How Succession Occurs When Environmental Factors Affect an Ecosystem Change

Ecological succession is the gradual process by which the mix of species in a community changes over time. Succession occurs when environmental factors affect an ecosystem change—such as a wildfire, flood, volcanic eruption, or human clearing—by resetting or altering the habitat and opening it to new colonists. The specific direction it takes depends on how severe that change was, what survived it, and how the surviving organisms themselves modify the site.

Two Pathways: Primary and Secondary Succession

The type of succession that follows depends on whether life and soil were already present before the change.

Primary succession begins on a newly formed surface that has no living organisms and no developed soil—bare rock exposed by a retreating glacier, a fresh lava flow, or newly formed sand dunes. Because there is no soil to hold water or nutrients, colonization starts with hardy organisms like lichens and mosses that can cling to rock and slowly break it down. Soil builds up over many decades, eventually allowing grasses, shrubs, and trees to establish.

Secondary succession begins after a disturbance destroys an existing community but leaves the soil intact. Fires, windstorms, floods, logging, and abandoned farmland all trigger this pathway. Because a seed bank, roots, and nutrients are already present, secondary succession proceeds far faster than primary succession. A burned forest may show regrowth within a single growing season, while primary succession on bare rock can take centuries. As Wikipedia’s overview of ecological succession notes, secondary succession follows disturbances that destroy a pre-existing community but do not strip away the soil.

After a fire, for example, some plants resprout from surviving root systems. Wind-blown and animal-carried seeds arrive within weeks. Fast-growing, sun-loving species—often grasses, weeds, and shrubs—dominate early, then give way as slower-growing trees shade them out. The soil’s stored nutrients give these early colonists a substantial head start.

The Stages of Succession

Ecologists describe succession as a sequence of stages. The classical framework, associated with early 20th-century ecologist Frederic Clements, breaks the process into recognizable phases:

• Nudation: a bare area forms, either newly created (primary) or exposed by disturbance (secondary).
• Migration: propagules—seeds, spores, or other dispersal units—arrive at the site.
• Ecesis: the arriving organisms establish and begin to grow.
• Competition: as vegetation spreads, species compete for space, light, and nutrients.
• Reaction: the community modifies its own habitat—for instance, by building up humus in the soil—which changes conditions for later arrivals.
• Stabilization: a relatively stable climax community forms.

Each intermediate community along this path is called a seral community. Different substrates and climates produce different seres, which is why a rocky slope and a river floodplain can follow quite different trajectories.

In classical theory, succession ends when the community reaches equilibrium with its physical and biological surroundings—a climax community. This stage is self-perpetuating: individuals that die are replaced by others of the same kind, so species composition stays roughly constant. Modern ecology treats this endpoint more flexibly. Disturbance is often frequent enough that a true steady state never fully arrives, and the same landscape may support several possible stable communities depending on local conditions.

How Environmental Factors Change the Outcome

Environmental factors influence succession in two broad ways: they determine what kind of disturbance occurs, and they shape how the recovering community develops.

A severe disturbance—lava flows, glacial retreat, deep mining spoils—removes soil entirely and pushes the site toward primary succession. A moderate disturbance—a surface fire, a flood that scours vegetation but leaves soil, a harvested woodlot—triggers secondary succession. The U.S. Forest Service and other land-management agencies study these patterns closely because post-fire and post-harvest recovery determines how quickly an area regains forest cover, wildlife habitat, and watershed protection.

Climate, soil type, slope, and available moisture further steer which species can colonize and which eventually dominate. In a moist, temperate region, a maple-beech community may represent the climatic climax; in a drier region, grasses and drought-tolerant shrubs may form the stable endpoint instead. The U.S. Geological Survey monitors how such factors interact across landscapes, especially where changing conditions shift the timing and severity of disturbances.

Human activity is now a dominant environmental factor. Clearing land, suppressing or igniting fires, introducing invasive species, and altering drainage all change the course of succession—sometimes accelerating it, sometimes locking a site into an early seral stage.

Consider a temperate forest hit by a severe crown fire. The fire kills most standing trees but leaves mineral soil and surviving root systems. Within a year, grasses and fireweed colonize. By year five, fast-growing shrubs and pioneer trees like aspen or birch dominate. Over several decades, shade-tolerant conifers establish beneath them. As the pioneers die, the conifers take over, and the site gradually approaches its former climax composition—provided no new disturbance intervenes.

Not every disturbance is a one-off event. Many ecosystems experience recurring disturbances at fairly regular intervals—fire-prone grasslands, floodplains that overflow each spring, coastal dunes reshaped by storms. Ecologists call this pattern a disturbance regime, and it helps explain why some communities never reach a single stable endpoint. The intermediate disturbance hypothesis proposes that species diversity tends to peak when disturbances are neither too rare nor too frequent: too rare and a few competitive dominants crowd out everything else; too frequent and only the fastest colonizers survive.

Different disturbance types leave different signatures. Fire tends to favor species with fire-resistant bark, resprouting roots, or seeds that germinate only after being heated. Floods often favor fast-growing riparian plants and scatter seeds downstream, accelerating colonization on freshly deposited sediment. Windstorms can create gaps in a canopy, allowing light to reach the forest floor and triggering small-scale, patchy succession within an otherwise mature stand. Two sites with similar climate and soil can follow different successional paths simply because they experience different kinds of disturbance.

Key Takeaways

• Succession occurs when environmental factors affect an ecosystem change, resetting or altering the habitat and allowing new species to colonize.
• Primary succession starts on surfaces with no soil or life, such as lava flows and bare rock; it is slow because soil must form first.
• Secondary succession follows disturbances that leave soil intact, such as fires, floods, and abandoned farmland; it proceeds much faster.
• The classical stages are nudation, migration, ecesis, competition, reaction, and stabilization, leading toward a climax community.
• Disturbance severity, climate, soil, moisture, and human activity all determine which species arrive, how quickly they establish, and what stable community eventually forms.
• Modern ecology views the climax as flexible rather than fixed, since repeated disturbances often prevent a true steady state.