Ecological Succession: Primary and Secondary

Ecological succession is the gradual, fairly predictable change in the species of a community over time, and it comes in two forms: primary succession on bare, lifeless surfaces and secondary succession after a disturbance that leaves soil intact. In brief: primary succession begins on new rock, such as cooled lava, with pioneer species like lichens, while secondary succession begins on soil, such as an abandoned field, and is much faster. Succession usually leads toward a more stable community, called a climax community.

Succession explains why a bare field turns into a forest and why newly formed islands slowly fill with life. This guide describes both types, the stages and the changing conditions, compares them in a table, works through a timeline for an abandoned farm field, and discusses ecosystem stability and common mistakes.

What is ecological succession?

Succession is the process by which one community replaces another over time in the same area. Each group of species changes the environment, for example by adding organic matter to the soil or by creating shade. The new conditions favor other species, which gradually replace the earlier ones.

The changes continue until the community becomes relatively stable. How long this takes depends on the starting point and the climate. It can be a few years for grassland and several hundred years for forest.

What is primary succession?

Primary succession starts where there is no soil and no previous community. Examples are fresh lava flows, bare rock exposed by a retreating glacier, new sand dunes and islands formed by volcanoes.

1. Pioneer species: Lichens and mosses colonize the bare rock. Lichens release acids that slowly break down the rock and trap dust.
2. Soil formation: Dead pioneers add organic matter, and the combination of rock fragments and humus forms thin soil.
3. Herbs and grasses: Hardy small plants take root in the new soil and add more organic matter.
4. Shrubs and young trees: Larger plants move in and begin to shade the smaller ones.
5. Mature community: In a forest region, trees form a stable community with a rich soil.

Primary succession is slow, often taking hundreds or thousands of years to build deep soil.

What is secondary succession?

Secondary succession occurs where a community has been removed or damaged, but soil remains with seeds, roots and organisms. Causes include forest fires, floods, storms, logging and abandonment of farmland.

Because soil and seeds are already present, recovery is much faster. Fast-growing annual weeds and grasses appear first, then perennial plants and shrubs, then trees that grow in the sun and finally slow-growing trees that tolerate shade. A forest may take 100 years or more to recover, but early stages appear within one or two years.

Primary and secondary succession compared

Does succession lead to a stable ecosystem?

Early ecologists believed that succession always ends in a single climax community determined by climate, such as deciduous forest in a temperate region. Modern ecologists treat this idea with caution. The end point can depend on chance, soil, which species arrive first and repeated disturbances.

Many ecosystems are kept in earlier stages by regular disturbance. Prairies depend on fire and grazing, and without them shrubs and trees move in. Some mature communities are also affected by small gaps made when old trees fall, which let in light and start local succession. Stability is therefore not static: communities change and recover, a quality called resilience.

As succession proceeds, biomass and species numbers typically increase for a time. The community becomes more complex, and the soil gains more organic matter. In late stages, diversity may level off or even fall as a few shade-tolerant species dominate.

Worked example: an abandoned field

A farmer stops plowing a field in a temperate region of eastern North America. Use a typical timeline to describe secondary succession. Actual timings vary with climate, soil and nearby seed sources.

1. Year 1: Annual weeds grow quickly from seeds already in the soil. They produce many seeds and cover bare ground.
2. Years 2 to 5: Perennial grasses and herbs, such as goldenrod and asters, replace the annuals.
3. Years 5 to 15: Shrubs and fast-growing, light-loving trees, such as pines, begin to appear.
4. Years 15 to 50: Pine trees form young forest. Their shade stops their own seedlings from growing, but shade-tolerant seedlings of hardwoods like oak and maple can grow beneath them.
5. After 100 years or more: Hardwood trees replace the pines and form a mature forest.

This pattern shows a general principle: species that colonize early grow fast and make many seeds, while later species grow slowly and tolerate shade. Each stage prepares conditions for the next, though sometimes early species also block later ones.

How did the idea of succession develop?

Henry David Thoreau described how forests regrow from seeds in the 1850s. Henry Chandler Cowles studied sand dunes at Lake Michigan and described the sequence from bare sand to forest in 1899. Frederic Clements proposed in the early twentieth century that communities develop toward a climax like an organism.

Henry Gleason argued that communities are looser collections of species responding to conditions. Today most ecologists accept that succession often follows patterns, but with significant variation.

Why does succession matter in practice?

Land managers use controlled burning to maintain grasslands and prevent forests from taking over. After mines or fires, restoration ecologists plant pioneer species to speed up recovery. They sometimes add topsoil to start from a secondary stage and save centuries of natural soil formation.

Succession also helps to explain the recovery after the 1980 Mount St. Helens eruption. Areas where some plants and soil survived recovered faster than areas covered by deep ash and pumice.

Which mistakes are common?

Mistake 1: Saying that secondary succession starts with bare rock

It begins where soil is already present. Bare rock indicates primary succession.

Mistake 2: Thinking that the climax community never changes

Even mature communities change through disturbance and climate shifts.

Mistake 3: Believing succession is always the same sequence

The sequence depends on the location, the seed supply and chance. The examples in textbooks are typical, not universal.

Mistake 4: Assuming that bigger plants simply replace smaller ones

Each stage changes soil, light and moisture, which makes the area suitable for the next stage. The reasons for replacement are changes in conditions and competition.