Ecological succession describes the natural, predictable change in the species structure of an ecosystem over time. This process is driven by organisms that progressively alter their environment, making it suitable for new species to establish. Succession occurs in two primary forms: primary succession begins where no soil exists, while secondary succession happens in a disturbed area where soil and some life remain. This progression includes a distinct middle phase where the community is highly dynamic, known as the intermediate stage.
Defining the Intermediate Stage
The intermediate stage, often termed the seral stage or mid-successional phase, represents the period of greatest directional change in the ecosystem’s development. It follows the pioneer community, which consists of the first species to colonize a barren or disturbed area, and precedes the final, stable climax community. This phase is characterized by shifts in species composition and community structure, leading to high turnover rates.
Organisms benefit from environmental modifications made by pioneer species, such as the initial accumulation of organic matter and soil formation. The intermediate community is unstable because the species present actively modify the habitat in ways that favor their own replacement by the next suite of species.
Key Ecological Characteristics
A defining feature of the intermediate stage is an increase in biomass compared to the pioneer phase. Net Primary Productivity (NPP), the rate at which an ecosystem accumulates biomass, is typically highest during this period. This rapid accumulation occurs because the community shifts toward larger life forms like shrubs and trees, which store energy more effectively than smaller organisms.
As succession advances, nutrient cycling becomes more robust. The increased biomass provides a greater source of dead organic matter, which enhances decomposition and the retention of essential nutrients like nitrogen and phosphorus. This maturing soil structure and improved nutrient availability support the growth of larger, more nutrient-demanding species.
The rapid growth of larger plants leads to intense competition for resources, particularly light. As the canopy begins to close, the amount of sunlight reaching the forest floor decreases significantly, creating a shaded understory environment. This competition filters which species can survive and contributes to the high rate of species turnover.
The intermediate stage is often the point where species diversity reaches its peak along the successional timeline. This is due to the co-existence of species from both the early pioneer community and later successional stages. However, as competition for light and nutrients intensifies, this peak is followed by a decline in diversity as competitive exclusion eliminates less-adapted species.
Typical Organisms and Community Structure
The plant life in the intermediate stage shifts from the small, annual plants of the pioneer phase to larger, more persistent forms. Initial colonizers are replaced by perennial grasses, dense shrubs, and eventually, fast-growing, shade-intolerant trees. These trees grow quickly to exploit the abundant light before the canopy closes completely.
This transition reflects a shift in life-history strategies from ‘r-selected’ to ‘K-selected’ species. The early species prioritize rapid reproduction and colonization. In the intermediate stage, these are replaced by species with K-selected traits, which favor slower growth, longer lifespans, and competitive ability.
The presence of shrubs and fast-growing deciduous trees creates multiple layers of vegetation for the first time. This physical structure supports a more complex food web with a greater variety of herbivores, predators, and decomposers. The three-dimensional structure of the developing forest begins to resemble the complexity of the future climax community.
The Drive Towards Climax
The intermediate stage ends due to the changes its inhabitants create, a process known as autogenic change. The species of this phase facilitate their own eventual demise by altering the environmental conditions in their habitat. For example, the build-up of organic matter and the physical presence of roots continue to deepen and enrich the soil, making it hospitable for future species.
The primary change is the reduction of light on the forest floor due to the formation of a dense canopy. The fast-growing, shade-intolerant trees of the intermediate stage cannot reproduce successfully under their own shaded conditions. This inhibition creates an opportunity for the slower-growing, shade-tolerant saplings of the climax community to establish themselves.
As the intermediate trees reach their lifespan and die, the established, shade-tolerant climax species take their place in the canopy. This mechanism of facilitation and inhibition ensures the orderly progression toward the final, more stable community structure. The ecological changes wrought by one group of species pave the way for the next, until the system reaches equilibrium with its climate.