Every landscape around us – a forest, a meadow, a rocky hillside – is in some stage of ongoing change. Left undisturbed, ecosystems don’t stay static; they move through a predictable sequence of community changes called ecological succession. But not all succession follows the same path. Depending on where it starts and what drives it, ecologists classify succession into several distinct types – each with its own logic, pace, and ecological significance.
Table of Contents
- Primary vs. secondary succession: where does it all begin?
- Primary succession: life on bare ground
- Secondary succession: rebuilding on existing ground
- Autotrophic and heterotrophic succession: who runs the show?
- Autotrophic succession
- Heterotrophic succession
- Allogenic vs. autogenic succession: what’s driving the change?
- Autogenic succession: organisms as architects
- Allogenic succession: change from outside
- Why these distinctions matter
Primary vs. secondary succession: where does it all begin?
The most fundamental way to classify succession is by the starting conditions of the habitat – specifically, whether soil and organic matter are already present.
Primary succession: life on bare ground
Primary succession occurs in areas that have never previously supported life, or where all previous biological material has been completely removed. There is no soil, no seed bank, no organic matter – just bare substrate. According to Clarke (1954), primary succession begins on bare areas where no life has existed or where the prior flora and fauna have been entirely destroyed.
Classic settings for primary succession include newly formed volcanic islands emerging from the ocean, glacial moraines left behind after a glacier retreats, and freshly deposited sand dunes. In these environments, the very first colonizers – called pioneer species – must be hardy enough to survive with almost nothing. On rocky surfaces, lichens are typically the pioneers. They slowly break down rock through chemical weathering and accumulate organic debris, gradually forming a thin layer of proto-soil that other organisms can exploit. Mosses follow, then herbaceous plants, then shrubs, and eventually trees. This process can take hundreds to thousands of years.
A well-documented example is hydrarch succession, where a bare pond or lake gradually fills with sediment and organic matter until it transitions into dry land hosting a terrestrial climax community. The sequence goes from phytoplankton and submerged aquatic plants, through rooted aquatics with floating leaves, emergent vegetation, and finally terrestrial species.
Secondary succession: rebuilding on existing ground
Secondary succession takes place in areas that were once colonized by a community but were then disturbed or cleared. Crucially, the soil and its seed bank remain partially intact, giving the recovering ecosystem a major head start. Secondary succession may be triggered by fire, flood, disease, or human activities such as farming, logging, or urban clearing.
Abandoned agricultural fields are the textbook example. Once farming stops, weedy annual plants rapidly establish on the exposed soil. Within a few years, perennial grasses take hold, followed by shrubs, fast-growing pioneer trees, and ultimately a mature forest – provided the climate supports one. This entire process typically unfolds over decades rather than centuries, because the soil structure, nutrient reserves, and nearby seed sources are already available. A study on secondary succession in landslide areas found that in zones where residual forest soil remained, trees and shrubs established within nine years, and after 72 years the vegetation was approaching the surrounding forest’s climax composition.
The end point for both primary and secondary succession is the same: a climax community – a relatively stable assemblage of species suited to the prevailing climate and soil conditions.
Autotrophic and heterotrophic succession: who runs the show?
Another way to classify succession is based on the type of organisms that dominate the early stages – specifically, whether the process is led by producers or by decomposers.
Autotrophic succession
Autotrophic succession starts in a predominantly inorganic environment and is characterized by the early and continued dominance of green plants and other photosynthetic producers. According to Odum (1969), in the early stages of autotrophic succession, the rate of gross photosynthesis (P) exceeds the rate of community respiration (R), meaning the system produces more energy than it consumes. This P/R ratio gradually approaches 1 as the community matures and biomass increases.
As succession proceeds, organic matter accumulates in the soil, species diversity increases, and the ecosystem becomes increasingly complex. Both primary and secondary succession, described above, fall under this category since both are ultimately driven by colonizing plants building biomass from inorganic resources. Energy flow in autotrophic succession is maintained or increased over time rather than declining, which is what allows the system to grow toward a stable climax community.
Heterotrophic succession
Heterotrophic succession, by contrast, occurs in environments that are already rich in organic matter. Here, the earliest colonizers are not plants but decomposers – bacteria, fungi, actinomycetes, and other consumers. These organisms break down the existing organic substrate, and because no new energy is being fixed by photosynthesis, the total energy in the system declines progressively as succession proceeds.
Classic examples of heterotrophic succession include the decomposition of a rotting log, the breakdown of animal dung, and communities developing in streams heavily polluted with sewage. In sewage-loaded waterways, the initial community is dominated by anaerobic bacteria and fungi consuming the organic load, succeeded by other organisms as the organic content diminishes. Ultimately, heterotrophic succession ends when the available organic energy has been fully consumed and the community disperses. As summarized by EasyBiologyClass, if succession begins with P > R, it is autotrophic; if it begins with P < R, it is heterotrophic.
While these two types operate differently, they are deeply interconnected in nature. Autotrophic organisms fix carbon and build organic matter; heterotrophic decomposers break that matter down and return nutrients to the soil – both processes occurring simultaneously in most real ecosystems.
Allogenic vs. autogenic succession: what’s driving the change?
A third classification looks at the source of the forces driving successional change – whether that force comes from inside or outside the community.
Autogenic succession: organisms as architects
Autogenic succession is driven by the organisms themselves. As a community establishes and grows, its members progressively modify the local environment in ways that make conditions less suitable for the current residents and more suitable for new arrivals. The principal force of change comes from within the community, and it operates on timescales tied to the lifespan of the organisms involved.
The mechanisms are varied: pioneer plants break down rock and add organic material, building soil that supports new plant species; as taller plants grow, they create shade and modify humidity, disadvantaging sun-loving pioneers; deep root systems open new soil layers; and fallen logs and leaf litter create microhabitats for fungi, invertebrates, and soil organisms. The transition from an abandoned agricultural field to a mature forest is a well-cited example of autogenic succession – the entire trajectory is directed by the living community itself, not by outside forces.
Allogenic succession: change from outside
Allogenic succession occurs when external environmental forces – rather than the resident organisms – drive changes in species composition. External forces may include climate change, shifts in temperature and precipitation, floods, fires, or other large-scale disturbances. In allogenic succession, organisms are responding to conditions imposed upon them, not creating those conditions themselves.
A river that periodically floods a floodplain delivers new sediments and nutrients from outside, reshaping which plant communities can establish. Similarly, as regional climate warms, cold-adapted species at higher elevations may be replaced by warm-adapted ones – not because the existing community changed its environment, but because the external temperature regime shifted. Ponds receiving heavy inputs of agricultural run-off from surrounding land represent another case: nutrient loading from outside drives rapid changes in aquatic community composition.
Because allogenic succession depends on external drivers, its pace and direction are less predictable than autogenic succession. A single severe flood or a years-long drought can abruptly reset or redirect a successional trajectory that organisms themselves had been building for decades.
Why these distinctions matter
Understanding the different types of succession is not just an academic exercise. These classifications have direct practical implications for conservation and ecosystem management. Restoring a site undergoing primary succession requires fundamentally different strategies than managing secondary succession on recently disturbed land – the former may need soil-building pioneer species introduced first, while the latter may only need disturbance removed to allow natural regeneration.
Recognizing whether succession is autogenic or allogenic is equally important. If a degraded ecosystem is recovering through autogenic processes, reducing human interference may be sufficient. But if allogenic forces – like ongoing pollution, erosion, or climate shifts – are the primary drivers, management must address those external pressures directly. Conservation efforts benefit enormously from this distinction, as it determines both how stable a restored habitat is likely to be and what interventions will actually work in the long term.
Similarly, distinguishing autotrophic from heterotrophic succession guides how we approach ecosystem restoration in polluted waterways or areas with high organic loading, where decomposer-led communities may need to run their course before photosynthesis-based succession can take hold.
Ecological succession, in all its forms, is nature’s way of continuously reorganizing biological communities in response to changing conditions. Whether driven by organisms or external forces, beginning on bare rock or recovering farmland, led by producers or decomposers – each type of succession reflects a different facet of ecosystem dynamics.
What do you think? If a forest recovers after a wildfire largely on its own, without any human intervention, which type(s) of succession would best describe that recovery – and why might the answer involve more than one category? And as climate change increasingly acts as an external driver of ecosystem change, how do you think the balance between autogenic and allogenic succession might shift in ecosystems near you?
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