Ecological succession is the natural and predictable process of change in the species structure of an ecological community over time. This process describes how groups of organisms replace one another in a sequence until the ecosystem reaches a mature, relatively stable state. It is a fundamental mechanism through which ecosystems recover from disturbance or colonize newly formed habitats.
When Life Begins Anew: Primary Succession
Primary succession occurs when a habitat is newly created or exposed, starting where no life or soil previously existed. This happens when a severe event completely removes all organic material and exposes bare rock or other sterile substrate. Examples include the cooling of lava flows, the exposure of land after a glacier retreats, or the formation of new volcanic islands.
The defining characteristic of primary succession is the complete absence of soil and a seed bank, meaning the process begins on an abiotic surface. Initial colonization is undertaken by pioneer species, such as lichens and mosses, which are highly adapted to survive in harsh conditions. These organisms initiate soil formation by weathering the rock through the secretion of acids and the accumulation of their own decaying organic matter.
This soil-building phase is extremely slow, often requiring centuries or millennia to produce enough material to support larger plant life. As thin layers of soil accumulate, small, hardy herbaceous plants begin to colonize the area, further adding to the organic content.
Recovery After Disturbance: Secondary Succession
Secondary succession occurs when a disturbance event destroys most of the existing community but leaves the underlying soil and nutrients largely intact. This form of succession happens in areas that were previously colonized and supported life, resulting in a much faster recovery trajectory. Common triggers include natural disasters like wildfires, severe floods, hurricanes, and human-caused events such as logging or the abandonment of agricultural fields.
Because the soil structure, essential nutrients, and a viable seed bank remain, the recovery process can bypass the long soil-building phase of primary succession. Seeds and roots from the previous community, which may have survived the disturbance underground, quickly germinate and sprout, allowing for the rapid establishment of fast-growing grasses and herbaceous plants.
The presence of residual organic matter and established soil means that secondary succession can progress to an intermediate community stage within decades, rather than the hundreds or thousands of years required for primary succession. Following a wildfire, nutrient-rich ash is left on the ground, and specialized species like the jack pine require the heat of the fire to open their cones and release seeds. The speed and trajectory of recovery are strongly influenced by these pre-disturbance conditions.
The Step-by-Step Timeline of Community Change
Once succession is initiated, the ecosystem proceeds through a sequential timeline of community change. The first species to colonize are the pioneer species, which are typically r-selected species characterized by rapid reproduction and efficient dispersal. These early colonizers are adapted to high-stress, low-resource environments and quickly maximize their presence in the newly opened habitat.
As the pioneer species grow and die, they modify the physical and chemical environment, a process known as facilitation. Shade-intolerant species create the initial canopy, which cools the soil and increases moisture retention, making conditions suitable for later species. This modification allows for the invasion and establishment of intermediate communities, often composed of shrubs and small trees.
The intermediate stages are characterized by a continuous shift in species composition driven by increased competition for light, water, and nutrients. New, slower-growing, and shade-tolerant species outcompete the initial pioneers, displacing them through a mechanism called inhibition. As the community develops, the total biomass and species diversity increase, leading to a more complex food web and habitat structure.
Achieving Stability: Climax Communities
Succession continues until the ecosystem reaches a climax community, a relatively stable and mature stage where the species composition remains constant over a long period. This community is considered to be in equilibrium with the prevailing regional climate and geographical conditions. The species present are those best adapted to the long-term environmental factors.
The state of a climax community is more accurately described as a dynamic equilibrium rather than a static endpoint. While the overall species composition may not change, individual species populations fluctuate in response to minor environmental variations. This stability persists until a major disturbance, such as a severe storm or human intervention, resets the process, potentially initiating a new sequence of secondary succession.