Ecological succession describes the process by which the mix of species and the habitat structure within an ecological community change over time. The process continues until a relatively stable community, often called a climax community, is reached, or until a new disturbance occurs. Ecologists categorize this transformative process into two main types based primarily on the starting conditions: primary succession and secondary succession.
The Initial Environmental Conditions
The most significant difference between the two types of succession lies in the state of the starting environment, specifically the presence or absence of soil. Primary succession begins in an area that is completely devoid of life and lacks any soil structure or organic matter. This happens on substrates like newly formed volcanic rock, exposed glacial till left behind by retreating ice sheets, or new land created by lava flows.
The initial environment for primary succession is barren and often extremely harsh, featuring bare mineral surfaces without the nutrient base required by most plant life. This necessitates the creation of a habitable layer before complex organisms can colonize the area. In contrast, secondary succession occurs in an area where a previous ecosystem has been disturbed or destroyed, yet the underlying soil structure and nutrient base remain largely intact.
The remaining soil in secondary succession often contains a biological legacy, including a rich seed bank, dormant roots, and underground vegetative organs. This pre-existing foundation means the environment is already favorable for the rapid re-establishment of plant communities. Disturbances that trigger secondary succession, such as wildfires, hurricanes, or clear-cut logging, remove the above-ground vegetation but do not eliminate the essential soil layer.
Pioneer Species and the Pace of Change
The difference in starting conditions directly dictates the type of organisms that colonize first and the overall speed of the recovery process. In primary succession, the pioneer species are organisms uniquely adapted to survive on bare rock, such as lichens, mosses, algae, and certain fungi. These hardy organisms are capable of initiating the long and difficult process of pedogenesis, or soil formation.
Lichens and mosses contribute to the weathering of the parent rock by trapping dust and moisture and releasing chemical compounds that break down the mineral surface. As generations of these pioneers live, die, and decompose, they contribute the first small amounts of organic matter, gradually creating pockets of rudimentary soil. Because this process of building a nutrient-rich soil layer from scratch is geological, primary succession is slow, often taking hundreds or even thousands of years to reach a stable, mature ecosystem.
The pioneers of secondary succession are vastly different, usually consisting of fast-growing annual plants, weeds, and grasses. These species can quickly sprout because the soil, which provides nutrients and moisture, is already present. The speed of recovery is significantly accelerated by the existing seed bank and root systems that survived the disturbance.
Secondary succession bypasses the lengthy soil-building phase, allowing the ecosystem to progress faster through its successional stages. What takes centuries in a primary succession event can often happen in mere decades during secondary succession, with a mature community potentially re-establishing in 50 to 200 years. This rapid recovery is a direct consequence of the biological resources that persist beneath the surface after the initial disturbance.
Real-World Examples of Each Process
Primary succession is observed in landscapes that have been newly formed or scoured clean by extreme geological forces. A classic example is the colonization of newly cooled lava flows, such as those that continually expand the Hawaiian Islands. Here, life must establish itself on sterile, igneous rock before any soil can begin to form.
Another example is the land exposed by the retreat of glaciers, which scrape bedrock surfaces completely bare of soil and life. The initial colonization of the area around Mount St. Helens following its 1980 eruption is also a well-documented case of primary succession on volcanic deposits.
Secondary succession is a common ecological occurrence, following events that disrupt established communities but spare the soil. Forest fires, which burn trees and surface vegetation, are a frequent natural trigger for secondary succession, as the ash provides nutrients to the surviving soil. Clear-cut logging operations, where all trees are removed but the ground remains, also initiate this faster recovery process.
The most accessible example is an abandoned agricultural field, where farming activities cease, allowing the land to revert to a natural state. The soil is already present, often containing seeds from surrounding areas, which allows grasses and shrubs to quickly take hold. Other examples include areas recovering from major windstorms or non-catastrophic floods that wash away surface growth but leave the root systems and soil structure mostly intact.