Ecosystems are never finished. A field left unploughed fills with weeds, then shrubs, then young trees. A lava flow is colonised by lichens, then mosses, then ferns and forest. A forest burnt by fire regrows. This gradual, directional change in the species that make up a community over time is called ecological succession, and the broader process of change in an ecosystem’s structure and functioning is called ecosystem development.
What you will learn
- The difference between primary and secondary succession
- The stages of succession and the role of pioneer species
- How facilitation, tolerance and inhibition drive succession
- Modern views of climax communities and disturbance
- Real-world examples and their importance for restoration
Primary and secondary succession
Primary succession begins on newly exposed surfaces where no soil exists: bare rock after a glacier retreats, fresh lava, new volcanic islands, sand dunes. It is slow, often taking centuries, because soil must form first.
Secondary succession begins where a community has been disturbed but soil remains: after a fire, flood, storm, logging or when farmland is abandoned. It is much faster, because seeds, roots and soil organisms are already present.
The stages of succession
Succession is usually described as a series of stages, called seral stages:
- Pioneer stage. Hardy organisms that can tolerate harsh conditions colonise first. On bare rock these are lichens and mosses, which weather the rock and trap dust, beginning soil formation. On disturbed soil, fast-growing annual weeds and grasses arrive first.
- Early stage. As soil builds up, grasses, herbs and nitrogen-fixing plants establish.
- Intermediate stage. Shrubs and fast-growing, sun-loving trees such as birch, pine or alder shade out the early plants.
- Late stage. Slower-growing, shade-tolerant trees such as oak, beech or maple eventually dominate.
- Climax community. A relatively stable community that persists until the next major disturbance.
Watch the stages unfold below.
Throughout succession, several trends are typical: soil deepens and becomes richer in organic matter, biomass increases, the number of species usually rises (often peaking in the intermediate stages), food webs become more complex, and nutrients are held more tightly within the ecosystem.
What drives succession?
In 1977, the ecologists Joseph Connell and Ralph Slatyer described three ways that earlier species affect later ones:
- Facilitation: early species make conditions better for later species, for example by building soil or adding nitrogen. This is typical of primary succession.
- Tolerance: later species are simply better at tolerating low resources, such as shade, and outlast earlier species.
- Inhibition: early species hold space and prevent later species from establishing until they die or are disturbed.
All three operate in real ecosystems. Succession can also be autogenic, driven by the organisms themselves (such as shading and soil building), or allogenic, driven by external forces (such as changes in climate, drainage or sediment).
The climax concept, then and now
In the early twentieth century, the American ecologist Frederic Clements described succession as a predictable march towards a single climax community determined by the regional climate, almost like the development of an organism. Henry Gleason challenged this in 1926, arguing that communities are loose collections of species responding individually to conditions and chance.
Modern ecology sits closer to Gleason’s view. Succession is influenced by which seeds arrive first, by grazing animals and by repeated disturbance, so the same starting point can lead to different outcomes. Many landscapes are kept in a shifting mosaic of stages by natural disturbances such as fire, floods, storms and the activities of animals like elephants and beavers.
Disturbance and diversity
The intermediate disturbance hypothesis, proposed by Joseph Connell in 1978, suggests that species diversity is highest at moderate levels of disturbance. With too little disturbance, a few dominant competitors take over. With too much, only a few hardy pioneers survive. In between, species from several stages coexist. The idea fits some systems, such as coral reefs and rocky shores, but not all, and remains debated.
Examples of succession
- Krakatau, Indonesia. The 1883 eruption sterilised the island’s remnants. Within three years ferns and blue-green algae were growing; within 50 years forest had returned, and birds, bats, reptiles and insects had recolonised from Java and Sumatra. Krakatau became a natural experiment in island colonisation.
- Glacier Bay, Alaska. As glaciers retreated over the last 250 years, scientists could study a sequence of surfaces of different ages, from bare rock near the ice to mature spruce and hemlock forest where the ice melted longest ago.
- Mount St Helens, USA. After the 1980 eruption, lupins (nitrogen-fixing plants), surviving gophers and fallen trees helped life return faster than expected. The eruption showed the importance of biological legacies: organisms and structures that survive a disturbance.
- Abandoned farmland in Europe and eastern North America, which has returned to forest over decades, a process that now supports rewilding.
Why succession matters for restoration
Restoration ecologists use succession to repair damaged ecosystems. Sometimes it is enough to remove the pressure, such as grazing or ploughing, and let natural regeneration take its course. Elsewhere, restoration can speed things up by planting nurse species, adding soil microbes, reintroducing seed-dispersing animals, or reinstating natural fire regimes. In tropical forests, natural regeneration on former farmland can often store carbon faster and more cheaply than planting trees, provided remnant forest and seed dispersers are nearby.
Case study: the Dust Bowl and the return of the prairie
In the 1930s, drought and ploughing of native grassland on the US Great Plains led to massive soil erosion, the Dust Bowl. Ecologists including John Weaver studied which prairie plants survived and how grassland recovered. Their work showed how deep-rooted native grasses stabilise soil, and influenced soil conservation and grassland restoration programmes that continue today.
Try it yourself
Find a patch of ground that has been disturbed recently, such as a cleared plot, a crack in pavement or a burnt area. Visit it every month for a season and record which plants appear. Which arrive first, and how do they differ from those that come later?
Common misconceptions
- “Succession always ends in forest.” In dry, cold or frequently burnt places, the stable community may be grassland, shrubland or tundra.
- “The climax community is permanent.” All ecosystems are eventually disturbed and change.
- “Disturbance is always bad.” Many species and ecosystems depend on fire, floods or grazing.
Key terms
- Primary succession: succession on surfaces with no soil.
- Secondary succession: succession after disturbance where soil remains.
- Pioneer species: the first organisms to colonise a site.
- Seral stage: one stage in a successional sequence.
- Climax community: a relatively stable late-successional community.
- Biological legacy: organisms or structures that survive a disturbance and aid recovery.
Quick quiz
Test what you have learned. Scoring 60 percent or more marks this module as complete.
Further reading
- Connell, J. H. and Slatyer, R. O. (1977). “Mechanisms of succession in natural communities.” The American Naturalist.
- Dale, V. H. et al. (eds.) (2005). Ecological Responses to the 1980 Eruption of Mount St. Helens.