Succession (AQA A-Level Biology): Revision Notes
Succession
Succession is the process describing how the variety of species occupying a particular area changes over time. Ecosystems are dynamic systems that undergo continuous change as populations fluctuate and environmental conditions alter. These changes can occur slowly over decades or rapidly following disturbances.
Pioneer species and colonisation
The initial colonisation of barren or inhospitable environments begins with pioneer species. These organisms form the first communities in areas such as bare rock exposed after glacial retreat, volcanic islands, or newly formed sand dunes.
Pioneer species possess several key adaptations that enable them to survive in harsh conditions:
- Asexual reproduction allows single organisms to rapidly multiply and establish populations without requiring mates
- Wind-dispersed seeds or spores enable easy colonisation of isolated or remote locations
- Rapid seed germination occurs without requiring dormancy periods, allowing quick establishment
- Photosynthetic ability provides independence from other organisms for food, as light is typically available even when nutrients are scarce
- Nitrogen fixation capability enables survival in nutrient-poor environments by converting atmospheric nitrogen into usable compounds
- Tolerance to extreme conditions such as drought, temperature fluctuations, and poor soil quality
Lichens exemplify ideal pioneer species, particularly on bare rock surfaces. Their ability to withstand severe drying conditions and extract nutrients directly from rock through chemical weathering makes them highly successful initial colonisers. This dual nature - being both algae and fungi in symbiosis - gives them unique advantages in hostile environments.
Stages of succession progression
Succession occurs through a series of distinct stages, with each community modifying the environment in ways that favour different species. This process creates a cascade of environmental changes that drives community development.
As pioneer species establish and eventually die, they release organic matter that begins soil formation. The weathering action of lichens on rock, combined with decomposing organic material, creates increasingly hospitable conditions. Mosses typically represent the next successional stage, followed by ferns as soil depth and organic content increase.
Key Succession Principle:
Each stage of succession creates conditions that are more favourable for the next stage but less favourable for itself. This is why pioneer species are eventually replaced - they literally "engineer" themselves out of their own habitat through environmental modification.
The accumulation of organic matter improves water retention and nutrient availability, enabling the establishment of flowering plants such as grasses. These plants provide additional food sources and habitat complexity, supporting more diverse animal communities. Subsequently, shrubs and small trees establish, creating further habitat diversity and food web complexity.
This progression continues until the community reaches a climax community - a stable, balanced assemblage of species that remains relatively unchanged over long periods. In lowland Britain, the typical climax community is deciduous oak woodland.
Environmental changes during succession
Several key environmental transformations occur throughout successional development:
- The abiotic environment becomes progressively less hostile. Soil formation, improved water retention, and shelter from wind and temperature extremes create more favourable growing conditions. This environmental amelioration allows species with different requirements to establish successfully.
- Habitat diversity and niche availability increase significantly, particularly during early to mid-succession stages. The structural complexity provided by different plant forms creates numerous microenvironments and ecological opportunities.
Biodiversity Pattern in Succession
Biodiversity initially increases as new species colonise available niches, reaching peak diversity during mid-succession. However, biodiversity may decline slightly as the climax community develops, due to competitive exclusion by dominant species. This creates a characteristic "hump-shaped" curve when biodiversity is plotted against succession time.
- Food web complexity increases as more species and trophic levels establish. The greater variety of producers supports diverse primary consumers, which in turn sustain higher-level predators and decomposers.
- Total biomass accumulates throughout succession, with the greatest increases occurring during mid-succession when rapid plant growth and establishment occur.
Primary versus secondary succession
Primary succession occurs on previously uncolonised surfaces such as bare rock, newly formed volcanic islands, or areas exposed by retreating glaciers. This process begins with no existing soil or organic matter, making it typically slow, often requiring decades to centuries for substantial development.
Secondary succession develops in areas where communities previously existed but were disturbed or removed through events like fire, disease, grazing, or human activities such as agriculture. This process occurs more rapidly than primary succession because soil, nutrients, and seed banks often remain, and immigration from surrounding areas can quickly replenish species.
Timing Difference: Secondary succession is typically 5-10 times faster than primary succession because it starts with established soil and nutrient cycling systems. While primary succession might take 100-1000 years to reach climax, secondary succession often reaches climax in 50-200 years.
Climax communities and stability
Climax communities represent the stable endpoint of succession, characterised by balanced species composition that remains relatively constant over extended periods. These communities exist in equilibrium with prevailing climate conditions, with the dominant species determined primarily by factors such as temperature, rainfall, and soil type.
In different climatic regions, various climax communities develop - deciduous woodland in temperate areas, tundra in arctic regions, or tropical rainforest in equatorial zones. The climax community maintains its stability through balanced competition, with few new species able to establish successfully.
Case study: Glacier bay succession
Worked Example: Glacier Bay, Alaska Succession Timeline
Research in Glacier Bay, Alaska, provides detailed evidence of primary succession following glacial retreat over the past 200 years. This natural experiment demonstrates clear successional stages:
Stage 1 - Pioneer Stage (0-10 years post-glaciation):
- Photosynthetic bacteria and lichens colonise exposed rock and sediment
- Nitrogen fixation begins, essential for ecosystem development in nutrient-poor glacial till
Stage 2 - Dryas Stage (~30 years post-glaciation):
- Continuous mats of the herbaceous plant Dryas dominate
- Root nodules containing nitrogen-fixing bacteria further enrich developing soil
- Additional species: Arctic poppy and moss campion establish
Stage 3 - Alder Stage (~60 years post-glaciation):
- Alder trees with nitrogen-fixing root nodules become dominant
- Decomposing alder leaves create nitrogen-rich humus
- Substantial improvement in soil quality occurs
Stage 4 - Spruce Stage (100+ years post-glaciation):
- Spruce trees gradually outcompete alders through superior height and canopy development
- Transitional phase towards eventual climax forest community
- Represents approach to climax conditions
Throughout this succession, soil nitrogen content, plant diversity, and total biomass show distinct patterns of change, with nitrogen and biomass generally increasing while diversity peaks during intermediate stages before declining slightly in mature forests.
Key Points to Remember:
- Succession is the predictable change in species composition over time in a particular area
- Pioneer species possess specific adaptations (rapid reproduction, dispersal ability, stress tolerance) for colonising harsh environments
- Environmental conditions become progressively less hostile during succession through soil development and habitat creation
- Primary succession starts from bare surfaces while secondary succession occurs in previously inhabited areas
- Climax communities represent stable endpoints determined by climate, maintaining equilibrium through balanced species interactions
- Each stage of succession modifies the environment to favour the next stage while becoming less suitable for itself