Ecological succession describes the natural process of change that occurs in an ecosystem over time, involving a predictable sequence of species replacement. Secondary succession is a specific type of this process, defined by its occurrence in an area where a previously existing community has been disturbed or partially removed. Crucially, this disturbance leaves the soil, substrate, and nutrient base intact. The situations that lead to secondary succession are those that destroy above-ground biomass while preserving the ecological foundation below the surface.
Natural Events that Trigger Succession
Wildfires are a frequent natural trigger for secondary succession, especially in fire-adapted ecosystems. Low to moderate ground fires typically clear away leaf litter and small shrubs but leave the underlying soil structure viable. These fires often return nutrients to the ground in the form of ash, which provides a boost for pioneer species. The heat can also stimulate the germination of seeds from certain species adapted to fire.
Severe weather events also initiate secondary succession by removing vegetation without sterilizing the ground. Hurricanes and tornadoes can uproot trees and flatten entire forest sections, creating large canopy gaps. Floods and landslides similarly strip away existing plant cover, but the soil often remains in place, retaining its organic material and seed bank.
Secondary succession is also observed following volcanic ash fall rather than lava flows. A thick layer of ash can bury and kill existing vegetation, but the original soil layer remains underneath, acting as a foundation for recovery. This contrasts with lava flows, which destroy the soil entirely and lead to primary succession.
Human Activities that Initiate Succession
Intentional human land use changes that are later abandoned create environments for secondary succession. This process, often called old-field succession, begins when agricultural land is no longer cultivated. The soil in these abandoned fields is highly disturbed but present, nutrient-rich from years of farming, and often compacted, providing a favorable base for rapid plant colonization.
Forestry practices, such as clear-cutting and logging, also initiate secondary succession when the site is not completely sterilized or paved over. The removal of trees opens the canopy, increasing light exposure and allowing pioneer species like grasses and shrubs to colonize the area quickly. Although the trees are gone, the soil, root systems, and a vast seed bank remain, facilitating fast forest regeneration.
Mining operations and urban development that are later reclaimed or abandoned can also lead to this process. In reclamation projects, topsoil is often replaced or restored after extraction, providing the necessary substrate for plants to take hold. These anthropogenic disturbances set the stage for ecological re-establishment as long as the foundation is preserved.
How Secondary Succession Differs from Primary Succession
The defining characteristic that separates secondary from primary succession is the starting condition of the habitat. Secondary succession begins in environments that already possess soil, which contains nutrients and organic matter from the previous ecosystem. Conversely, primary succession must start on a sterile substrate, such as bare rock exposed by a retreating glacier or a new lava flow, where no soil previously existed.
The presence of pre-existing soil in secondary succession provides an immediate advantage because it often contains an established seed bank. These buried seeds, along with surviving roots and underground vegetative organs, allow fast-growing, opportunistic species like grasses and weeds to sprout rapidly. This allows the recovery process to bypass the time-consuming initial stages of soil formation mandatory for primary succession.
Pioneer species in secondary succession are typically herbaceous plants that quickly cover the exposed ground, whereas primary succession’s first colonizers are slow-growing organisms like lichens and mosses. Due to the intact substrate and biological foundation, secondary succession is significantly faster, often taking only decades to reach an advanced stage. Primary succession, which requires the breakdown of rock to form soil, can take hundreds to thousands of years for similar development.