Leave a patch of bare rock alone for a few hundred years and it will grow a forest. Nobody plants it. Each set of species changes the conditions just enough to make life possible for the next set — and, usually, impossible for itself. That self-driven, one-way handover is succession.
📚 What you need to know
Ecosystems are dynamic: they change from simple to relatively complex over time. That process is succession.
Primary succession starts on newly formed or newly exposed land with no species and no soil.
Secondary succession starts on bare soil where a community used to live, so it is much faster.
Pioneer species colonise first; each temporary stage is a seral community (a sere); the final stable stage is the climax community.
As succession proceeds, soil deepens, biomass, gross productivity, diversity and nutrient cycling complexity all increase.
Net productivity is the odd one out: it peaks in the middle stages, then falls as respiration catches up with gross productivity.
Each community changes the environment in a way that lets the next one outcompete it.
Two kinds of succession
The difference is entirely about what you start with.
Question
Primary succession
Secondary succession
Starting point
Bare, lifeless surface with no soil
Bare soil, with seeds, roots and nutrients still in it
How it is created
Cooled lava and new volcanic islands, retreating glaciers leaving bare rock or moraine, land exposed by falling sea level or a lake drying up, new coastal sand dunes
Abandoned farmland, land cleared by an intense fire, a forest after a storm blowdown
First colonisers
Mosses and lichens; marram grass on dunes
Grasses and fast-growing weeds already present as seed
Speed
Very slow — soil has to be built from nothing
Much faster — the hardest step is already done
End point
Climax community
Climax community, usually reached sooner
Why marram grass? It is the pioneer on sand dunes because it has deep roots that reach water other plants cannot, and it tolerates the high sodium and calcium levels in sea spray. Pioneers are not the strongest competitors — they are simply the ones that can survive where nothing else can.
The stages, one at a time
A seral community (a sere) is any temporary stage on the way. Each one alters the environmental conditions, and those changes let the next community replace it, usually through competition, until a stable climax is reached.
Notice what is really changing: the soil. Every stage adds dead organic matter, and the deeper, richer soil is what allows the next, larger set of plants to move in and shade the previous one out.
Walking through it
Pioneers arrive. Wind-blown spores and seeds land on bare rock. Mosses and lichens can grip it and survive with almost no water or nutrients.
The first soil forms. Pioneers die and decompose. That dead organic matter, called humus, mixes with weathered rock fragments to make a thin basic soil.
Small plants and grasses move in. Their seeds arrive on the wind or in bird droppings. They cope with shallow, nutrient-poor soil, and their roots bind it so it is not washed away.
The soil deepens and enriches. More plants dying means more humus, so the soil holds more water and more nutrients.
Shrubs and small trees take over. They need deeper soil and more water, both of which now exist. They shade out the smaller plants beneath them.
A climax community forms. The soil finally supports large trees, which become the dominant species. The community stops changing in any big way — it is stable and relatively complex.
Every stage sets up its own replacement. The grasses build the soil that lets shrubs beat them; the shrubs build the soil that lets trees beat the shrubs. Write that sentence into an exam answer and you have the whole mechanism.
What changes as succession runs
Structure and species composition are not the only things that change. So do energy flow, productivity, diversity and nutrient cycling.
Feature
Early stages
Late stages
Why
Energy flow
Low
High
Few species and little biomass at first; a complex food web later
Gross productivity
Low
High
More plants and more leaf area capturing light
Net productivity
Rises quickly to a peak
Falls back towards zero
Respiration of a large standing biomass eats up most of what is fixed
Species diversity
Low
High
More habitats and more niches, so more species can coexist
Nutrient cycling
Simple, dominated by abiotic processes
Complex
Many species, each with its own nutrient needs and cycling routes
Soil
Thin or absent
Deep and nutrient rich
Centuries of dead organic matter building up
At a mature climax community, gross productivity is high but nearly all of it is used up in respiration, so net productivity is close to zero. The system is running fast and standing still — which is exactly what “stable” looks like.
The trap in this graph. “Productivity increases during succession” is only half right. Gross productivity increases; net productivity peaks in the middle and then declines. Examiners write questions specifically to catch that.
Worked examples
EXAM Q1
Distinguish between primary and secondary succession. [2]
Primary
Starts on a newly formed or newly exposed surface with no soil and no species, for example cooled lava or bare rock left by a glacier.
Secondary
Starts on bare soil where a community existed before, for example abandoned farmland or a burnt forest.
The difference is whether soil and seed are already there“distinguish” means give both sides, not define one and stop
EXAM Q2
Explain the role of pioneer species in primary succession. [3]
Point 1: they can live where nothing else can
Mosses and lichens tolerate bare rock, little water and almost no nutrients.
Point 2: they start the soil
When they die and decompose they add humus, which mixes with weathered rock to form the first thin soil.
Point 3: they set up their own replacement
That soil holds water and nutrients, so grasses can germinate and outcompete them.
Pioneers make the environment habitable for the next seral communitythe word “humus” is worth having in the answer
EXAM Q3
Explain why net productivity falls in the later stages of succession, even though gross productivity is high. [3]
Step 1: define the gapnet productivity = gross productivity − respiration.
Step 2: what happens to biomass
A climax community has a very large standing biomass of trees, roots and animals.
Step 3: what that costs
All of that living tissue respires, so respiratory losses rise until they nearly match gross productivity.
The gap between the two closes, so net productivity approaches zeroa mature forest still fixes huge amounts of carbon; it just does not accumulate much more
💡 Exam tip
Use the exact terms: pioneer species, seral community (or sere), climax community. Vague words like “first plants” lose marks.
Any unfamiliar example (a volcanic island, a landslide, a demolished car park) still follows the same stages. Apply the principles rather than panicking about the setting.
When explaining a stage, always say what it does to the soil — that is the mechanism linking one stage to the next.
Keep gross and net productivity apart, and be ready to explain why they behave differently.
Diversity rising is usually explained by more niches becoming available. Use that phrase.
Succession is change over time in one place. If the question shows bands across a landscape, that is zonation.
⚠ Common mix-up
Saying productivity simply increases. Gross does; net peaks and then falls.
Thinking a climax community is dead or unchanging. It is dynamic — species are constantly replaced, but the overall structure stays the same.
Calling grasses pioneers on bare rock. On rock the pioneers are mosses and lichens; grasses need at least a thin soil.
Assuming succession always ends in forest. The climax depends on the climate — in tundra or desert it is nothing like a forest.
Forgetting that pioneers are outcompeted. They do not simply die out; they are shaded and crowded out by the species they made room for.
Muddling succession with zonation. Time versus space, once more.
Up next: What Makes an Ecosystem Stable — why some systems bounce back from a disturbance and others tip into something completely different and stay there.
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