Succession
Succession is the gradual process of change in ecosystems, where one community replaces another due to shifts in biotic and abiotic conditions.
- Succession is the process by which ecosystems develop and change over time, transforming from a simple community with few species to a complex, stable ecosystem with high biodiversity and intricate trophic relationships.
- It represents the temporal (time-based) change in an ecosystem’s structure and species composition, unlike zonation, which describes spatial changes along an environmental gradient.
- Succession is a key ecological process that demonstrates how biotic (living) and abiotic (non-living) factors interact to shape the dynamic nature of ecosystems.
Zonation is a spatial phenomenon, succession is a temporal phenomenon.
Stages of Succession (Seral Stage)
Seral community
A seral community (sere) is a stage in the process of succession, where one community modifies the environment, making it more suitable for the next community.
- Each stage of succession is called a seral stage or sere, representing a distinct community that modifies the environment, paving the way for the next stage.
- The community that occupies it is a seral community.
1. Pioneer Stage
- The first colonizers are hardy species like lichens, mosses, and algae.
- They tolerate harsh conditions (temperature extremes, desiccation, lack of nutrients).
- These species weather rocks and add organic matter when they die, forming the first thin layer of soil.
Lichens on volcanic rock produce weak acids that chemically weather rock surfaces, forming the first soil particles.
2. Intermediate Stage
- As soil forms, small plants and grasses establish, increasing nutrient cycling and water retention.
- Roots stabilize the soil and microorganisms enrich it with nitrogen and organic matter.
- Over time, shrubs and small trees replace grasses due to competitive advantages like taller growth and deeper roots.
3. Climax Stage
Climax community
A climax community is the final, stable community in succession, in equilibrium with its environment and characterized by high biodiversity, complex food webs, and efficient nutrient cycling.
- The final stable community, in equilibrium with the environment.
- Species diversity, biomass, and nutrient cycling are highest.
- Soil is deep and rich, and complex food webs stabilize energy flow.
- The climax community depends on climate (climatic climax) and local conditions (edaphic climax).
In temperate regions, the climax community is often a deciduous forest, while in tropical areas it is a rainforest, and in dry zones, a grassland.
Types of Succession
Primary Succession (Starts from Bare Substrate)
- Occurs in areas where no previous life existed (e.g., volcanic lava, retreating glaciers, bare rock).
- Pioneer species (lichens, mosses) colonize first, breaking down rock to form soil.
- Over time, grasses, shrubs, and trees replace earlier species as soil quality improves.
The island of Surtsey (Iceland), formed by volcanic eruptions between 1963 and 1967, is a classic site of primary succession where life colonized bare volcanic rock.
Krakatau, Indonesia
Background:
- The volcanic eruption of Krakatau in 1883 destroyed all life on the island.
- The landscape was left as sterile volcanic ash and rock.
- Over time, it became a textbook example of primary succession.
Stages Observed:
- Pioneer Stage: spores of algae and lichens arrived via wind and sea currents.
- Early Colonization: ferns and grasses developed as organic matter accumulated.
- Intermediate Stage: shrubs and small trees stabilized soil.
- Climax Stage: dense tropical forest now dominates, supporting birds, bats, and insects.
Secondary Succession (Starts from a Disturbed Ecosystem)
- Occurs in areas where an ecosystem was disrupted but soil remains (e.g., after a wildfire, flood, or abandoned farmland).
- Faster than primary succession since soil and seed banks already exist.
- Grasses and small plants appear first, followed by larger shrubs and trees.
Broadbalk Wilderness, UK
Background:
- Abandoned farmland in Rothamsted, England, has been left undisturbed since the 1880s.
- Scientists have tracked ecosystem changes over 140+ years.
Stages Observed:
- Early Stage: Grasses and herbaceous plants colonize the bare soil.
- Intermediate Stage: Shrubs such as bramble and hawthorn appear.
- Climax Stage: Oak and ash trees dominate, forming mature woodland.
Abiotic Changes Observed:
- Soil organic carbon increased.
- Nutrient cycling became more closed and efficient.
- Biomass and species diversity rose steadily over time.
Changes During Succession
- Succession affects every component of ecosystem structure and function.
- Over time, energy flow, productivity, diversity, soil formation, and nutrient cycling all evolve.
1. Energy Flow
- Early stages: low energy flow due to few producers and limited biomass.
- Intermediate stages: energy flow increases as primary productivity rises.
- Climax stage: energy flow stabilizes; GPP ≈ R, maintaining dynamic equilibrium.
2. Productivity
- Gross Productivity (GPP): increases as plant cover and photosynthetic area expand.
- Net Productivity (NPP): high during early and middle stages when growth outpaces respiration, but decreases near climax as the system matures.
3. Species Diversity
- Begins low with few pioneer species.
- Increases with time as niches expand and habitats diversify.
- Stabilizes or slightly decreases in the climax community as competition limits new species establishment.
4. Soil Depth and Quality
- Starts as bare substrate (rock or sand).
- Organic matter accumulates through decomposition.
- Deeper soils form with greater water retention, aeration, and nutrient content.
- Root systems improve soil structure and prevent erosion.
5. Nutrient Cycling
- Early: open system with nutrients entering and leaving the ecosystem freely.
- Later: closed, self-sustaining system with internal nutrient recycling.
- Decomposers play a major role in maintaining fertility and supporting long-term stability.
In a sand dune succession, humus and nitrogen levels increase inland, while soil pH decreases, supporting the shift from grasses to shrubs and trees.
Alternative Stable States
- Different climatic conditions, soil types, and disturbance regimes can lead to multiple possible climax communities in the same region.
- These are known as alternative stable states.
In similar temperate climates, climax communities can vary from deciduous forest to grassland, depending on moisture and disturbance frequency.
Disturbance and Secondary Succession
- Events like fires, floods, or human interference can reset succession.
- After disturbance, succession restarts, but usually faster, as soil and seed banks remain.
- Fire-adapted ecosystems (e.g., pine forests) rely on periodic secondary succession for regeneration.
Mapping and Monitoring Succession
- Ecologists use GIS (Geographic Information Systems) and Remote Sensing (RS) to study changes in vegetation and land cover over time.
- These technologies help identify seral stages and track succession progress using satellite images and aerial photography.
Applications:
- Mapping vegetation recovery after wildfires.
- Studying reforestation patterns in abandoned farmland.
- Monitoring soil organic carbon and biomass growth.
The use of LIDAR (Light Detection and Ranging) can generate 3D vegetation maps showing forest canopy height, aiding in the assessment of successional maturity.
- Define ecological succession and differentiate between primary and secondary succession.
- Describe the stages of primary succession on a volcanic island.
- Compare the rate and causes of change in primary vs. secondary succession.
- Using examples, distinguish between succession and zonation.
