Ecological succession describes the gradual process of change in the species composition of an ecosystem over time. Among the types of ecological change, secondary succession stands out as a common process of recovery and re-establishment in environments that have experienced disruption. Understanding the events that initiate secondary succession helps to appreciate how ecosystems adapt and rebuild.
What Defines Secondary Succession’s Start
Secondary succession occurs where a disturbance has removed most existing vegetation, but the underlying soil, seed bank, and some life remain intact. Unlike primary succession, which begins on newly formed land without any pre-existing soil, secondary succession benefits from the residual fertility and biological remnants. This includes dormant seeds buried in the soil, roots, spores, and other underground plant organs. New plant life can emerge more quickly. This existing biological foundation allows for a faster re-colonization and rapid ecological change, as resources are already available.
Natural Events Initiating Secondary Succession
Natural events can initiate secondary succession by clearing existing vegetation while largely preserving the soil and its biological components. Wildland fires, such as forest fires, are a common trigger, burning above-ground material but often leaving soil, roots, and seeds unaffected. The ash produced by fires returns nutrients to the soil, creating favorable conditions for new growth. For instance, the 1988 Yellowstone National Park wildfires led to secondary succession with the re-establishment of vegetation, including lodgepole pines and aspen trees.
Floods also initiate secondary succession, particularly in riparian zones where overflowing rivers clear vegetation but deposit nutrient-rich sediments. Powerful wind events like hurricanes and tornadoes cause windthrow, uprooting trees and damaging forests, yet the soil structure usually remains undisturbed. This opens the canopy, allowing sunlight to reach the forest floor and stimulating growth from the existing seed bank.
Landslides also lead to secondary succession. While they strip away existing plant cover, the soil often remains present, facilitating regrowth. The specific trajectory of succession on a landslide can vary depending on factors like soil stability and nutrient availability. Large-scale insect outbreaks or disease epidemics can decimate plant populations, creating gaps and opportunities for other species to colonize and thrive, initiating a successional pathway.
Human Activities Initiating Secondary Succession
Human activities frequently cause disturbances that initiate secondary succession by removing vegetation but leaving the soil and its biological remnants intact. Deforestation and logging operations, where forests are cleared for timber but the land is not converted, are prime examples. This process opens up the forest canopy, allowing sunlight to reach the ground and stimulating the growth of pioneer species from the existing seed bank. Studies show that cleared tropical forests can regrow significantly within 20 years due to retained soil.
The abandonment of agricultural fields is another common human-caused event leading to secondary succession. When cultivated land is no longer farmed, the soil retains nutrients and seeds from previous activities, allowing natural vegetation to reclaim the area. For example, in the U.S. Midwest, abandoned farmlands transition from weeds and grasses to shrubs, and eventually to young forests within decades. Urban development, particularly the demolition of buildings that leaves cleared lots, also creates spaces where secondary succession can begin as plants colonize the disturbed ground.
Even certain mining operations, specifically surface mining where topsoil is carefully removed and later replaced, can set the stage for secondary succession. When such sites are rehabilitated, the replaced topsoil provides a medium for plant growth. Similarly, the construction of roads or trails that are subsequently abandoned can lead to secondary succession, as the cleared land, with its disturbed but present soil, becomes available for recolonization by plant communities. These human-induced changes demonstrate how disturbances, whether intentional or unintentional, can trigger nature’s rebuilding processes.