The Stages of Ecological Succession: From Nudation to Climax

Every ecosystem you see around you – a forest, a wetland, a rocky hillside – was once something far simpler. Left undisturbed, bare land and open water don’t stay that way forever. Through a predictable, step-by-step process called ecological succession, nature gradually builds complexity, replacing simple pioneer communities with increasingly diverse ones until a stable endpoint is reached. Ecological succession was first systematically studied by Frederic Clements in 1916, who laid out the sequential phases that ecologists still reference today. Understanding these phases – from the creation of a bare area right through to climax – reveals how ecosystems assemble themselves from scratch.

Nudation: where succession begins 🔗

Nudation is the development of a bare area with no life forms present. It is the essential starting point, because without a cleared space, there is nothing for new communities to colonize. This bare condition is created by three broad categories of causes.

Topographic causes include soil erosion, landslides, volcanic eruptions, and sand deposition – all of which physically strip an area of existing vegetation and soil. Climatic causes involve destructive events such as glacial advance, prolonged drought, or powerful storms that kill off existing communities. Biotic causes are human-driven, including deforestation, agriculture, and disease epidemics that eliminate the organisms in a given area. Regardless of the cause, the outcome is the same: a bare substrate – rock, sediment, or open water – that sets the stage for new life to move in.

Invasion: life moves in 🔗

Once a bare area exists, invasion begins – the successful establishment of species in that previously empty space. This happens through three distinct sub-steps.

Migration 🔗

The first sub-step is migration, which refers to the arrival of seeds, spores, or other reproductive bodies (collectively called propagules) at the site. Dispersal agents include wind, which carries lightweight spores and seeds over long distances; water currents, which transport propagules along rivers and coastlines; and animals, which carry seeds in fur, feathers, or digestive tracts. The type and speed of migration depends heavily on what dispersal agents are active in that environment.

Ecesis 🔗

Ecesis is the establishment phase – where arriving propagules germinate, grow, and reproduce successfully in the new environment. Not every migrating propagule survives. The seedling must withstand local conditions including temperature extremes, drought, low nutrients, and competition from other arriving organisms. Ecesis is considered complete only when an organism can reproduce sexually in its new environment, confirming it has truly established itself rather than merely survived temporarily.

Aggregation 🔗

Aggregation follows ecesis. Established pioneer individuals reproduce, and their offspring grow nearby, gradually increasing the population density. What begins as isolated individuals spread across bare ground becomes denser clusters of pioneer species, which then begin to alter local conditions – setting up the next stage of succession.

Competition and coaction: species interact 🔗

As aggregation increases population density, resources become limited. Space, light, water, and nutrients are finite, and multiple species are now competing for all of them. Competition in succession is of two kinds: interspecific competition occurs between different species, while intraspecific competition (also called coaction) occurs among individuals of the same species.

This competitive pressure is not simply destructive – it is selective. Species better adapted to local conditions outcompete others, determining which organisms persist into later seral stages. The success of plants depends on seed dormancy and the capacity of seedlings to withstand adverse conditions like drought, low light, and pest damage. Those that survive shape the next transformation: reaction.

Reaction: organisms reshape their environment 🔗

Reaction is arguably the most ecologically significant stage in succession. It refers to the modification of the habitat by the organisms living in it. Clements considered reactions the primary driving force of succession, because each community alters conditions in ways that make the environment less suitable for itself and more suitable for the next wave of species.

Practical examples of reaction are easy to observe. Pioneer plants bind loose soil particles and prevent erosion. Their roots break up compacted substrate. When they die and decompose, they add organic matter, increasing the moisture-holding capacity and nitrogen content of the soil. Shading by taller plants lowers ground-level temperature and raises humidity. This accumulation of metabolic waste and organic matter alters the environment, making it increasingly suitable for later-arriving species. Each successive community modifies conditions further, triggering replacement by the next seral community.

Climax: the stable endpoint 🔗

After a series of seral communities have replaced one another through repeated cycles of invasion, competition, and reaction, the community eventually reaches climax – a stable, self-sustaining state in equilibrium with the prevailing climate. The climax community is final, terminal, and self-perpetuating, representing a steady state of species composition, community structure, and energy flow.

Several defining features distinguish a climax community from earlier seral stages. There is no net annual accumulation of organic matter – production and decomposition are balanced. Individuals of climax species are replaced by others of the same species, so the overall composition remains stable. The community tolerates its own reactions rather than being displaced by them. It has high species diversity, complex food webs, and well-defined spatial structure. Climax communities are generally determined by the regional climate, which is why ecologists refer to climatic climax as the theoretical endpoint when local edaphic or disturbance factors don’t intervene.

There are, however, variations. An edaphic climax forms when soil conditions – moisture, texture, or nutrients – prevent the regional climatic climax from developing. A disclimax (or anthropogenic subclimax) occurs when human activity, such as overgrazing or repeated burning, maintains a community below its natural climax potential. A subclimax is the seral stage that immediately precedes the climatic climax, sometimes persisting for a very long time under local constraints.

Hydrosere: succession in aquatic environments 🔗

A hydrosere is a primary succession sequence that begins in a freshwater body – a pond, lake, or wetland – and progresses toward a terrestrial climax community. The succession from open water to climax woodland takes centuries or millennia, driven by the gradual accumulation of sediment and organic matter that progressively shallows the water body.

The process unfolds through distinct seral stages. In the initial phytoplankton stage, pioneer colonizers such as cyanobacteria, green algae like Spirogyra and Oedogonium, and diatoms populate the open water. Their life activities add organic matter and nutrients, and when they die, their remains settle as a layer of muck on the bottom. This raises the substrate slightly and creates conditions for the next stage.

In the submerged plant stage, rooted hydrophytes such as Elodea, Potamogeton, and Ceratophyllum establish in the shallow muddy bottom. These plants trap sand and silt around their stems, steadily raising the bottom level further. As water depth decreases, floating-leaved plants like water lilies colonize the area. Their broad leaves shade the water below, suppressing submerged plants and adding further leaf litter to the accumulating sediment.

Shallower water allows reed swamp species – cattails (Typha), bulrushes, and reeds – to take hold. Their rhizomes knit the soil, their leaves transpire large quantities of water, lowering the water table, and their seasonal die-back adds to the peat layer. Eventually, the substrate rises above the water level entirely, and sedge meadow species such as Carex and Juncus colonize the drier ground. Mesic conditions develop further, allowing invasion by shrubs and trees such as willow (Salix), alder (Alnus), and poplar (Populus). The climax forest stage sees herbs, shrubs, mosses, and shade-tolerant plants coexisting, with trees dominant and bacteria and fungi enriching the soil with organic matter.

Lithosere: succession on bare rock 🔗

A lithosere is a type of xerosere succession that originates on bare rock surfaces – areas exposed by glacial retreat, volcanic eruption, or tectonic uplift. The defining challenge of a lithosere is the complete absence of soil, which rules out most plants entirely. The driving force of each seral progression in a lithosere is the impact that current species have on their own environment – each community builds the soil conditions that allow the next to establish.

The sequence begins with the crustose lichen stage. Crustose lichens – symbiotic associations of fungi and algae – are the only organisms capable of colonizing bare, mineral rock. They require no soil, absorbing moisture and dissolved minerals directly from the rock surface. They produce organic acids that slowly weather the rock, releasing minerals and creating the first fine particles of proto-soil. As crustose lichens die and decay, foliose lichens (larger, leaf-like forms) invade and further deepen the surface layer.

The moss stage follows once a thin layer of soil and organic matter has accumulated. Mosses such as Polytrichum have rhizoids that penetrate rocky crevices, absorbing water and deepening the substrate. Their dense, velvet-like growth retains moisture, moderates temperature, and adds substantial organic matter on decomposition.

With improved soil, the annual grass and herb stage begins. Hardy annual grasses like Aristida and Poa take hold. Their root systems physically fragment the rock further, accelerating soil formation and increasing humus content. Over time, these are replaced by perennial grasses and shrubs, which have deeper root systems and greater biomass, enriching the soil even further.

Eventually, the substrate can support woodland and forest. Fast-growing pioneer trees like birch establish first, providing canopy shade that allows the forest floor to develop humidity and humus. The climax of a lithosere is marked by fast-growing trees such as birch and then by slower-growing but larger trees like oak, which form the final stable community in equilibrium with the regional climate. Lithosere succession is notably slower than hydrosere, often requiring centuries to millennia depending on rock type, climate, and species availability.

What hydrosere and lithosere share 🔗

Despite beginning in completely different substrates – water versus bare rock – both hydrosere and lithosere converge on the same fundamental pattern. Pioneer species modify the environment through reaction, making it less suitable for themselves but more hospitable for the next seral community. Each stage builds on the biological and physical legacy of the last. Both sequences ultimately move toward a mesophytic climax forest under similar climatic conditions, demonstrating that the same ecological process operates regardless of starting conditions. In plant succession, the last stage tends to be a climax forest regardless of whether the sere began as a xerosere or hydrosere – they differ only in their intermediary stages.

This convergence has important implications for restoration ecology, where understanding which seral stage an ecosystem is currently in helps land managers accelerate recovery after disturbance – by introducing the right pioneer species at the right time, or by protecting early-stage communities from further disruption so that natural succession can proceed uninterrupted.

What do you think? If human activity can create a disclimax – a community held below its natural climax potential – what responsibility does that place on us to actively manage or restore those ecosystems? And given that both hydrosere and lithosere eventually converge on the same climax community type, does the starting substrate of an ecosystem really matter in the long run?

References

1. https://en.wikipedia.org/wiki/Ecological_succession
2. http://www.kaliganjgovtcollege.ac.in/studyMaterial/01036Stages-of-Ecological-Succession.pdf
3. https://biosmartnotes.com/steps-of-ecological-succession/
4. https://ebooks.inflibnet.ac.in/esp01/chapter/20-ecological-succession-part-2/
5. https://en.wikipedia.org/wiki/Hydrosere
6. https://www.biologydiscussion.com/plants/7-successional-stages-of-hydrosere-with-diagram/6814
7. https://www.savemyexams.com/a-level/geography/aqa/18/revision-notes/6-ecosystems-under-stress/6-4-ecosystems-in-the-british-isles/6-4-1-vegetation-succession-in-the-uk/
8. https://www.vedantu.com/question-answer/correct-succession-of-stages-of-a-lithosere-class-12-biology-cbse-5f982df4e7dec55df5c1d168
9. https://biosmartnotes.com/xerosere-succession-stages/