Ecological succession describes the process by which the species structure of an ecological community changes over time. It is a fundamental concept in ecology, illustrating how ecosystems recover and develop following various events. This natural progression involves a series of shifts in species composition, often spanning many years. Understanding these changes helps explain how diverse communities, such as forests, emerge from different starting conditions.
What is Primary Succession?
Primary succession occurs in environments where no soil or life previously existed. This includes newly formed volcanic islands, bare rock exposed by retreating glaciers, or fresh lava flows. In these barren areas, colonization must begin from scratch, as no pre-existing organic matter or established soil is present.
The initial colonizers, known as pioneer species, are organisms capable of surviving in harsh, nutrient-poor conditions. Examples include lichens and mosses, which attach to rock surfaces and gradually break them down. As these pioneer species grow and die, their decomposition contributes to rudimentary soil formation. Over extended periods, this slow accumulation of organic material allows for the establishment of larger, more complex plant life, eventually supporting a diverse ecosystem.
What is Secondary Succession?
Secondary succession takes place in areas where a community previously existed but has been disturbed or removed, yet soil and some life forms remain intact. This type of succession often follows events like wildfires, logging, or abandoned agricultural fields. Unlike primary succession, foundational elements for new growth, such as soil and a seed bank, are already present.
Because soil structure and nutrients are largely preserved, the recovery process in secondary succession can proceed more quickly. The existing soil provides an immediate substrate for plants to grow, and dormant seeds can germinate rapidly. This allows for quick re-establishment of plant communities and other organisms within the ecosystem.
Why Secondary Succession is Faster
Secondary succession proceeds at a faster rate than primary succession primarily due to existing soil. This established soil is rich in nutrients and organic matter, providing an immediate, fertile substrate for plant growth. In contrast, primary succession requires the slow creation of soil from bare rock, a process that can take hundreds or thousands of years.
The availability of seeds and spores also accelerates secondary succession. Many seeds from the previous community may remain dormant within the soil, forming a seed bank, or be dispersed from nearby undisturbed areas. These available propagules allow for quick germination and establishment of new plant populations, bypassing the long colonization phase seen in primary succession.
Some organisms often survive the disturbance in secondary succession. Microorganisms, fungi, and even small animals may persist in the soil or underground, contributing to the quick re-establishment of ecological processes. These surviving organisms help maintain nutrient cycling and soil health, which are important for supporting new growth.
Existing soil in areas undergoing secondary succession retains nutrients from the previous community. This immediate nutrient availability supports strong plant growth from the outset, unlike primary succession where nutrients must be accumulated slowly through pioneer species decomposition. In some cases, root systems of certain plants may even survive the disturbance, allowing them to resprout quickly and speed ecosystem recovery.