Ecological succession

1. Meaning, scope and ecological vocabulary

Ecological succession is a change in community composition and structure over time in which populations establish, expand, decline or are replaced. It often follows a disturbance or the creation of a new substrate. The process concerns communities and ecosystems rather than genetic change within a species. Seasonal replacement of annual plants or recurring seasonal plankton peaks is not, by itself, evidence of long-term succession.

A pioneer community comprises the organisms that first establish in a habitat. These organisms must tolerate the site's initial conditions, such as unstable sediment, drought, nutrient scarcity or strong exposure. Lichens and mosses are familiar pioneers on some bare rock surfaces, but microbes, grasses, herbs or woody plants may be pioneers elsewhere. Successful colonisation also depends on nearby source populations and the arrival of seeds, spores or other propagules.

The complete sequence of communities is called a sere, and its intermediate communities are seral stages. A relatively persistent late-successional community is traditionally called a climax community. Persistence is relative: populations continue to turn over, gaps form and environmental conditions change. Succession therefore should not be understood as a staircase that always ends in a permanently fixed community.

• Habitat refers to where an organism lives; succession concerns changes in the community occupying a habitat.
• Ecological succession operates through ecological time; evolution involves inherited changes across generations.
• Early colonisation and later community replacement may occur simultaneously in different patches of the same landscape.

2. Primary and secondary succession

Primary succession begins where a developed soil and a previously established terrestrial community are absent. Examples include fresh lava, recently exposed rock following glacial retreat and newly formed mineral surfaces. Organisms must establish despite limited nutrients and often poor water retention. Weathering, deposition of wind-blown material, biological activity and the accumulation of dead organic matter gradually help create a soil environment suitable for additional species.

On suitable rock surfaces, lichens contribute to weathering through chemical and physical processes. Microbes and plants add organic material, while roots influence substrate stability and soil formation. Nitrogen-fixing organisms can improve nitrogen availability in some primary successions, although not every pioneer is a nitrogen fixer. Soil development may be slow, especially in cold or arid settings; there is no universal duration for completing primary succession.

Secondary succession follows disturbance of an existing community where soil and some biological legacies survive. Abandoned fields, forests after many fires and storm-damaged woodlands provide examples. Seed banks, surviving roots, underground stems, microorganisms and nearby vegetation commonly accelerate recovery. Secondary succession is therefore generally faster than primary succession, but repeated burning, invasive species, erosion or severe degradation can delay or divert it.

The classification depends on what remains, not merely on the name of the disturbance. A fire usually initiates secondary succession if the soil persists. A disturbance that removes soil and exposes fresh mineral substrate may instead create conditions resembling primary succession. Similarly, mine reclamation may involve either retained topsoil or largely soil-free spoil, requiring different restoration approaches.

• Primary succession: development of a community on a substrate lacking developed soil.
• Secondary succession: reorganisation of a community with surviving soil and biological legacies.
• Faster recovery does not necessarily mean restoration of the original species composition.

3. Processes and mechanisms of succession

Classical descriptions identify nudation, invasion, competition and coaction, reaction, and stabilisation. Nudation is the creation of an open area. Invasion includes migration to the site, successful establishment or ecesis, and multiplication. As populations grow, interactions intensify. Reaction refers to organisms modifying their environment, for example through shading, litter deposition or changes in soil chemistry. These terms offer a useful framework, but actual processes overlap rather than follow rigid boundaries.

Autogenic succession is driven substantially by changes generated by the community itself. Accumulating litter may build soil organic matter, while a developing canopy reduces light and favours shade-tolerant plants. Allogenic change is driven by external factors such as flooding, sediment deposition, erosion or climatic shifts. Most real successions reflect both internal feedbacks and external influences, so these categories are analytical distinctions rather than mutually exclusive situations.

Three influential mechanisms explain species replacement. Facilitation occurs when early occupants improve conditions for later species. Tolerance occurs when later species establish without requiring such improvement and eventually persist because of their environmental tolerances or competitive abilities. Inhibition occurs when established occupants suppress newcomers until damage or death creates opportunities. Different mechanisms can operate together within one site.

Priority effects arise when the order of arrival influences the community that develops. An early coloniser may monopolise space or alter soil conditions, favouring some later arrivals and excluding others. Consequently, identical physical environments need not produce identical communities. Dispersal barriers, grazing pressure and the availability of suitable establishment sites also help explain why succession can be delayed or redirected.

4. Hydrarch, xerarch and climax concepts

Hydrarch succession begins in a wet or aquatic habitat, whereas xerarch succession begins under dry conditions. A lithosere starts on rock; a psammosere develops on sand. A textbook hydrosere may progress from open-water organisms through submerged and floating-leaved plants to emergent vegetation and marsh communities. If sediment and organic matter accumulate sufficiently, less waterlogged communities may develop.

The familiar rock-to-lichen-to-moss-to-herb-to-shrub-to-forest sequence is likewise an illustrative lithosere, not a universal rule. Traditional accounts suggest that hydrarch and xerarch successions tend towards more mesic conditions. This is a simplified generalisation: hydrology, climate, fire, grazing and substrate can maintain wetlands, grasslands or shrublands indefinitely. Natural grasslands and wetlands must not automatically be classified as incomplete forests.

Frederic Clements emphasised an orderly progression towards a climax largely determined by regional climate. Arthur Tansley's polyclimax perspective gave greater importance to factors such as soil, topography, fire and other local conditions. Modern ecology stresses patch dynamics, continuing disturbance and multiple possible trajectories. A mature forest can contain young gaps and old stands together, while feedbacks may sometimes maintain alternative ecosystem states.

• Hydrarch and xerarch describe starting conditions, not guaranteed endpoints.
• A climax community is relatively persistent, not absolutely unchanging.
• Natural disturbance can maintain biodiversity rather than merely interrupt ecosystem development.

5. Ecosystem trends and conservation significance

During many successions, standing biomass, vegetation height, litter and soil organic matter increase. More complex vegetation creates additional habitats and modifies temperature, humidity and light. However, these trends are context-dependent. Species richness can rise, plateau or decline, and early open-habitat specialists may disappear as a canopy closes. A later stage is therefore not automatically more valuable for every conservation objective.

Gross primary productivity is the total rate at which producers fix energy as organic matter. Net primary productivity equals gross primary productivity minus producer respiration. Biomass is a stock, whereas productivity is a rate: a high-biomass old forest need not have the highest net primary productivity. Classical models suggest that ecosystem production and respiration become more closely balanced in mature systems, but mature forests can still remain carbon sinks.

Successional knowledge supports ecological restoration by identifying barriers to recovery. Passive restoration may suffice where propagules and soil remain; assisted natural regeneration may require controlling repeated fires, excessive grazing or invasive plants. Severely altered sites may need erosion control, hydrological repair or native planting. India's National Plan for Conservation of Aquatic Eco-systems supports wetland conservation, where maintaining appropriate water regimes may be more important than promoting terrestrial vegetation. Restoration should target the appropriate reference ecosystem, not maximise tree cover everywhere.

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