — making it suitable for the next, more complex community. The process continues until the climax community is established.
Nudation
Bare area formed
Migration
Propagules arrive
Ecesis
Species establish
Aggregation
Numbers increase
Competition
Species compete
Reaction
Habitat modified
Climax
Stable community
🔑 Detailed Explanation of Each Stage
- Nudation: A bare area is created — by volcanic eruption, glaciation, fire, flood, or human activity. This is the starting point of succession.
- Migration (Invasion): Seeds, spores, and propagules of organisms arrive at the bare area through wind, water, or animals.
- Ecesis (Establishment): Some of the arriving organisms successfully germinate, grow, and reproduce. These become the pioneer species.
- Aggregation: Pioneer species multiply and increase in number. Their populations grow and they begin to cover the area.
- Competition and Coaction: As populations grow, species compete with each other for space, light, water, and nutrients. Some species begin to dominate.
- Reaction: The living organisms modify their physical environment — changing soil chemistry, humidity, light availability, and temperature. This makes the area suitable for a new, different community, causing the current community to be replaced.
- Climax: A final, stable community establishes itself that is in balance with the local climate. This community no longer changes unless disturbed — it is self-sustaining.
When lichens grow on bare rock, they release acids that slowly break down the rock, creating a thin layer of soil. Mosses can then grow in this soil. Mosses add more organic matter, building up soil further. Grasses follow, then shrubs, then trees. Each community reacts on the environment to make it ready for the next. This is the core mechanism that drives succession forward.
🔑 What Changes During Succession?
- Species diversity increases from pioneer to climax stage.
- Biomass (total weight of living organisms) increases.
- Food web becomes more complex.
- Net primary productivity (NPP) initially increases, then stabilises in the climax.
- The ratio of Gross Primary Production (GPP) to Respiration (R) moves toward 1 as the community matures.
- Soil depth, organic matter, and nutrient content increase.
- Community becomes more resistant to external disturbances (more stable).
The stages of succession — especially nudation, ecesis, and reaction — can appear in UPSC as match-the-following or statement-based questions. Know that the reaction stage is the most important — it is the mechanism that actually drives succession forward by changing the habitat. Also remember: species diversity and biomass both increase through succession.
Primary Succession
Primary succession is the process of ecological succession that begins in a completely barren area where no soil exists and no living organisms or seeds are present. It starts from scratch — on newly formed land.
🔑 Key Features
- Begins on surfaces with no soil and no biological legacy (no previous organisms).
- Takes an extremely long time — hundreds to thousands of years.
- Pioneer species are typically lichens (on rock) or algae/phytoplankton (in water).
- Soil is built from scratch through the weathering of rock and accumulation of organic matter.
- Involves the largest changes in species composition over time.
Bare rock after a volcanic eruption (e.g., newly formed islands like Surtsey in Iceland). Glacial moraines left behind after a glacier retreats. Sand dunes newly formed along coastlines. Lava flows that solidify and cool. Newly formed ponds or lakes created by geological events. All of these are completely bare areas with no soil — and primary succession slowly builds life from nothing.
🔑 Sequence of Primary Succession on Rock (Xerosere)
- Crustose lichens — the very first colonisers; grow directly on bare rock; produce acids that slowly break down rock.
- Foliose lichens — more complex lichens; add more organic matter.
- Mosses — grow in the thin soil formed; hold moisture and add more organic matter.
- Herbaceous plants (grasses, herbs) — as soil deepens, grasses and small herbs establish.
- Shrubs — shade out herbaceous plants; further modify soil.
- Trees (climax forest) — large trees eventually dominate; a stable forest climax community is reached.
Primary succession is like building a house on an empty plot — you first need to lay the foundation (soil), then the walls (pioneer plants), then the roof (canopy trees) — everything is built from zero.
Know that lichens are the pioneer species in primary succession on rock — this is a frequently tested fact. Also know that primary succession is very slow because soil must be built from scratch. The newly formed volcanic islands (like Surtsey) are real-world examples of primary succession that UPSC may use as context in questions.
Secondary Succession
Secondary succession is the process of ecological succession that occurs in an area where a community has been destroyed or removed but the soil, seed bank (dormant seeds in the soil), and some organisms still remain. It is a restart, not a fresh start.
🔑 Key Features
- Begins on surfaces where soil already exists and may contain seeds, roots, or dormant organisms.
- Much faster than primary succession — takes decades to a few centuries, not millennia.
- Pioneer species are often grasses, weeds, and fast-growing herbs (not lichens — soil is already present).
- The existing seed bank in the soil speeds up recovery considerably.
- Soil does not need to be created — it already has nutrients and organic matter.
After a forest fire — soil and roots remain; grasses grow back within weeks, shrubs within years, trees within decades. After a flood — once waters recede, pioneer plants regrow from the existing seed bank. Abandoned farmland — when farming stops, wild plants colonise the field; over time it returns to woodland. After a landslide — the exposed area has soil from below and seeds blown in from nearby areas. Cleared forests — areas where trees were cut down begin secondary succession naturally.
Primary = No soil, no seeds, starts from scratch. Secondary = Soil present, seeds present, rebuilding after damage. Think: Primary is like building a new house on empty land. Secondary is like renovating a damaged house — the foundation is already there.
Secondary succession is more relevant to current conservation scenarios — forest fires, deforestation, and flood-damaged ecosystems all trigger secondary succession. UPSC links this to India’s ecosystem restoration policies. The key fact: Secondary succession is always faster than primary succession because soil is already present. This is a frequently tested comparison.
Autogenic and Allogenic Succession
Ecological succession can be driven by two different forces — the organisms themselves, or outside physical forces. These give us Autogenic Succession and Allogenic Succession.
Autogenic succession is driven by the organisms within the community itself. The community modifies its own physical environment through biological processes — and these modifications make the area suitable for a new, different community to replace it.
🔑 Key Points — Autogenic
- The cause of change is internal — the organisms are the engine of change.
- Example: Lichens break down rock and create soil → soil allows mosses to grow → mosses add organic matter → this allows grasses to grow → and so on. Each community creates the conditions for the next.
- Most natural successions (on rock, in ponds) are primarily autogenic.
- The word auto = self; the community changes itself.
Allogenic succession is driven by external physical or environmental forces — not by the organisms themselves. The community changes because the external environment around it changes.
🔑 Key Points — Allogenic
- The cause of change is external — an outside force disturbs or changes the environment.
- Examples of external forces: climate change, glaciation, fire, floods, drought, volcanic eruption, human deforestation.
- Example: A river changes its course and dries up a pond → the pond’s aquatic community disappears and a terrestrial community begins — this is allogenic succession driven by physical change.
- Example: A long drought causes a forest to change into grassland — driven by external climate, not by the organisms themselves.
- The word allo = other/external; the change comes from outside.
Autogenic: A pond fills up with sediment and organic matter (added by aquatic plants and animals over time) — the pond slowly becomes shallower, then marshy, then a meadow. The organisms themselves filled it in.
Allogenic: A river floods and deposits a thick layer of sand over a forest — the forest community is replaced by sand-tolerant plants. The change was caused by an external force (flood), not the forest organisms.
Autogenic vs Allogenic is a conceptual distinction that can appear in match-the-following or statement-based questions. Key rule: Autogenic = community changes itself from within. Allogenic = external forces change the community from outside. Human activities like deforestation and dam construction are classic examples of allogenic succession triggers.
Succession in Plants
Xerosere is the type of ecological succession that occurs in a dry environment — on bare rock, sand dunes, or dry land where water is scarce. It is a type of primary succession on land. The word comes from Greek: xero = dry, sere = series of communities.
🔑 Stages of Xerosere (Succession on Bare Rock → Climax Forest)
-
Crustose Lichen Stage (Pioneer Stage):
Crustose lichens (flat, crust-like) are the first colonisers. They attach directly to bare rock. They secrete acids that chemically weather the rock. On death, they add a thin layer of organic matter. Extremely slow stage. -
Foliose Lichen Stage:
Slightly more complex lichens with leaf-like structures colonise the area. They add more organic matter and accelerate rock weathering. A very thin humus-like layer begins to form. -
Moss Stage:
Mosses (bryophytes) grow in the thin soil formed. They can hold water and significantly add to organic matter. Soil depth increases. The environment becomes moister. -
Herbaceous Stage (Herb/Grass Stage):
As soil becomes deeper and nutrient-rich, grasses and herbs colonise. They have proper root systems. Annual plants appear first, then perennials. Shade increases and soil quality improves rapidly. -
Shrub Stage:
Shrubs (bushy woody plants) replace grasses. They create significant shade, making it difficult for grasses to survive. Soil is now deep, rich, and moist. Insects, birds, and small animals begin to appear. -
Woodland/Forest Stage (Climax):
Trees (first small trees, then larger ones) eventually dominate. A complex, multi-layered forest develops — with a canopy, understorey, shrub layer, and forest floor. This is the climax community — stable, diverse, and self-sustaining.
Sand dunes on a coastline undergo their own version of xerosere: bare sand → sea couch grass (stabilises sand) → marram grass (deeper roots) → other grasses and herbs → shrubs → eventually a coastal scrub or woodland. Each plant stabilises the sand more and adds organic matter, enabling the next stage.
The sequence Lichens → Mosses → Herbs → Shrubs → Trees is the most tested succession sequence in UPSC. Know that lichens are ALWAYS the pioneer species on bare rock. The climax vegetation depends on the local climate — in a tropical region the climax is a tropical rainforest; in a temperate region it is a temperate deciduous forest; in an arid region it may be a thorn forest or savanna.
Succession in Water
Hydrosere is the type of ecological succession that begins in a freshwater environment — a lake, pond, or slow-moving water body. Over time, the water body fills up with sediments and organic matter, becoming progressively shallower, until it eventually turns into a terrestrial ecosystem. It is a type of primary succession in water.
🔑 Stages of Hydrosere (Pond → Forest)
-
Phytoplankton Stage (Pioneer Stage):
In a newly formed, deep, clear lake — microscopic floating algae (phytoplankton) are the first colonisers. They begin to add organic matter as they die. Zooplankton and microscopic animals follow. -
Rooted Submerged Plant Stage:
As sediment accumulates and the lake becomes shallower, rooted aquatic plants (like Hydrilla, Potamogeton) grow on the bottom. They add more organic matter and further raise the lake bottom. -
Rooted Floating Plant Stage:
The lake becomes shallower still. Plants with floating leaves appear — water lilies (Nymphaea), water lettuce (Pistia), lotus (Nelumbo). These shade the water below, reducing deeper aquatic plant growth. -
Reed Swamp Stage (Amphibious Stage):
Tall emergent plants like reeds (Phragmites), bulrushes, and cattails (Typha) grow at the water’s edge. Their roots trap more sediment. The water body shrinks. Swamp conditions develop. -
Marsh/Meadow Stage:
The area becomes very shallow or waterlogged. Sedges, grasses, and marsh plants dominate. The area is periodically dry. It is now a wetland or marsh — not open water any more. -
Woodland/Forest Stage (Climax):
As the area dries completely, terrestrial shrubs and trees colonise. Over time, a forest develops on what was once a lake. The climax community is now a terrestrial forest — the hydrosere has become a xerosere-type climax.
Many small lakes and ponds around the world are gradually undergoing hydrosere. Wetlands like Loktak Lake (Manipur) and Chilika Lake (Odisha) show signs of succession — their edges are progressively being colonised by reeds and marsh plants. The famous floating phumdi (heterogeneous mass of vegetation) in Loktak is essentially a stage in the hydrosere process.
Xerosere = starts DRY (bare rock/sand) → ends in forest. Hydrosere = starts WET (open water) → ends in forest. Both end in the same destination — a stable terrestrial climax forest — but they start from opposite extremes.
Hydrosere is linked to wetland conservation — a hot UPSC topic. The filling up of lakes and ponds (eutrophication) can accelerate hydrosere artificially. Know that phytoplankton are the pioneer species in aquatic succession — just as lichens are in terrestrial (rock) succession. The wetland-to-forest conversion also has implications for carbon sequestration and climate, relevant to current affairs questions.
Master Comparison
Primary vs Secondary Succession
| Parameter | Primary Succession | Secondary Succession |
|---|---|---|
| Starting point | Completely bare area — no soil, no life | Disturbed area — soil and seeds still present |
| Cause | New land formation (volcano, glacier retreat) | Destruction of existing community (fire, flood, farming) |
| Speed | Very slow — hundreds to thousands of years | Relatively fast — decades to centuries |
| Pioneer species | Lichens (on rock), phytoplankton (in water) | Grasses, weeds, fast-growing herbs |
| Soil formation | Soil must be built from scratch | Soil already exists; nutrients available |
| Seed bank | None present initially | Present in existing soil |
| Climax community | Same — a stable forest or grassland | Same — returns to original or similar climax |
| Example | Bare lava, newly exposed glacial rock, new sand dunes | After forest fire, abandoned farmland, post-flood area |
Autogenic vs Allogenic Succession
| Parameter | Autogenic Succession | Allogenic Succession |
|---|---|---|
| Driver of change | Organisms within the community (internal) | External physical/environmental forces |
| Mechanism | Community modifies its own environment | Outside forces alter the environment |
| Examples of causes | Soil formation by lichens, silt deposition by aquatic plants | Climate change, fire, flood, glaciation, human clearing |
| Most natural successions | Yes — most natural successions are autogenic | Less common; often involves disturbance events |
| Word origin | Auto = self | Allo = other/external |
Xerosere vs Hydrosere
| Parameter | Xerosere | Hydrosere |
|---|---|---|
| Meaning | Succession on dry land | Succession in freshwater |
| Starting environment | Dry bare rock or sand | Open water — lake or pond |
| Pioneer species | Crustose lichens | Phytoplankton (algae) |
| Key stages | Lichens → Mosses → Herbs → Shrubs → Forest | Phytoplankton → Submerged plants → Floating plants → Reeds → Marsh → Forest |
| End point (Climax) | Terrestrial forest | Terrestrial forest (same!) |
| Example | Bare volcanic rock, sand dunes | Ageing lake, pond, slow river |
