NCERT grounding
Biotic communities are not static. The NIOS Biology module on the Principles of Ecology defines ecological succession as the process by which the communities of plant and animal species in an area are replaced by another over a period of time. Both biotic and abiotic components drive this change — it is brought about partly by the activities of the communities themselves and partly by the physical environment of that area. The physical environment, in turn, influences the nature, direction, rate and optimal limit of these changes. Across the sequence, both plant and animal communities are transformed.
The process by which communities of plant and animal species in an area are replaced by another over a period of time is known as ecological succession.
NIOS Biology · Principles of Ecology
The same source establishes the two recognised types — primary succession and secondary succession — and the moisture-based classes of xerarch (dry-start) and hydrarch (water-start) succession. This page treats each of those terms in depth, because NEET examiners reward exact recall of the vocabulary far more than vague description.
What ecological succession means
Succession is best understood as an orderly, directional and largely predictable replacement of one community by the next. A bare or disturbed patch of land is not invaded by its final vegetation in a single step. Instead a series of communities arrive, modify the habitat, and are themselves displaced — each one preparing conditions that favour the community that comes after it. This self-driven modification is the engine of succession: the organisms present at any stage change the soil, moisture, light and nutrient regime in ways that make the site less suitable for themselves and more suitable for their successors.
Because the physical environment sets the direction and rate, succession is not random drift. It moves towards a community that is increasingly stable and increasingly in tune with the regional climate. Along the way, the community grows more complex: species number, biomass and the number of niches all tend to rise from the bare beginning towards the mature end-state.
Types of succession
Succession is classified into primary succession (begins where no community ever existed) and secondary succession (begins where an existing community was destroyed). By water availability it is further described as hydrarch or xerarch.
The very first organisms to arrive on a previously lifeless site are the pioneer species. The assemblage of these pioneers is the pioneer community. Pioneer species are biologically distinctive: they generally show a high growth rate but a short life span, and they tolerate the harsh, resource-poor conditions of an uncolonised habitat. On bare rock the classic pioneers are lichens, which can survive extreme dryness and which secrete acids that dissolve and weather the rock surface. That chemical weathering, together with the trapping of dust and the decay of dead lichen tissue, produces the first traces of soil — without which no higher plant could establish.
Figure 1. Primary succession on bare rock. Lichens act as the pioneer community; mosses, herbs, grasses and shrubs form the intermediate seral stages; shade-tolerant trees form the stable climax community. Each stage modifies the habitat for the next.
Sere, seral stages and the climax
Three terms must be kept crisply separate, because NEET frequently tests them as distractors of one another. Each transitional, temporary community that forms and is then replaced during succession is called a seral community (also called a stage in succession or a seral stage). The terminal, final stage of succession is the climax community. And the entire sequence of communities that succeed one another in a given area, from the pioneer community through every seral stage to the climax, is called a sere.
The sere — from pioneer community to climax
-
Stage 1
Pioneer community
First colonisers of the bare or disturbed area; high growth rate, short life span.
e.g. lichens -
Stage 2
Early seral stage
Mosses and small annual plants establish on the thin soil left by pioneers.
transitional -
Stage 3
Mid seral stage
Perennial herbs and grasses deepen the soil and add organic matter.
transitional -
Stage 4
Late seral stage
Shrubs and shade-intolerant trees give way to shade-tolerant trees.
transitional -
Stage 5
Climax community
Stable, mature, complex, long-lasting; in dynamic equilibrium with the climate.
terminal stage
The climax community has a defined biological character. It is stable, mature, more complex and long lasting compared with every seral community before it. As long as it remains undisturbed, it stays in dynamic equilibrium with the prevailing climate and habitat factors — meaning species are continually born and die, but the overall composition does not change. The climax persists for as long as the environment itself remains unchanged; a major disturbance such as fire, flood or clearing resets the site and a new succession begins.
Animals are part of this story too. The animal community of any successional stage also exhibits succession, and to a large extent the animal succession is determined by the plant succession — vegetation supplies the food and shelter that decide which animals can persist. Animal succession is additionally shaped by which species are able to migrate in from neighbouring communities.
Primary vs secondary succession
The most heavily examined division of succession is by the starting condition of the site. Primary succession takes place over bare or unoccupied areas where no community has ever existed before — rock outcrops, newly cooled lava flows, emerging volcanic islands, newly formed deltas and sand dunes, and glacial moraines (the muddy ground exposed by a retreating glacier). A newly created pond or reservoir is the aquatic equivalent of such a fresh, lifeless site.
The defining difficulty of primary succession is that soil is initially absent. Before any rooted plant can grow, the slow work of soil formation must be done by the pioneer community. This is why primary succession takes a very long time, and why it is far harder to observe in nature than secondary succession — there are relatively few places on earth that do not already carry communities of organisms.
Secondary succession is the development of a community on land where a natural biotic community already existed but was removed, disturbed or destroyed. The trigger may be a natural event such as a hurricane or forest fire, or a human-related event such as the tilling, harvesting or abandonment of farmland. The crucial point is that the soil is already present, complete with nutrients and with a large pool of seeds and other dormant stages of organisms. Because the site does not have to wait for soil to form, secondary succession is relatively fast compared with primary succession.
Primary succession
No soil
starts on a never-colonised site
- Begins where no community ever existed
- Sites: bare rock, cooled lava, volcanic islands, sand dunes, glacial moraines, new pond
- Soil must be formed first by pioneers
- Very slow — takes a very long time
- Hard to observe; few such sites exist
Secondary succession
Soil present
starts on a cleared but soil-rich site
- Begins where an existing community was destroyed
- Sites: abandoned farmland, burned or cut forest, flooded land
- Soil already holds nutrients, seeds, dormant stages
- Relatively fast
- Common and easier to observe
The contrast is therefore not about the destination — both can end at a comparable climax — but about the head start. Secondary succession inherits a functioning soil and a seed bank, so it skips the longest and slowest part of the journey.
Hydrarch vs xerarch succession
The second classification describes succession by the water availability of the starting habitat. Hydrarch succession takes place in a water body — a pond, lake or newly created reservoir. Its successional series begins under hydric (water) conditions: open water is gradually colonised by submerged plants, then floating and emergent vegetation, sediment and organic matter accumulate, the basin slowly fills, and the series progresses towards drier ground. Xerarch succession takes place in a dry area such as bare rock or a sand dune, where moisture content is low. Its series begins under xeric (dry) conditions and progresses as soil and moisture-holding capacity build up over time.
The single most testable idea is the direction of convergence. A hydrarch series moves away from excess water; a xerarch series moves away from extreme dryness. They start at opposite extremes, yet both progress towards medium-moisture, mesic conditions. The climax community of both a hydrarch and a xerarch sere reflects this balanced, moderate water environment — succession smooths out the extremes of the starting habitat.
Figure 2. Hydrarch succession begins in hydric (wet) conditions and xerarch succession begins in xeric (dry) conditions. Although they start at opposite extremes, both progress towards and end at mesic, medium-moisture conditions.
Memory hook: the prefix tells you the start, never the end. Hydrarch = wet start, xerarch = dry start — and both roads lead to mesic.
Hydrarch
Start: hydric — open water body, pond or lake.
Direction: water-to-land; the basin fills and dries.
End: mesic, medium-moisture climax.
Xerarch
Start: xeric — bare rock or sand dune, low moisture.
Direction: dry-to-moist; soil and water capacity build.
End: mesic, medium-moisture climax.
A useful linking idea ties these classes together: primary succession on bare rock is a xerarch series, while primary succession in a newly formed pond is a hydrarch series. The two classifications — primary/secondary and hydrarch/xerarch — describe different aspects of the same event and can therefore be combined.
Worked examples
Arrange the following in the correct order of a xerarch primary succession on bare rock: perennial herbs and grasses; lichens; shade-tolerant trees; mosses; shrubs and shade-intolerant trees.
The correct sequence is lichens → mosses → perennial herbs and grasses → shrubs and shade-intolerant trees → shade-tolerant trees. Lichens are the pioneer community on bare rock because they secrete acids that weather the rock and begin soil formation. Mosses and small annuals follow on the thin soil; perennial herbs and grasses deepen it; shrubs and shade-intolerant trees come next; and shade-tolerant trees form the stable climax community. Each intermediate community is a seral stage, and the whole sequence is the sere.
A forest is completely cleared by a fire. Within a few decades a comparable forest re-establishes on the site. Identify the type of succession and explain why it is faster than succession on a newly cooled lava flow.
This is secondary succession, because it begins on land where a natural community already existed and was destroyed by a disturbance (fire). It is faster than succession on a lava flow — a case of primary succession — because the burned forest retains its soil, complete with nutrients and a large pool of seeds and dormant stages. On the lava flow there is no soil at all, so the very slow process of soil formation by pioneer species must happen first, making primary succession take a very long time.
Hydrarch succession begins in a pond and xerarch succession begins on a dry rock. Towards what common condition do both progress, and what does this reveal about succession?
Both progress towards mesic — medium-moisture — conditions. Hydrarch succession moves from hydric (wet) conditions and xerarch succession moves from xeric (dry) conditions, but the two series converge on the same balanced, moderate water environment. This reveals that succession smooths out the extremes of the starting habitat: regardless of whether the site began too wet or too dry, the directional process drives it towards a stable mesic climax community in equilibrium with the regional climate.
Common confusion & NEET traps
Succession questions punish loose vocabulary. Three confusions account for most lost marks: the sere/seral distinction, the direction of hydrarch and xerarch series, and the assumption that primary and secondary succession differ in their endpoint rather than their starting point.