Ecological succession describes the natural process of change in the structure of a biological community over time. This occurs when an ecosystem faces an alteration, such as a natural disaster or human activity. The process involves progressive changes where one community of organisms is gradually replaced by another, leading toward a more stable assemblage of species adapted to the local environment.
Defining Primary and Secondary Succession
Ecologists categorize the recovery process into two main types based on the starting conditions of the disturbed habitat. Primary succession is a long-term process that begins in an environment devoid of life and, most importantly, without pre-existing soil. This occurs on newly formed or exposed surfaces, such as cooled lava flows, areas exposed by a retreating glacier, or volcanic islands. Pioneer species, like lichens and mosses, must colonize bare rock and slowly break it down to begin the initial formation of soil.
Secondary succession occurs in an area that was previously inhabited but has been disturbed by an event that did not remove all of the soil and biological remnants. Examples include a forest fire, a severe windstorm, or an abandoned agricultural field. Because the soil remains, the initial conditions are far more favorable for life. The existence of a seed bank, remaining root systems, and established organic matter allows for a much faster recovery.
The presence or absence of established soil is the definitive difference. Primary succession must spend a long time creating soil from scratch, which can take thousands of years to support complex life. Secondary succession bypasses this initial, slow stage, starting instead with an environment that already contains the fundamental building blocks for ecosystem recovery.
The Role of Substrate in Post-Disturbance Recovery
The ground material, or substrate, dictates the speed and nature of post-disturbance recovery because it holds the biological and chemical memory of the ecosystem. Established soil is a complex mixture of weathered minerals, organic matter, water, air, and billions of microorganisms. When a disturbance leaves this soil intact, the recovery process is significantly accelerated.
Organic matter provides energy and nutrients for soil microbes, which are responsible for the cycling of nutrients like nitrogen and phosphorus. The most important biological component remaining is the seed bank, a collection of dormant seeds from the previous plant community. This allows for the rapid establishment of early successional plants, which quickly stabilize the ground and begin accumulating new biomass.
When the soil is stripped away, as happens in primary succession, all these elements are lost, leaving only bare rock or fresh, unweathered sediment. It can take decades for hardy pioneer species, such as lichens and cyanobacteria, to break down the substrate and accumulate enough organic matter to form a thin, nutrient-poor soil layer. Without the seed bank, recovery must wait for seeds and spores to migrate into the area from surrounding ecosystems. The preserved chemical and biological foundation of the soil is why secondary succession is often completed in decades, while primary succession takes centuries.
Determining Succession Type After a Flood
A flood event is not inherently classified as one type of succession or the other; its classification depends entirely on the degree of destruction it causes to the existing substrate. The vast majority of common riverine floods are examples of secondary succession. These events typically involve water overflowing banks, depositing a layer of nutrient-rich silt or sediment over the existing ground.
In less severe floods, the underlying soil, root systems, and seed bank remain largely undamaged, or are simply buried under new fertile material. The existing biological components quickly sprout and colonize the new sediment layer, leading to a relatively fast recovery. This is secondary succession because the disturbance alters the community but preserves the essential biological foundation of the soil.
A flood can initiate primary succession if it completely removes the soil layer. Rare, high-velocity events like massive flash floods or tsunamis can scour the landscape, removing all topsoil, organic matter, and vegetation down to the bedrock or sterile subsoil. This process leaves behind a barren surface of sand, gravel, or exposed rock, which is functionally equivalent to a new lava flow or glacial retreat.
When a flood leaves a landscape stripped down to a fresh, sterile substrate, the recovery must begin from the initial stages of soil formation, defining it as primary succession. The determining factor is the flood’s intensity and its effect on the ground: silt deposition over existing soil means secondary succession, while complete erosion of soil and organic matter means primary succession.