Succession and Conservation
Succession is the process by which communities of organisms change over time in a predictable sequence, ultimately leading to a stable climax community. Understanding succession is important for ecology, conservation, and land management. This lesson covers primary and secondary succession, the concept of climax communities, deflected succession, and conservation strategies.
Key Definition: Succession is the progressive change in the species composition of a community over time, as the environment is modified by the organisms themselves and by external factors, leading towards a stable climax community.
Primary Succession
Primary succession occurs on a surface that has never previously supported a community — there is no existing soil. Examples include bare rock, volcanic lava, sand dunes, and newly formed islands.
Stages of Primary Succession
1. Pioneer Stage (Colonisation)
- Pioneer species are the first organisms to colonise the bare surface.
- They must be tolerant of extreme conditions: exposure, desiccation, nutrient deficiency, temperature extremes.
- Examples of pioneer species:
- Lichens on bare rock — they secrete acids that weather the rock surface, beginning the process of soil formation.
- Mosses — colonise small pockets of weathered rock and dead lichen material.
- Marram grass on sand dunes — tolerant of salt spray, drought, and sand burial; its roots stabilise the sand.
2. Early Seral Stages
- Pioneer species modify the environment (biotic interactions):
- Their dead remains add organic matter (humus) to the developing soil.
- Roots help bind particles and prevent erosion.
- The soil layer deepens and becomes more fertile (more nutrients, better water retention).
- Conditions become suitable for slightly less hardy species to colonise (e.g., grasses, herbaceous plants, small flowering plants).
- These new species may outcompete the pioneers, replacing them. This is because they are better competitors for resources in the now-modified environment.
3. Mid Seral Stages
- Soil continues to develop, becoming deeper and richer.
- Shrubs and eventually small trees colonise.
- Species diversity increases.
- Microhabitats develop (e.g., shade, leaf litter), providing niches for a wider range of organisms, including animals (insects, small mammals, birds).
- The environment becomes more sheltered and less extreme.
4. Climax Community
- The final, stable stage of succession.
- The community is in dynamic equilibrium — it is self-sustaining and does not undergo further directional change (though fluctuations occur).
- Species diversity is typically at its maximum.
- The dominant species of the climax community depend on the climate of the region (hence the term climatic climax).
- In lowland Britain, the climatic climax is typically deciduous oak woodland.
Key Changes During Succession
| Feature | Early Succession | Late Succession / Climax |
|---|
| Species diversity | Low | High |
| Biomass | Low | High |
| Soil depth | Thin or absent | Deep, fertile |
| Soil organic matter | Little | Abundant |
| Food web complexity | Simple | Complex |
| Nutrient cycling | Slow, open | Rapid, closed (nutrients recycled within ecosystem) |
| Abiotic conditions | Harsh, extreme | Moderated, stable |
| Productivity (gross) | Low | High |
| Net primary productivity | Can be high (relative to biomass) | Lower (more energy used in respiration of large biomass) |
Secondary Succession
Secondary succession occurs on a surface where a community has been partially or completely destroyed, but soil and seed bank remain. Examples include abandoned farmland, forest after a fire, areas recovering from flood or storm damage.
Differences from Primary Succession
- Secondary succession is faster than primary succession because:
- Soil is already present (with nutrients, organic matter, and microorganisms).
- A seed bank exists in the soil.
- Root systems and underground organs may survive.
- Propagules (seeds, spores) arrive rapidly from surrounding communities.
- The stages are similar to primary succession, but the pioneer stage is shorter or absent.
Deflected Succession and Plagioclimax
In many habitats, succession is prevented from reaching the climatic climax by human activity or natural disturbance. The community is maintained at a stage before the climax — this is called a plagioclimax (or deflected climax).
Causes of Deflected Succession