Ecological succession is the predictable process of change in an ecological community over time, occurring following a major disturbance or the formation of entirely new land. This process leads to a sequence of species replacement. Understanding this phenomenon requires differentiating between the two main types, primary and secondary succession, which are fundamentally defined by their starting conditions. The distinction between these two pathways explains why some landscapes recover in decades while others take many centuries to develop complex life.
The Process of Primary Succession
Primary succession begins in an environment completely devoid of life and established soil. This occurs on newly created or exposed substrates, such as bare rock left by a retreating glacier, newly formed volcanic rock, or cooled lava flows. Since no organic matter or previous biological community exists, the initial stages involve the slow work of breaking down rock into a habitable substrate.
The first organisms to colonize these barren areas are known as pioneer species, typically hardy life forms like lichens, mosses, and certain microbes. Lichens secrete acids that chemically weather the rock surface. As these pioneer species grow, die, and decompose, their organic remains mix with the weathered rock particles to create the first, thin layer of soil.
This slow accumulation of organic material is pedogenesis, or the formation of soil, which is required for the next successional stages. Once a thin layer of soil can hold moisture and nutrients, small, hardy vascular plants like grasses and ferns can establish themselves. The environment transitions from an inhospitable mineral surface to a more supportive environment, enabling colonization by larger shrubs and eventually trees.
The Process of Secondary Succession
Secondary succession takes place where a biological community previously existed but was recently disrupted by an event like a forest fire, logging, or abandoned agricultural land. Unlike primary succession, the environment retains established soil containing organic matter and nutrients. The intact soil structure provides an immediate advantage for new growth.
The presence of a soil seed bank is a major accelerator, allowing dormant seeds to quickly germinate. Pioneer species are often fast-growing annual plants, grasses, and weeds that quickly colonize the disturbed area. These early colonizers are typically sun-loving and reproduce rapidly, establishing a dense cover in the first few seasons.
The initial plant community quickly stabilizes the soil, preventing erosion and beginning nutrient cycling. As these annual and perennial plants mature, they are gradually replaced by intermediate species like shrubs and small, softwood trees. Existing root systems and residual fertility allow the ecosystem to recover significantly faster than a barren landscape.
Fundamental Differences in Progression
The most pronounced difference is the overall time scale required to reach a stable, mature ecosystem. Primary succession is an exceedingly slow process that requires centuries, and sometimes millennia, to complete because it must build soil from scratch. The weathering of rock and the gradual accumulation of humus demands vast amounts of time.
Secondary succession is a much faster process, often taking only decades or a century or two to establish a stable community. The pre-existing nutrient-rich substrate, including organic matter and a microbial community, bypasses the time-consuming step of soil formation. This foundation means the energy input required for colonization is significantly lower compared to the process of rock breakdown in primary succession.
The nature of the pioneer species also highlights the difference. Primary succession pioneers, such as lichens, are specialized organisms capable of surviving in nutrient-poor environments. Secondary succession pioneers are often fast-growing, highly dispersive plants that exploit the existing, favorable soil conditions immediately. The presence of viable propagules in the soil bank further accelerates the re-establishment of vegetation, which is entirely absent at the start of primary succession.
Illustrative Examples of Both Types
Primary succession occurs following a volcanic eruption that creates new land or covers existing land with sterile lava rock. The formation of the island of Surtsey off the coast of Iceland in 1963 provided a laboratory for observing this process as life colonized the cooled, bare basalt. Another instance is the retreat of a glacier, which exposes barren bedrock or glacial till that must be colonized by lichens and mosses before vascular plants can take hold. New sand dunes, which lack organic matter, also represent a slow progression toward soil formation and primary succession.
Secondary succession is exemplified by the immediate regrowth seen after a forest fire. While the fire destroys the above-ground vegetation, the root systems and the soil remain, often enriched by the ash and nutrients released from the burned biomass. Another common example is an abandoned agricultural field, where farming ceases and the land is quickly colonized by annual weeds, then perennial grasses, before shrubs and trees move in. A clear-cut logging operation also initiates secondary succession, as the disturbance removes the forest canopy but leaves the soil, seed banks, and residual organic matter intact.