Ecological succession describes the natural process by which the structure of a biological community changes over time. It represents a gradual development or recovery of an ecosystem following a disturbance or the colonization of new terrain. This process illustrates how species within an area are replaced by others, leading to a more stable and complex community. Understanding succession explains the resilience and adaptation of natural environments.
Understanding Primary Succession
Primary succession begins in areas where no life has previously existed, or where life has been completely removed, leaving no soil. This can occur on newly formed volcanic islands, after lava flows cool, or on bare rock exposed by retreating glaciers. In these environments, the substrate is initially devoid of organic matter and nutrients.
Pioneer species, such as lichens and mosses, are the first organisms to colonize these barren landscapes. These hardy organisms can grow directly on rock surfaces and begin breaking down the rock through physical and chemical weathering. Their decomposition also adds organic material, gradually forming a thin layer of rudimentary soil over hundreds or even thousands of years. This soil development then allows for the establishment of more complex plant life, such as grasses and small shrubs.
Understanding Secondary Succession
Secondary succession occurs in areas where a pre-existing community has been disturbed or removed, but the soil remains intact. Common examples include areas affected by wildfires, clear-cut logging, abandoned agricultural fields, or severe floods. Unlike primary succession, the foundation for new life, including seeds, spores, and nutrients, is already present in the soil.
Because the soil base is already established, secondary succession proceeds at a much faster rate than primary succession. Residual seeds in the soil or those dispersed from nearby can quickly germinate and grow. This allows for a more rapid re-establishment of plant communities. The process generally takes decades to a few centuries to reach a more stable state.
Core Differences
The fundamental distinction between primary and secondary succession lies in the initial environmental conditions. Primary succession begins on land without pre-existing soil, necessitating soil formation. Secondary succession occurs where soil and some organic matter remain intact after a disturbance.
Pioneer species also differ significantly between the two processes. In primary succession, initial colonizers are typically extremophiles like lichens and mosses, capable of surviving on bare rock. For secondary succession, the early colonizers are often fast-growing plants, such as annual weeds and grasses, which can quickly take advantage of the existing soil and available light. These differences influence the timeline for each process, with primary succession often taking hundreds to thousands of years to develop mature ecosystems. Secondary succession is considerably faster, usually completing its major changes within decades to a few centuries.
The availability of nutrients and the complexity of the early stages also set these processes apart. Primary succession begins with virtually no available nutrients, requiring pioneer species to gradually build up organic matter. Secondary succession benefits from the residual nutrients already present in the soil from the previous ecosystem. Therefore, primary succession involves creating a habitable environment, while secondary succession focuses on rebuilding an established foundation.
Illustrative Examples
A classic example of primary succession is the colonization of new volcanic islands, such as Surtsey, off the coast of Iceland. Over decades, lichens and mosses established themselves on the barren lava rock, slowly contributing to soil formation before vascular plants could take root. Similarly, areas of bare rock exposed by the retreat of glaciers, like those in Glacier Bay, Alaska, also undergo primary succession as pioneer species begin to colonize the newly revealed land.
In contrast, secondary succession can be observed in a forest ecosystem after a wildfire. Although the fire may destroy much of the vegetation, the soil remains, often enriched by ash. Within weeks or months, new grasses and fire-resistant shrubs begin to sprout from seeds or surviving roots in the soil. Another common example is an abandoned agricultural field. Weeds and grasses quickly colonize the tilled soil, followed by shrubs and eventually trees, as the land gradually reverts to a more natural state.