Ecological succession is the directional, sequential process through which the composition, structure, and ecological functioning of a biological community change over time. It occurs because organisms modify their physical environment, because different species have different tolerances and resource requirements, and because disturbances repeatedly alter ecological conditions. A succession can involve changes in species abundance, species richness, biomass, soil properties, nutrient availability, microclimate, food webs, and rates of primary production and decomposition.
The term does not necessarily imply that an ecosystem moves towards a permanently superior or perfectly stable endpoint. Instead, succession describes a changing trajectory in which particular organisms become established, grow, reproduce, interact, alter environmental conditions, and are subsequently replaced or supplemented by other organisms.
Ecologists commonly distinguish primary succession, which begins where no developed soil or biological community previously exists, from secondary succession, which begins after a disturbance removes much of an existing community while leaving soil, organisms, seeds, roots, or other biological legacies behind. Succession is therefore a process rather than simply a list of communities. The word succession derives from the Latin succedere, meaning to follow or come after, reflecting the observation that ecological communities often exhibit recognisable sequences of change.
Primary succession develops on newly exposed or newly formed surfaces where soil is absent or extremely limited. Examples include newly cooled volcanic lava, freshly exposed rock following glacial retreat, newly formed sand or mineral surfaces, and some environments created by major geological disturbances. Early colonists, sometimes called pioneer species, are organisms capable of tolerating harsh conditions and dispersing effectively to unoccupied sites. Depending on the environment, pioneers may include lichens, cyanobacteria, algae, microbes, mosses, grasses, or specialised vascular plants.
These organisms can contribute organic matter, trap particles, weather mineral surfaces, alter moisture retention, and participate in processes that gradually make the environment more suitable for additional species. As organic residues accumulate and mineral material is transformed, soil development can accelerate. Larger plants may then establish, followed by more complex communities containing shrubs, trees, fungi, animals, and diverse microbial populations where climate permits.
Primary succession can be extremely slow because soil formation, nutrient accumulation, and biological colonisation initially proceed under severe constraints. The exact sequence is not universal because climate, substrate chemistry, dispersal, disturbance history, available species pools, and interactions among organisms strongly influence the trajectory. A volcanic surface in a wet temperate region therefore need not follow the same sequence as a newly exposed surface in an arid environment.
Secondary succession begins after a disturbance affects an established ecosystem but leaves some of the conditions or biological materials necessary for recolonisation. Forests recovering after fire, agricultural fields abandoned after cultivation, vegetation regenerating after storms, and ecosystems recovering after some forms of logging or flooding can undergo secondary succession. Because soil is generally already present, secondary succession can occur much more rapidly than primary succession.
Seeds stored in soil, surviving roots, bulbs, rhizomes, dormant organisms, fungi, microorganisms, and nearby populations can provide sources for recovery. The process may begin with annual plants and other rapidly growing species, followed by perennial herbs, grasses, shrubs, and eventually trees in suitable climates. However, the sequence is not predetermined. A fire that leaves mature trees alive may produce a very different trajectory from a severe fire that removes nearly all aboveground vegetation.
Similarly, repeated grazing, invasive species, altered drainage, soil compaction, nutrient enrichment, or continued human disturbance can divert succession into an alternative stable or persistent state. Ecologists therefore distinguish succession from simple regrowth. Regrowth describes biological recovery in a general sense, whereas succession refers more specifically to systematic changes in community composition and ecosystem organisation through time. Secondary succession also demonstrates the importance of ecological memory, because remnants of the previous ecosystem can strongly influence which organisms arrive and establish.
Several mechanisms contribute to ecological succession, including facilitation, tolerance, inhibition, competition, dispersal, predation, herbivory, disease, and environmental modification. Facilitation occurs when earlier colonists modify conditions in ways that improve the environment for later species. Nitrogen fixing organisms, for example, can increase biologically available nitrogen in nutrient poor environments, while vegetation can increase shade, soil organic matter, water retention, and habitat complexity.
Tolerance occurs when later species can establish despite conditions created by earlier species, particularly when they can withstand limited resources or shade. Inhibition occurs when established organisms prevent or suppress potential competitors through competition for light, water, nutrients, space, or through chemical effects and physical occupation. These mechanisms often operate simultaneously. A rapidly growing plant may initially facilitate soil development while later competing strongly with other plants for sunlight.
Animals can influence succession through grazing, seed dispersal, predation, burrowing, and disturbance. Fungi and microorganisms are equally important because decomposition, nutrient mineralisation, symbiosis, and soil formation influence which plants can persist. Succession is consequently an ecosystem level process arising from interactions between organisms and their physical environment rather than a simple replacement of one plant species by another.
The concept of succession has a substantial history in ecology. Early ecological observations identified recurring patterns in vegetation development, and the American ecologist Frederic Clements was particularly influential in developing the classical idea of succession as an orderly process leading towards a relatively stable climax community. Clements viewed communities as highly integrated entities that developed in somewhat analogous ways to organisms.
Later ecologists, including Henry Gleason, emphasised the individuality of species responses and the importance of chance, dispersal, environmental heterogeneity, and species specific interactions. Modern ecology generally treats succession as more variable and contingent than the strict classical climax model suggested. Disturbance regimes, climate variability, landscape connectivity, biological invasions, herbivory, fire, drought, human land use, and stochastic events can all redirect ecological trajectories. The idea of a single inevitable climax community is therefore considered too simplistic for many ecosystems. Some systems experience continual disturbance and may never approach a single equilibrium state. Others can possess multiple persistent community states, with transitions influenced by thresholds and feedbacks. Contemporary succession theory consequently combines deterministic mechanisms with historical contingency and probabilistic processes.
Ecological succession affects ecosystem properties beyond species composition. Early communities often have relatively simple food webs and limited structural complexity, while later stages in many ecosystems develop greater vertical layering, biomass, habitat diversity, and opportunities for specialised organisms. Primary productivity can change substantially through succession, although its precise pattern depends on environmental conditions and ecosystem type. Decomposition and nutrient cycling also change because litter quantity, chemical composition, microbial communities, and soil conditions evolve.
Carbon can accumulate in vegetation and soils, while nitrogen and other nutrients can become increasingly incorporated into biological material. Water movement can be altered by vegetation through interception, transpiration, infiltration, and soil development. Habitat complexity can increase, creating additional niches for insects, birds, mammals, fungi, and microorganisms. Nevertheless, succession does not always produce increasing diversity, biomass, or productivity indefinitely. Disturbance can repeatedly reset or redirect succession, and some specialised early successional species may decline as later vegetation develops. Consequently, ecological succession should be understood as a changing pattern of ecosystem organisation rather than a universal march towards greater complexity.
Human activities can accelerate, interrupt, redirect, or reverse succession. Agricultural cultivation repeatedly removes vegetation and disturbs soil, preventing many systems from progressing towards later successional stages. Grazing can maintain grass dominated communities, while fire management can alter woodland and grassland trajectories. Urbanisation replaces natural substrates with artificial surfaces, substantially changing the physical basis for succession. Conversely, ecological restoration may deliberately encourage succession by removing invasive species, restoring hydrology, reducing disturbance, reintroducing key species, or establishing native vegetation.
Conservation practitioners sometimes allow natural succession to proceed without intervention, while in other circumstances they maintain an earlier habitat stage because it supports species that depend on open ground, young woodland, heathland, wetlands, or other particular conditions. This is important because later successional communities are not automatically more valuable for biodiversity than earlier ones. Many organisms are adapted specifically to disturbance and early successional habitats. Effective management therefore depends on identifying the ecological processes and species assemblages that a conservation objective seeks to maintain rather than assuming that succession should always be allowed to proceed to mature vegetation.
A common misconception is that ecological succession always follows one fixed sequence and inevitably ends in a permanent climax community. In reality, succession varies among ecosystems and can be interrupted by disturbance, altered by climate, redirected by species interactions, and influenced strongly by the surrounding landscape. Another misconception is that primary succession and secondary succession are distinguished simply by whether plants are present.
The more important distinction concerns the condition of the substrate and whether a developed soil system and biological legacies remain after disturbance. Succession should also be distinguished from ecological evolution. Evolution describes heritable changes in populations across generations, whereas succession describes changes in ecological communities and ecosystems through time, although evolutionary processes can influence successional dynamics. Succession is also distinct from ecological development in the broad sense because it specifically concerns changing community composition and ecosystem organisation.
Modern understanding recognises that succession can have multiple pathways and that disturbances are not necessarily external interruptions to an otherwise linear process. Fire, storms, herbivory, floods, droughts, and other disturbances can be intrinsic components of the dynamics of many ecosystems. Ecological succession is therefore best understood as a context dependent process of community and ecosystem change generated by interactions among organisms, resources, environmental conditions, disturbance, dispersal, and time.
