What is ecological succession? [Explainer]

Ecological succession is the gradual, natural transformation of ecosystems through a change in their organism structure and composition over time. This transformation happens in steps as organisms arrive in a predictable sequence over large timescales. It starts with the first or pioneer species settling in barren areas, initiating the process of ecological succession. They are followed by the intermediate species, and finally, stable and long-lived climax species that represent the end point of the process. At this point, ecosystems mature and stabilise and are self-sustaining.

Operating silently in the background, succession determines how ecosystems recover from natural and human-induced disasters such as volcanic eruptions, floods, landslides, deforestation, fires, and more.

What are the different stages of succession?

Ecological succession is classified into two types — primary and secondary — determined by whether the ecosystem is new or disrupted by disturbance.

Primary succession occurs in areas that previously had no biological life, like cooled lava flows, newly formed sand dunes, or land newly exposed by retreating glaciers. Here, hardy pioneer species such as mosses, lichens, fungi, and algae establish themselves first. These organisms can survive in nutrient-poor conditions and begin the slow process of creating and stabilising soil by breaking down rocks and organic matter.

Very few primary forests remain today, due to constant disturbance from development and other human-driven activities. Most can only be found in very remote areas or isolated hills in the Himalayas, Western Ghats, and parts of the Andaman and Nicobar Islands where the old forests have been subjected to very little disturbance. A prominent global example is the Amazon basin which is home to a significant portion of the world’s remaining primary forests.

Secondary succession happens in areas where a disturbance — like a wildfire, flood, or human activity — has disrupted an existing ecosystem but left behind some biological remnants. Since soil and some seed banks remain, secondary succession progresses faster, occurring over several decades as compared to the hundreds to thousands of years that primary succession takes.

In India, prominent secondary forests include those in the Western Ghats, and most of the Central Himalayan forests which have regenerated after years of logging and disturbance to form a mostly stable assemblage of pines and shrubs, and climax hardwood oaks. However, not every climax community may follow the same species succession. An example of secondary succession is the sholas in the high-altitude regions of the Western Ghats. They are considered by some to be a stable “sub-climax” stage rather than a true climax community due to the grassland-forest mosaic since typically, grasslands are early-stage vegetation while forests come in much later.

Between the pioneer and climax stages lie various seral stages (or seres), which are intermediate community assemblages that gradually prepare the environment to support the next stage of more complex and stable communities.

Ecological succession determines resource availability for wildlife and communities and affects ecosystem services like carbon storage.

How is climate change disrupting ecological succession?

Climate change is altering the frequency, intensity, and nature of ecological disturbances. Extreme, unpredictable weather events and shifting seasons are affecting the rate and direction of ecological succession, often resetting the system to earlier stages. While ecosystems are inherently resilient, they rely on relatively stable climatic conditions and regular recovery periods, both of which are now increasingly uncertain.

The timing of key biological events such as flowering, seed dispersal, and animal migrations is also shifting — a phenomenon known as phenological mismatch or shift. When pollinators and flowering do not sync in time, reproduction fails, disrupting succession at early stages when regeneration and colonisation are critical. Additionally, changing soil properties and environmental conditions are making ecosystems inhospitable for native species, further undermining essential ecosystem services like carbon sequestration, water retention, and soil stability.

Repeated disturbances make the ecosystem increasingly vulnerable to invasion by non-native species as incomplete or stalled succession leaves ecological gaps that are often filled by hardy invasive species like Lantana camara, now widespread across India. Frequent disturbances also prevent natural regeneration, leading to long-term degradation and reduced ecological resilience.

How is ecological succession being affected in the Himalayas?

The disruption in ecological succession in the Himalayas is causing the treeline to shift upwards.

Recurring fires, grazing, and logging are blocking the natural progression of forest succession in mid-montane Central Himalaya, where the hardwood oak forests (Quercus leucotrichophora or banj oak) form a now-threatened late-successional community. While early-stage grasslands and pine forests (pioneers) show strong seedling recruitment, chronic disturbances are affecting regeneration of oak saplings, halting the transition towards stable, hardwood oak-dominated (climax) forests.

Similarly, rising temperatures and shifting precipitation patterns are altering soil biochemistry, further affecting species composition and processes such as plant regeneration and animal migration. As a result, the treeline is shifting upwards towards cooler climates. The altered growing seasons and shifting flowering periods are limiting regeneration and reducing seed survival. Species like Abies spectabilis (East Himalayan fir), Rhododendron campanulatum, and Betula utilis (Himalayan birch) are among those showing significant range shifts.

How is ecological succession being affected in the Sundarbans?

The iconic Sundarbans, already under pressure from frequent cyclones, are experiencing increased exposure to salinity due to rising sea levels and decreased rainfall.

As a result, the larger and salinity-sensitive late-successional species are decreasing in density, altering the native species composition. Studies have further linked increasing salinity to impaired regeneration and survivability among some late-successional species including Heritiera fomes (sundari mangrove).

Succession in the Sundarbans typically progresses from shrub-like, salt-tolerant pioneer species like Avicennia officinalis to less salt-tolerant, specialist species like Heritiera spp. But due to rising salinity and the resultant soil enrichment, these early-successional species are beginning to dominate the landscape, upsetting natural succession, and reducing long-term resilience, biomass carbon, and biodiversity.

How is ecological succession being affected in the Western Ghats?

With fire events increasing in recent years due to changing climate regimes, the Western Ghats and the Himalayas are becoming increasingly vulnerable.

Historically, fires played a natural role in forests, helping grasses regenerate and allowing fire-adapted species to pave the way for late-successional hardwood trees. However, this balance is now being disrupted. Recurring fires are halting succession by preventing these species from establishing, repeatedly ‘resetting’ the ecosystem. Instead, fire-resistant invasives like Pteridium aquilinum (bracken fern) are beginning to take over the grasslands, with invasive trees like exotic wattle (Acacia spp.) posing further threats. Lantana camara, an aggressive, fire-tolerant invader is now widespread across Karnataka among other states. These invasives outcompete native flora and permanently change forest structure, function, species composition, forage for native herbivores, and soil characteristics.

With each fire cycle, forests lose species that either lack fire resistance or the capacity to regenerate after burning, particularly affecting young seedlings which are more vulnerable than mature trees, and eventually halting succession.

Why must we rethink conservation and restoration?

To build true resilience, ecological restoration needs to mimic successional sequences.

For example, efforts in montane habitats to boost ecosystem services should ideally support the natural succession of the current pine-oak mosaics to late-successional, stable dense-oak forest communities.

However, India’s ecosystem restoration policies often overlook the principles of ecological succession, focussing instead, on simply expanding green cover. Forest restoration programmes tend to favour a small number of fast-growing, easily propagated species over native species to meet prescribed carbon goals. Restoration efforts in mangroves also typically involve the uniform approach of planting hardy, early-successional species like Avicennia marina, regardless of the community composition.

A meta-analysis of studies on ecological restoration highlights the importance of large-scale collaboration among governments, scientists, industries, and local communities. The research indicates that prioritising passive and natural recovery is crucial, and that limiting and tailoring generic active restoration to only necessary site-relevant scenarios is needed. Supporting this approach is a study on mangrove-specific interventions, which found that site-specific approaches informed by local sediments, salinity, hydrology, and species assemblage to restore mangroves outperformed conventional mono‑species mangrove plantations.

Ecological succession forms the base for ecosystem development, recovery, and resilience. Therefore, understanding and integrating knowledge of native seral stages and species into conservation, restoration, and climate adaptation strategies will be essential, not just for biodiversity, but for the millions of people who depend on stable, functioning ecosystems.

Read more: Tracking seasons through changing tree behaviour [Commentary]

Banner image: An aerial view of the Sundarbans, showing the species-wise zonation. Image by Touhid Biplob via Wikimedia Commons (CC BY-SA 4.0).