Ecological succession describes how the species structure of a biological community changes over time. This natural progression involves different communities replacing one another in a given area. Understanding how ecosystems evolve and recover is central to ecology, and this process is broadly categorized into primary and secondary succession, depending on the starting conditions.
Defining Primary Succession
Primary succession is the development of an ecosystem in an environment completely devoid of life that has never sustained soil. This process often begins on a clean slate, such as newly formed volcanic rock, bare rock exposed by a retreating glacier, or a new sand dune. The initial habitat is harsh, lacking the organic matter and nutrients necessary to support complex plant life.
The first organisms to colonize these barren landscapes are known as pioneer species, adapted to extreme conditions. These initial colonizers are typically hardy, slow-growing organisms like lichens and mosses, which attach directly to bare rock surfaces. Lichens release weak acids that chemically weather the rock, slowly breaking down the mineral substrate.
As pioneer organisms live and die, their decaying organic remains mix with the weathered rock particles. This slow accumulation of organic material and fine sediment marks the first stage of true soil formation. Over centuries, this nascent soil layer thickens enough to support larger plants, such as small grasses and ferns, progressing the ecosystem toward greater complexity.
Defining Secondary Succession
Secondary succession describes the redevelopment of a biological community after a disturbance, where the soil and its underlying nutrient structure remain intact. Because the foundation of the previous ecosystem—including organic matter, the microbial community, and the seed bank—is preserved, this process is faster than primary succession.
This ecological change is triggered by events that remove above-ground vegetation without sterilizing the soil. Common natural disturbances include wildfires, floods, hurricanes, or landslides. Human activities, such as logging or abandoning an agricultural field, are also frequent causes of secondary succession.
The pioneer species in secondary succession are typically fast-growing annual plants, weeds, and grasses that quickly utilize available light and nutrients. These early colonizers rapidly stabilize the exposed soil, preventing erosion and adding new organic material through their rapid life cycles. The pre-existing soil structure allows the ecosystem to progress toward a mature community more quickly.
Key Comparative Factors
Starting Substrate
The fundamental difference between the two processes lies in their starting environment. Primary succession begins on a sterile, abiotic substrate, such as bare rock or cooled lava, where no soil has ever existed. Secondary succession always begins on a substrate that contains pre-existing, developed soil, rich in organic matter and nutrients from the prior ecosystem. The presence or absence of mature soil is the defining characteristic separating the two types.
Pioneer Species
The initial colonizers, or pioneer species, also differ significantly due to environmental constraints. In primary succession, pioneers must survive on bare rock and initiate soil creation; these are typically non-vascular organisms like lichens and mosses. Secondary succession pioneers are generally vascular plants, such as annual weeds, perennial grasses, and small shrubs, which sprout quickly from the existing soil or seed bank.
Time Scale
The duration required for each process to reach a stable, mature community varies dramatically. Primary succession is a protracted process because the entire soil profile must be created from mineral rock, which can take centuries or even millennia. Conversely, secondary succession proceeds rapidly; because the soil is already present, recovery to a complex ecosystem can occur within decades to a few hundred years.
Illustrative Global Examples
A classic example of primary succession is the colonization of new land formed by volcanic activity, such as in Hawaii or Iceland. When lava flows cool and solidify, they form vast expanses of barren rock lacking organic material. Lichens and mosses must first colonize these surfaces, slowly weathering the rock and building the first pockets of soil over immense periods before larger plants can establish a foothold.
An example of secondary succession is the regeneration of a forest after a severe wildfire, common in ecosystems like the oak and hickory forests of North America. While the fire destroys above-ground trees and vegetation, the root systems and nutrient-rich ash are left behind, preserving the soil structure. This allows fast-growing annuals and grasses to rapidly colonize the area, quickly followed by the re-sprouting of shrubs and trees from surviving roots or the existing seed bank.