You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
This lesson covers primary and secondary succession, pioneer species, climax communities, and deflected succession, as required by the Edexcel A-Level Biology specification (9BI0), Topic 10 -- Ecosystems.
Ecological succession is the process by which the structure of a community changes over time in a predictable manner. It involves a series of changes in species composition, diversity, and ecosystem structure, eventually reaching a stable end point called a climax community.
Each intermediate stage is called a sere (or seral stage), and the whole sequence from bare ground to climax community is called a sere or prisere.
Primary succession occurs on land where there has been no previous soil or community -- bare, sterile surfaces such as:
flowchart LR
A["Bare rock /\nnew surface"] --> B["Pioneer species\ncolonise\n(lichens, mosses)"]
B --> C["Soil begins to\nform from\nweathered rock +\ndead organic matter"]
C --> D["Small herbaceous\nplants and grasses\ncolonise"]
D --> E["Shrubs colonise\nas soil deepens"]
E --> F["Small trees\nestablish\n(birch, willow)"]
F --> G["Climax community\n(e.g. mature\noak woodland)"]
Pioneer species are the first organisms to colonise a bare surface. They must be tolerant of extreme conditions (exposure, poor nutrients, little water).
Examples: lichens (on rock), marram grass (on sand dunes), algae (on bare soil)
Characteristics of pioneer species:
| Characteristic | Explanation |
|---|---|
| Tolerant of harsh conditions | High light, extreme temperatures, drought, exposure to wind |
| Can fix nitrogen or grow in nutrient-poor conditions | Lichens are a mutualism between fungi and algae/cyanobacteria; some cyanobacterial partners fix N2 |
| Produce large numbers of seeds/spores | Ensures dispersal to new sites |
| Rapid reproduction | Colonise quickly before conditions change |
| Small size | Can grow in thin soil or on rock surfaces |
| Weathering ability | Lichens produce acids that help weather the rock surface, beginning soil formation |
As pioneer species grow, die, and decompose:
The climax community is the final, stable stage of succession. It is in dynamic equilibrium with the climate and environment.
| Feature of Climax Community | Description |
|---|---|
| Stable | Species composition does not change significantly over time (in the absence of disturbance) |
| High biodiversity | Many species and complex food webs |
| High biomass | Large total mass of living organisms |
| Complex structure | Multiple vertical layers (e.g. canopy, understorey, herb layer, ground layer in a woodland) |
| Determined by climate | The type of climax community is determined by the regional climate (e.g. temperate deciduous woodland in the UK) |
Exam Tip: During succession, the pioneer species change the abiotic environment (e.g. by forming soil, adding nutrients, retaining water), making it more suitable for the next species. This is called facilitation. The new species often outcompete the pioneers, which is why community composition changes. You must be able to use the term "facilitation" in your answers.
Sand dune succession (a psammosere) at Studland Bay, Dorset, is one of the most commonly examined examples:
| Zone | Distance from Sea | Species | Soil Conditions |
|---|---|---|---|
| Embryo dune | 0-20m | Sea couch grass, sea rocket | Almost pure sand; very dry; salty; no humus |
| Yellow (mobile) dune | 20-50m | Marram grass (dominant) | Sandy; dry; little humus; pH ~8 (alkaline, calcium from shells) |
| Grey (semi-fixed) dune | 50-100m | Marram grass, red fescue, mosses, lichens | Some humus developing; slightly more moisture |
| Dune grassland | 100-200m | Diverse grasses, heather, gorse | More humus; deeper soil; pH decreasing |
| Dune slack | Low-lying areas | Creeping willow, marsh orchids, rushes | Waterlogged; high water table |
| Dune scrub / woodland | 200m+ | Birch, willow, oak (climax) | Deep soil; rich in humus; neutral pH |
As you move inland from the sea, the succession shows increasing biodiversity, soil depth, humus content, and biomass, and decreasing soil pH (from alkaline to neutral/acidic).
Secondary succession occurs on land where a community has been destroyed or disturbed but the soil remains intact.
Examples: After a forest fire, flood, deforestation, abandonment of farmland
Key differences from primary succession:
| Feature | Primary Succession | Secondary Succession |
|---|---|---|
| Starting point | Bare rock / sterile surface | Soil already present (with seeds, roots, nutrients) |
| Speed | Slow (can take hundreds of years) | Faster (soil provides a head start; seed bank in soil) |
| Pioneer species | Lichens and mosses | Often grasses and herbaceous plants |
| Soil | Must form from scratch | Already exists with a nutrient supply and microbial community |
Deflected succession occurs when human activity or other factors prevent succession from reaching the natural climax community.
The resulting stable community is called a plagioclimax -- it is maintained by the continued influence of the deflecting factor.
| Deflecting Factor | Resulting Plagioclimax | Location Example | What Would Happen Without It |
|---|---|---|---|
| Grazing (sheep, cattle) | Grassland (e.g. chalk grassland) | South Downs, Sussex | Would succeed to scrub and woodland |
| Mowing | Maintained lawn or meadow | Urban parks | Would succeed to scrub and woodland |
| Burning (heather moorland) | Heather-dominated moorland | North York Moors | Would succeed to birch/oak woodland |
| Coppicing | Managed woodland with diverse ground flora | Kent and Sussex | Would develop into mature high forest with less light at ground level |
| Draining | Drained farmland | East Anglian Fens | Would return to wetland/bog |
Exam Tip: A plagioclimax is NOT a natural climax community. It requires continuous human management to maintain it. If the management stops, succession will resume towards the natural climax. Many UK habitats that we consider valuable for conservation (chalk grassland, heather moorland, hay meadows) are actually plagioclimax communities -- they are maintained by traditional management practices.
As succession progresses from pioneer to climax, predictable changes occur:
| Property | Early Succession | Late Succession (Climax) | Explanation |
|---|---|---|---|
| Biodiversity | Low | High | More niches available as habitat structure increases |
| Biomass | Low | High | Larger organisms accumulate more organic matter |
| Soil depth | Minimal | Deep and humus-rich | Decomposition of dead matter builds soil over time |
| Nutrient availability | Low | High | Nutrient cycling becomes more efficient |
| Food web complexity | Simple | Complex | More species = more feeding relationships |
| Habitat diversity | Low | High (many niches) | Multi-layered vegetation creates varied microhabitats |
| Species | Pioneer species (r-strategists) | Climax species (K-strategists) | Links to population dynamics lesson |
| NPP | Low initially, then high | Moderate (GPP high but respiration also high) | Mature forests have high respiration demands |
Question: A farmer abandons a wheat field. Describe the sequence of changes you would expect to see over the next 200 years.
Answer:
Throughout this process, biodiversity, biomass, and soil depth increase, and food webs become more complex.
Question: A student carries out a belt transect across sand dunes. The percentage cover of marram grass decreases from 80% on mobile dunes to 5% on fixed dunes, while species richness increases from 3 species to 15 species. Explain these changes in terms of succession.
Answer:
On the mobile dunes (early succession), marram grass is the dominant pioneer species because it is adapted to harsh conditions: it has deep roots to reach the water table, rolled leaves to reduce transpiration, and it can tolerate burial by windblown sand. Few other species can survive these conditions, so species richness is low (3 species) and marram dominates (80% cover).
As succession progresses inland to fixed dunes, the soil becomes deeper, richer in humus, and retains more water. These improved conditions allow many more species to colonise (15 species). These species outcompete marram grass for light and other resources -- marram is adapted to harsh conditions and cannot compete in richer soils. Its percentage cover falls to 5% as it is replaced by a more diverse community of grasses, herbs, shrubs, and eventually trees.
This illustrates the principle of facilitation: pioneer species change the environment (build soil, add nutrients), making it suitable for new species that then outcompete the pioneers.
Students sometimes write that "pioneer species die out and are replaced." In fact, pioneer species do not necessarily die out -- they are outcompeted by later-arriving species that are better competitors in the altered environment. The key mechanism is competition, driven by the environmental changes caused by the pioneers themselves (facilitation).
Succession is the directional, broadly predictable change in community composition over time as one assemblage modifies the abiotic environment (soil, light, water, nutrients) in ways that allow a different assemblage to outcompete and replace it. The Edexcel 9BI0 treatment turns on three moves: the distinction between primary and secondary succession (whether soil pre-exists); the mechanism of facilitation by pioneer species; and the human-managed deviation that produces a plagioclimax rather than a natural climax. This deep dive deepens each with quantitative ecology, named UK case studies and the mark-scheme literacy needed for Paper 2 Section B.
The Edexcel 9BI0 specification places ecological succession in Topic 5: On the Wild Side — Photosynthesis, Energy and Ecosystems, on Paper 2 (Energy, Exercise and Coordination). This is the time-axis lesson of Topic 5: where lessons 1–2 set out community structure and energy flow at a snapshot and lessons 3–4 quantify nutrient cycling at steady state, lesson 5 introduces directional change in community composition over years to centuries. Statements concern: the definition of succession as a sequence of seral stages culminating in a climax community; the distinction between primary succession (on previously bare or sterile surfaces — bare rock, sand, lava, glacial till) and secondary succession (where a community has been disturbed but soil and seed bank remain); the role of pioneer species (lichens, mosses, marram grass, fireweed) in colonising harsh substrates and initiating soil formation through weathering, organic-matter accumulation and nutrient capture; the mechanism of facilitation, by which each seral stage modifies the abiotic environment to favour the next stage and disadvantage itself; the increase in biodiversity, biomass, soil depth and food-web complexity through successive seres; the concept of deflected succession producing a plagioclimax maintained by continuous management (grazing, mowing, burning, coppicing, draining); and named UK case studies — sand dune succession (psammosere, e.g. Studland Bay), pond succession (hydrosere), bare rock (lithosere), and abandoned-field old-field succession (refer to the official Pearson Edexcel 9BI0 specification document for exact wording). Synoptic links radiate to Topic 4 — Biodiversity (Simpson's D for tracking diversity through seres), lesson 6 — Population Dynamics (r/K-strategist contrast distinguishing pioneer from climax species), lesson 2 — Energy Transfer (NPP rises through early-mid succession then plateaus as respiration costs rise in mature forest), lessons 3–4 — Carbon and Nitrogen Cycles (succession builds the soil N and C pools that sustain the climax), lesson 7 — Investigating Ecosystems (belt transects, quadrats, kite-diagram methodology), and lesson 10 — Conservation and Sustainability (UK plagioclimax habitats — chalk grassland, heather moorland, hay meadow — are conservation priorities precisely because they require active management to persist).
Question (8 marks):
A sand-dune transect at a UK coastal nature reserve (a psammosere) records the following data along a 200 m belt running inland from the strand line. At 20 m: marram grass Ammophila arenaria dominant, percentage cover 78%, species richness 4, soil depth 2 cm, soil pH 8.1, soil organic matter 0.3%. At 80 m: marram cover 18%, species richness 11, soil depth 9 cm, soil pH 7.2, soil organic matter 3.4%. At 200 m (dune scrub / early woodland): marram cover 1%, species richness 22, soil depth 28 cm, soil pH 6.0, soil organic matter 11.8%, with birch and willow saplings present.
(a) Identify the seral stage at 20 m and explain two xerophytic adaptations that allow marram grass to act as a pioneer on mobile dunes. (3)
(b) Calculate the percentage increase in soil organic matter between 20 m and 200 m, and explain why soil organic matter rises so steeply across the transect. (3)
(c) Predict, with reasoning, what would happen to species richness at the 200 m point if grazing by rabbits and roe deer were excluded for fifty years. (2)
Solution with mark scheme:
(a) M1 (AO1.1) — the seral stage at 20 m is the yellow (mobile) dune; marram is the characteristic pioneer species and 78% cover is diagnostic. M1 (AO1.2) — adaptation 1: rolled (involute) leaves with stomata sunken in pits on the inner surface trap humid air, reducing the water-vapour gradient and so reducing transpiration on a windy, water-poor substrate. A1 (AO1.2) — adaptation 2: an extensive rhizome and deep root system reaches the freshwater table well below the surface; stoloniferous rhizomes spread laterally and stabilise the sand (accept thick waxy cuticle / hairs on inner leaf surface as alternatives).
(b) M1 (AO2.1) — percentage increase = ((11.8 − 0.3) / 0.3) × 100 = 3833% (accept ~3800–3900%; ~39×). M1 (AO1.2) — soil organic matter accumulates because each seral stage adds dead leaves, roots and microbial biomass that saprotrophic decomposers partially break down to humus; humus accumulates as long as litterfall input exceeds mineralisation losses. A1 (AO3.1a) — this is the central facilitation mechanism: rising organic matter raises water-holding and cation-exchange capacity and lowers pH, shifting conditions to favour later-seral species and outcompete marram.
(c) M1 (AO2.1) — without grazing, the open dune-scrub at 200 m would proceed towards the regional climax of temperate deciduous woodland; existing birch and willow saplings would mature, the canopy would close, and shade would suppress the light-demanding herbaceous flora. M1 (AO3.2a) — species richness would fall (perhaps 22 → 8–12) because the diverse open-grassland flora is a plagioclimax maintained by grazing; removal returns the system to succession and typically reduces local diversity (the intermediate-disturbance hypothesis — moderate grazing maximises diversity; both removal and overgrazing reduce it).
Total: 8 marks.
Question (6 marks): Explain how primary succession proceeds from bare rock to a temperate-deciduous-woodland climax community, and evaluate why some ecologically valued UK habitats are best understood as plagioclimaxes rather than as natural climaxes.
Mark scheme decomposition by AO:
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.