Leave a car park alone for two hundred years and you will not find a car park. You will find woodland. Ecosystems do not sit still: bare ground gets colonised, the colonisers change the conditions, and those new conditions suit somebody else. That slow handover is succession.
📘 What you need to know
Succession is the change in the community at one place over time, going from simple to more complex.
Primary succession starts on brand new bare ground with no soil and no living things — cooled lava, land left by a retreating glacier, a new sand dune.
Secondary succession starts where soil is already there, such as after a fire or on an abandoned field. It is much faster.
Pioneer species are the tough first arrivals. They survive harsh conditions and start building soil.
Each stage is a seral community (a sere). Each one changes the conditions and then gets replaced by the next.
As succession runs: soil gets deeper and richer, diversity rises, biomass rises, food webs get more complex, and nutrient cycling becomes tighter.
The end point is the climax community, which stays roughly the same unless something disturbs it.
What succession is
Succession is a change in one place across time. That is the difference from zonation, which is a change across space at one moment. If a question mentions “years” or “over time” you are in succession territory. If it mentions “up the shore” or “with altitude” you are in zonation territory.
The key thing to hold onto is this: the organisms themselves cause the change. They are not just passengers waiting for conditions to improve. Lichens break up rock. Dead plants rot down into soil. Roots hold that soil in place. Trees cast shade. Every stage makes the site a bit different, and usually a bit better for something else.
Definition
Succession = the orderly change in the species making up a community at a site over time
Primary and secondary succession
The only real difference is your starting line.
Primary begins on ground that has never had a community on it and has no soil. New volcanic rock, rock left behind by a melting glacier, new sand dunes, bare rock exposed when a lake dries up.
Secondary begins where a community has been wiped out but the soil survived. A burnt forest, a field left to go wild, land cleared by a storm.
Secondary succession is far quicker for two reasons. The soil is already there, so nobody has to spend decades making it. And there is usually a seed bank in that soil, plus roots and fungi that survived, so plants come back within weeks rather than centuries.
Soil is the bottleneck. Anything that saves the soil after a disturbance saves the ecosystem hundreds of years of work.
The stages, using bare rock as the example
🧩 From bare rock to woodland
Bare rock. No soil, no water held, big temperature swings, no nutrients. Almost nothing can live here.
Pioneers arrive. Spores of mosses and lichens blow in. Lichens grip the rock and their weak acids slowly break the surface into grains.
The first soil. Pioneers die and rot. That dead organic matter (humus) mixes with the rock grains to make a thin, poor soil.
Grasses and small plants. Seeds land on the thin soil and can now root. Their roots hold the soil so rain does not wash it away, and when they die the soil gets deeper and richer.
Shrubs and small trees. The soil now holds enough water and nutrients for bigger, deeper-rooted plants. They shade out many of the smaller plants.
Woodland. The soil is deep enough for large trees. They become the dominant species and the community stops changing much. This is the climax community.
Notice that the plants get taller as the soil gets deeper. Height needs roots, roots need depth, and depth is made by whatever died there before.
Why does one stage replace another?
This is the part worth understanding properly, because it turns a list you have memorised into an argument you can write.
First, facilitation. Each community makes the site better for the next one. Lichens create grains of rock and humus. Grasses deepen and bind the soil. Shrubs add shade and shelter. Each group builds the conditions the next group needs.
Then, competition. Once conditions improve, the newcomers are simply better at living there. A tree is a terrible pioneer — it cannot grow on bare rock. But once there is deep soil, a tree beats a moss easily, because it grows tall and takes the light. The pioneers are not killed off by the weather any more; they are shaded out.
Here is a neat way to remember it: pioneers are good at surviving; climax species are good at competing. Early on, survival is what matters, so pioneers win. Later, when survival is easy, competition is what matters, so the big competitors win.
What changes as succession runs
Feature
Early stages (pioneer)
Late stages (climax)
Soil
Absent or very thin, few nutrients
Deep, holds water, nutrient-rich
Species diversity
Low, only a few tough species
High, many niches to fill
Total biomass
Low
High, stored mostly in wood
Energy flow
Simple, short food chains
Complex food webs, many links
Nutrient cycling
Simple and leaky, nutrients wash away
Complex and tight, nutrients recycled
Size of organisms
Small
Large
Stability
Easily knocked back
More resistant to disturbance
Why does nutrient cycling get tighter? On bare rock there are few roots and no humus, so rain washes nutrients straight out of the system. In woodland, deep roots catch them, fungi and decomposers return them to the soil, and the same atoms get used again and again. Less is lost, so the system needs fewer inputs from outside.
Worked examples
WORKED EXAMPLE 1
A wildfire burns a pine forest to the ground. Twenty years later there are young trees. State which type of succession this is and explain why it is faster than succession on new lava.
Identify the typeSecondary succession — a community was destroyed, but the site was not new ground.
Give the reason that scores the mark
The soil survived the fire, so the slowest step (making soil from bare rock) has already been done.
Add a second reason
Seeds, roots and fungi survive in that soil, so plants regrow from the seed bank instead of having to arrive from outside.
Secondary; soil and seed bank are already in placeAsh also adds nutrients, which speeds up the first few years further.
WORKED EXAMPLE 2
Explain how lichens growing on bare rock make it possible for grasses to grow there later. (3 marks)
Mark 1: what the lichens do to the rock
Lichens release weak acids and grip the surface, breaking the rock into small mineral grains.
Mark 2: what happens when they die
Dead lichens are decomposed into humus, which mixes with those grains to form a thin soil.
Mark 3: why that matters to a grass
Grass seeds need soil to root in and to hold water and nutrients, so they can only germinate once that soil exists.
Rock broken down + humus added = a soil grasses can root inNotice the chain: cause, consequence, therefore. Three linked steps, three marks.
WORKED EXAMPLE 3
Marram grass is the pioneer on coastal sand dunes. Suggest two features that make it suited to that role.
Feature 1: getting water
Very long, deep roots reach water far below the dry surface sand, which most plants cannot do.
Feature 2: coping with salt
It tolerates high salt levels from sea spray, so the salty conditions that kill other plants do not stop it.
Bonus point worth adding
Its roots trap and hold the loose sand, which stabilises the dune and starts soil building for later species.
Deep roots for water, salt tolerance, and it stabilises the sand
💡 Exam tip
Say which type of succession it is and give the soil reason. That single sentence usually carries the mark.
Write in chains: the pioneers do X, which causes Y, which is why Z can now grow. Loose lists of facts score badly.
You may be given an unfamiliar example (a mine spoil heap, a new island, a rooftop). The principles are identical — apply them, do not panic.
Use the proper words once each: pioneer species, seral community, climax community, humus, seed bank.
If asked what increases during succession, remember diversity, biomass, soil depth and food web complexity all rise together.
Time scales matter. Primary succession on rock is centuries. Secondary succession is decades.
⚠ Common mix-up
Zonation and succession. Space versus time. Do not describe bands up a shore as “stages of succession”.
Thinking pioneers are weak. They are the toughest species there. They lose later because they are poor competitors, not because they are fragile.
Calling a burnt forest primary succession. The soil is still there, so it is secondary.
Saying conditions “get better”. Better for what? Say what improves: soil depth, water held, nutrients, shelter.
Assuming succession always ends in forest. The climax depends on climate and soil. In a dry or cold place it may be grassland or tundra.
Forgetting animals. Succession is not only about plants; insects, birds and mammals arrive as the habitat becomes suitable.
Up next: What Makes an Ecosystem Stable — why some ecosystems bounce back from a disturbance and others never do.
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