🌱 Ecological Succession
In one sentence: Succession is the gradual change in an ecosystem over time, where one community of species is replaced by another until a stable climax community is reached.
Think of it like a relay race — different species take over at different stages, each one changing the environment to make it suitable for the next.
📖 Key definitions
Succession: The process of change in species composition and community structure over time.
Pioneer species: The first organisms to colonise a barren environment. They're tough, survive harsh conditions, and start changing the environment for others.
Climax community: The final, stable community that remains in equilibrium with the environment. It has maximum biodiversity for that area.
🔄 Two types of succession
Primary Succession
- Starts on BARE rock or land
- No soil exists yet
- Very slow (hundreds of years)
- Examples: volcanic lava, retreating glaciers, new islands
Secondary Succession
- Starts where soil ALREADY exists
- After disturbance (fire, flood, farming)
- Much faster (decades)
- Examples: abandoned farmland, after forest fire
Primary = from scratch (bare rock). Secondary = soil already there (just disrupted).
🪨 Primary Succession — step by step
- 1️⃣ Bare rock — no life, no soil
- 2️⃣ Pioneer species arrive — lichens and mosses colonise rock
- 3️⃣ Soil starts forming — pioneers break down rock + dead matter accumulates
- 4️⃣ Grasses & small plants — can now grow in thin soil
- 5️⃣ Shrubs — soil deepens, more nutrients available
- 6️⃣ Trees — larger plants establish as soil matures
- 7️⃣ Climax community — stable forest (or other biome) with max biodiversity
Classic example: After a volcanic eruption (like Mount St. Helens), succession starts from bare lava rock → lichens → mosses → grasses → shrubs → forest.
🔥 Secondary Succession — faster recovery
- 1️⃣ Disturbance — fire, flood, or land cleared by humans
- 2️⃣ Pioneer plants return — grasses, weeds, fast-growing plants
- 3️⃣ Shrubs & small trees — outcompete pioneers for light
- 4️⃣ Climax community — stable ecosystem returns
Secondary succession is faster because soil, seeds, and nutrients already exist in the ground.
🦎 Pioneer species — the tough ones
Pioneer species are specially adapted to survive harsh, barren conditions:
- Tolerant of extreme temperatures, drought, low nutrients
- Reproduce quickly — r-strategists, lots of offspring
- Disperse easily — seeds spread by wind or animals
- Fix nitrogen — some add nutrients to poor soil
- Break down rock — lichens produce acids that weather rock into soil
Pioneer species examples: Primary: Lichens, mosses, algae · Secondary: Grasses, dandelions, fireweed
🌳 Climax community — the end goal
The climax community is the final stable stage. It stays in balance unless disturbed.
- Maximum biodiversity for that climate/location
- Stable — species composition doesn't change much
- In equilibrium — inputs roughly equal outputs
- K-strategists dominate — slow-growing, long-lived species
- Complex food webs — many trophic levels
The climax community depends on climate — a tropical climax is rainforest, a temperate climax might be oak woodland, a dry climax might be grassland.
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📈 Changes during succession
As succession progresses, these things generally increase:
- 📊 Biodiversity — more species over time
- 🌿 Biomass — more living material
- 🪴 Soil depth & nutrients — soil develops and enriches
- 🕸️ Food web complexity — more trophic levels
- 🏠 Habitat variety — more niches available
🚜 Human impacts on succession
- Deforestation — resets succession, often leads to secondary succession
- Agriculture — prevents succession by keeping land at early stage (fields)
- Urbanisation — stops succession permanently (concrete)
- Grazing — keeps ecosystems at grassland stage
- Fire management — some ecosystems need fire to maintain balance
Humans often arrest succession — keeping ecosystems at an early stage for farming, grazing, or development.
🔗 Succession and sustainability
Understanding succession helps us:
- Restore degraded ecosystems — knowing what stage comes next
- Predict recovery time after disturbance
- Design rewilding projects — let succession do the work
- Manage invasive species — they often dominate early stages
📝 What examiners actually ask
- Define succession and climax community
- Compare primary and secondary succession
- Explain the role of pioneer species
- Describe changes in biodiversity/biomass during succession
- Explain how humans affect succession
Exam Tips:
- Know the difference between primary and secondary — examiners love this comparison
- Be able to give examples of pioneer species and explain their adaptations
- Understand that climax community depends on climate and location
- Link succession to biodiversity changes — it increases over time
- Remember humans can arrest, reverse, or accelerate succession
Succession vs zonation (easy comparison)
- Succession = change over time
- Zonation = change over space
- Both are caused by abiotic and biotic factors
Succession explains how ecosystems develop, not where organisms are found.
Big idea: An ecosystem’s ability to cope with disturbance depends on its diversity, size, and internal structure.
What is ecosystem resilience?
Resilience Resilient ecosystems can absorb change without collapsing.
Resilience is about recovery, not preventing disturbance.
Resilience and equilibrium
Mature ecosystems often exist in dynamic equilibrium These systems can adjust after disturbance and return to stability.
Why storages increase resilience
Large storages act as buffers. If inputs are disrupted temporarily, the ecosystem can continue functioning using stored resources.
- Large biomass provides energy reserves
- Nutrient-rich soils support regrowth
- Multiple storages reduce reliance on one resource
Succession and increasing resilience
As succession progresses, ecosystems usually become more resilient because biodiversity, soil depth, and complexity increase.
Early successional stages are less resilient than climax communities.
How biodiversity improves resilience
- More species interactions create complex food webs
- Energy can flow through multiple pathways
- The system responds faster to change
In complex food webs, the loss of one species is less damaging because other species can fill similar roles.
Redundancy in ecosystems
Redundancy Redundancy increases resilience because if one species is lost, another can replace its function.
Simple food chains are less resilient than complex food webs.
Genetic diversity and resilience
Genetic diversity allows some individuals to survive disturbances such as disease, drought, or fire, ensuring the species can continue.
Feedback loops and resilience
Resilient ecosystems rely on negative feedback to maintain stability after disturbance.
- Negative feedback promotes stability
- Positive feedback amplifies change
- Too much positive feedback can reduce resilience
Key exam takeaways
- Resilience is the ability to recover from disturbance
- Biodiversity increases ecosystem resilience
- Large storages act as buffers
- Complex food webs are more resilient than simple chains
- Negative feedback helps maintain equilibrium