Succession (OCR A-Level Biology A): Revision Notes

Succession

What is succession?

Succession is the progressive change in ecological communities that occurs over time. This process takes place when new land becomes available for colonisation (primary succession) or when existing vegetation is cleared or disturbed (secondary succession). The development follows a predictable sequence as species composition and environmental conditions change.

In exposed environments such as high altitudes, cliff faces, or newly formed land, physical factors initially control which organisms can survive. Early colonisers gradually modify the environment, making conditions suitable for additional species. As the community develops, biological interactions between organisms become increasingly important in determining community structure.

Over extended periods, communities are progressively replaced until the ecosystem resembles others in the region experiencing similar climatic conditions. This series of changes occurs on previously uncolonised land and following natural disturbances like storms, floods, fires, or human activities including deforestation and agriculture.

Primary succession

Primary succession describes the colonisation of land that has never previously supported life, followed by progressive community development until a stable climax community becomes established.

The image above shows glacial moraine in Glacier Bay, Alaska, where retreating ice exposes bare rock suitable for primary succession. This provides an excellent example of how succession proceeds in natural environments.

Pioneer community

The first organisms to colonise bare substrates are termed pioneer species. These organisms possess specific adaptations enabling survival in harsh, exposed conditions:

• Microorganisms including photosynthetic bacteria and algae establish first, representing the initial producers
• Lichens and mosses colonise surfaces requiring no soil development
• Small flowering plants produce numerous lightweight seeds easily dispersed by wind
• Many pioneer plants are self-pollinating, as pollinating insects may be scarce or absent
• These species can germinate and grow where soil is minimal or absent

Pioneer organisms begin modifying physical conditions through their presence and activity. Soil starts forming from wind-transported particles combined with decaying plant material. This soil development enables deeper-rooted plants such as grasses to become established.

The role of nitrogen fixation

Newly exposed substrates often contain almost no fixed nitrogen in forms plants can absorb. This nitrogen limitation restricts growth of most plant species. However, plants hosting nitrogen-fixing bacteria in their root systems can thrive despite nitrogen scarcity.

Examples include:

• Dryas octopetala (mountain avens) - forms low-growing shrub carpets
• Alnus sinuata (alder trees) - develops into thickets

These nitrogen-fixing plants significantly modify environmental conditions, particularly by enriching soil nitrogen content as leaves die and decompose. Plant roots stabilise soil structure, and soil characteristics progressively change. Species from early seral stages become outcompeted and are replaced. Plants with more demanding nutrient requirements can now establish and, in turn, outcompete earlier colonisers like mountain avens and alder.

Seral stages and community development

Each distinct community during succession represents a seral stage. The progression through these stages follows a predictable pattern:

The diagram above illustrates the sequence of communities from bare ground through to climax forest. Each plant community creates conditions leading to its eventual replacement by species better adapted to the modified environment.

Environmental factors that change during succession include:

• Light availability (reduced as vegetation develops)
• Moisture retention (improved with organic matter accumulation)
• Soil organic content (humus)
• Mineral ion concentrations

Developing vegetation provides habitats for animal colonisation. Increasing varieties of microorganisms become established, particularly decomposers and nitrifying bacteria essential for carbon and nitrogen cycling.

Climax community

The climax community represents the final, stable stage of natural succession. At this point, no further directional changes occur - the community remains relatively constant in composition. In the Glacier Bay example, the climax community consists of long-lived coniferous trees including hemlock (Tsuga heterophylla) and spruce (Picea sitchensis), along with numerous coexisting species.

Changes during succession

Several measurable ecosystem properties change predictably during succession:

Feature	Pioneer	Early succession	Mid succession	Climax
Dominant plants	Mosses, lichens, herbs	Herb species, grasses	Shrubs, alder trees	Trees
Number of plant species	$11$	$17$	$18$	$31$
Niches	Few	More	More	Most
Food chains	Very short	Short	Longer	Longest
Food webs	Simple	Becoming complex	Complex	Complex
Biomass	Very low	Increasing	Increasing	Very high
Primary productivity	Very low	Highest	High	High
Soil depth	None (rock/sand)	Shallow soil	Deep with layers	Deep with topsoil/subsoil
Soil pH	$7.2$	$7.0$	$6.8$	$3.5$
Soil organic matter	None	Some	More	Large quantity
Soil nitrogen / $\text{g} \cdot \text{m}^{-2}$	$3.8$	$5.3$	$21.8$	$53.3$
Recycling rate	Rapid	Slower	Slow	Slow

Key patterns include:

Biomass increases progressively until the climax community establishes, then remains relatively constant. This reflects accumulation of living material as larger, longer-lived organisms dominate.

Primary productivity increases to peak levels during early succession stages, then decreases slightly before stabilising. Early fast-growing species achieve maximum productivity, but this declines as slower-growing, longer-lived species dominate.

Species diversity increases throughout succession until reaching maximum levels in the climax community. Greater habitat complexity and resource availability support more species.

Soil development progresses from absent or minimal to deep, layered soil rich in organic matter and nutrients. This results from weathering processes and organic matter accumulation from dead organisms.

Types of primary succession

Primary succession type depends on the substrate being colonised. Four main categories are recognised:

Type of new land	Name of succession
Wetland that is silting up	Hydrosere
Sand dunes	Psammosere
Mud deposits in estuaries	Halosere
Bare rock	Lithosere

Each succession type follows similar principles but involves different pioneer species and seral communities adapted to specific substrate conditions. The Glacier Bay succession is a lithosere as it begins with bare rock exposed by glacial retreat.

Deflected succession

In many ecosystems, succession does not reach its natural climax community. Deflected succession occurs when factors prevent progression beyond early or mid-seral stages.

Approximately $10\,000$ years ago, humans began extensively clearing forests across Britain for agriculture. Forest loss would naturally be followed by secondary succession, but farming activities maintain communities at early seral stages. The maintained seral stage is termed a plagioclimax.

Maintaining plagioclimax communities

Several factors prevent succession reaching climax:

Grazing by livestock reduces competition from fast-growing grass species, allowing many other plant species to survive. Grazing also prevents shrub and tree seedling establishment, blocking progression to later seral stages.

Mowing produces similar effects to grazing but is less selective in which species are removed. Regular mowing maintains grassland communities including lawns, playing fields, and some semi-natural habitats.

Burning (particularly in moorland management) removes woody vegetation and maintains heathland communities.

Examples of plagioclimax communities

In the UK, several important ecosystems are plagioclimax communities maintained by management:

• Moorland - maintained by grazing and burning
• Heathland - requires grazing or periodic clearance
• Chalk grassland - requires sheep grazing to prevent scrub development
• Lawns and playing fields - maintained by regular mowing

These deflected successions often exhibit higher productivity than climax communities would achieve, and support different species assemblages. Without continued management, these ecosystems would progress through remaining seral stages toward woodland climax communities.

Case study: Sand dune succession

Sand dunes demonstrate primary succession (psammosere) on coastal environments. Only species adapted to unstable, mobile sand can colonise areas where wind constantly deposits material.

The image shows progression from mobile yellow dunes (foreground) to stabilised grey dunes (background), illustrating spatial representation of succession stages.

Species distribution across sand dune systems shows clear zonation:

Distance / m	$0$	$2$	$4$	$8$	$16$	$32$	$64$	$128$	$256$	$512$
Marram grass	$0$	$0$	$0$	$0$	$30$	$24$	$15$	$0$	$0$	$0$
Sea couch grass	$11$	$8$	$9$	$35$	$6$	$18$	$4$	$0$	$0$	$0$
Lyme grass	$0$	$0$	$0$	$0$	$0$	$8$	$13$	$0$	$0$	$0$
Red fescue grass	$0$	$0$	$0$	$0$	$0$	$0$	$0$	$1$	$26$	$0$
Lesser hawksbit	$0$	$0$	$0$	$0$	$0$	$10$	$15$	$2$	$1$	$1$
Bell heather	$0$	$0$	$0$	$0$	$0$	$0$	$0$	$0$	$40$	$0$
Bramble	$0$	$0$	$0$	$0$	$0$	$0$	$0$	$0$	$0$	$16$

Sea couch grass dominates closest to the strand line, tolerating mobile sand and salt spray. Marram grass achieves maximum abundance at $16$-$32$ m distance, where it stabilises dunes through extensive root systems. As dunes stabilise further, other species including red fescue grass, bell heather, and eventually bramble become established, indicating progression toward climax community.

Abiotic factors measured across the transect show:

Distance / m	$0$	$2$	$4$	$8$	$16$	$32$	$64$	$128$	$256$	$512$
Soil pH	$8$	$8$	$8$	$7$	$8$	$8$	$8$	$8$	$8$	$8$
Wind speed / $\text{m} \cdot \text{s}^{-1}$	$3.9$	$4.0$	$3.4$	$1.7$	$3.5$	$3.1$	$4.9$	$5.8$	$6.3$	$2.6$
Air temperature / °C	$17.1$	$17.5$	$16.8$	$18.1$	$16.2$	$18.6$	$15.8$	$18.0$	$19.0$	$15.9$
Vegetation height / m	$0.23$	$0.14$	$0.12$	$0.91$	$1.10$	$0.40$	$0.76$	$0.12$	$0.09$	$0.25$
Soil moisture (0-10 scale)	$1$	$2$	$2$	$2$	$1$	$1$	$1$	$1$	$1$	$1$

The relatively constant pH reflects alkaline conditions from shell fragments in the sand. Wind exposure varies with vegetation development and dune topography. These abiotic factors interact with biotic factors to determine species distribution patterns across the dune system.