Ecological succession describes the process of change in the species structure of an ecological community over time. Secondary succession specifically refers to the ecological changes occurring in an area where a previously existing community has been removed, but the soil or substrate remains intact. It typically follows a disturbance that clears established vegetation.
Understanding Secondary Succession
Secondary succession begins after disturbances remove existing vegetation without obliterating the soil. Common examples include wildfires, logging operations, or agricultural fields. Natural events like floods, landslides, or strong winds can also initiate secondary succession.
Unlike primary succession, which starts on newly formed or exposed land without pre-existing soil, secondary succession benefits from the presence of a developed soil layer. This existing soil often contains a seed bank, dormant roots, or microorganisms, accelerating the recolonization process. The remnants of the previous community, even if microscopic, provide a foundation for new growth.
Characteristics of Early Colonizers
The first species to colonize disturbed sites in secondary succession are pioneer species. These organisms possess specific traits that allow them to thrive in harsh, newly exposed environments. They exhibit rapid growth rates. Many pioneer plants also have short life cycles, completing their reproduction within a single growing season.
These early colonizers are highly effective at dispersing their seeds over wide areas. Their seeds are often small and lightweight, allowing for efficient wind dispersal, or they may be easily carried by water or animals. Pioneer species also typically produce a large number of seeds, increasing the probability that some will land in a suitable location and germinate. They can tolerate harsh conditions, such as poor soil quality, high light intensity, and temperature fluctuations.
These species often play a role in modifying the environment, making it more suitable for subsequent colonizers. For instance, their roots can help stabilize the soil, reducing erosion. As they grow and eventually die, pioneer plants contribute organic matter to the soil, improving its nutrient content and structure. This initial modification paves the way for a more diverse community to develop.
Common Examples of Pioneer Species
The initial colonizers after a disturbance are annual weeds and grasses. For example, species like ragweed (Ambrosia artemisiifolia) or crabgrass (Digitaria spp.) are common in abandoned agricultural fields or recently cleared land due to their prolific seed production and rapid germination. Dandelions (Taraxacum officinale) are another ubiquitous example, known for their wind-dispersed seeds.
Following these herbaceous plants, fast-growing shrubs like sumac (Rhus spp.) or blackberry (Rubus allegheniensis) establish, which can quickly form dense thickets. These shrubs often have root systems that further stabilize the soil and begin to add more substantial organic matter. Their presence provides shelter for other species.
Certain tree species also act as early colonizers. Trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) are common pioneer trees in northern latitudes, thriving in full sunlight and quickly occupying disturbed areas. In some ecosystems, various pine species (Pinus spp.) are among the first trees to appear after a fire, as their cones may require heat to release seeds.
Factors Driving Initial Colonization
Several environmental and biological factors influence which pioneer species arrive first. The proximity of existing seed sources, such as nearby mature plants, significantly determines the initial colonizers. Wind-dispersed seeds from adjacent areas are often among the first to arrive.
The presence of a soil seed bank is another factor. Many soils contain a reservoir of viable seeds from previous plant communities, which can lie dormant for years and germinate rapidly once conditions become favorable after a disturbance. The type and severity of the disturbance also play a role; a mild disturbance might leave more living plant parts or seeds, while a severe one might require colonization from external sources.
Remaining nutrients in the soil and its moisture content are important for establishment. Pioneer species are tolerant of low nutrient levels, but the existing soil structure can still provide an advantage. Regional climatic conditions, including temperature, precipitation, and light, dictate which species are physiologically capable of surviving and growing in the disturbed environment.