Ecological succession is the process by which the structure of a biological community changes over time following a disturbance or the creation of new land. Primary succession occurs on substrates entirely devoid of life, such as newly formed volcanic rock or land exposed by a receding glacier, where no soil previously existed. Secondary succession takes place in an area where a previous community was disturbed—by events like a wildfire or flood—but where the underlying soil and nutrient bank remain intact. Despite these distinct starting points, both trajectories are governed by the same overarching ecological mechanisms that drive communities toward a state of long-term balance.
Shared Initial Mechanisms
Both primary and secondary succession rely on the functional similarity of the first species to colonize the area, known as pioneer species. These opportunistic organisms arrive rapidly, often via wind-dispersed seeds or spores, and thrive in resource-poor or harsh environments. The initial species in both scenarios alter the abiotic environment, making it hospitable for subsequent, more complex life forms.
In primary succession, pioneers like lichens and mosses initiate pedogenesis, or soil formation. Lichens secrete organic acids that chemically weather the bare rock surface, and their decomposition provides the first inputs of organic matter necessary to bind mineral particles into rudimentary soil.
Pioneers in secondary succession, such as fast-growing grasses and weedy plants, rapidly stabilize the existing soil. These species establish a dense root network that prevents wind and water erosion, conserving nutrient capital and moisture. They also change the light environment by creating shade, which filters out light-demanding pioneers and facilitates the establishment of shade-tolerant shrubs and tree seedlings.
The Directional Progression
Once colonization occurs, both types of succession proceed through a predictable sequence of transient communities known as seral stages. This sequential replacement is driven by a continuous interplay between facilitation and competition among species. Facilitation occurs when one group of species modifies the local environment, making it more suitable for the next group, which then outcompetes the predecessors.
For instance, the establishment of nitrogen-fixing plants, such as legumes or alders, enriches the impoverished soil with usable nitrogen. This change benefits later successional species that require higher soil fertility. As seral stages advance, the physical structure of the ecosystem becomes increasingly complex, moving from simple herbs and grasses to shrubs and eventually to a multi-layered forest canopy. This structural complexity creates a greater variety of niches, which correlates with an increase in species diversity and total accumulated biomass.
Competition for resources intensifies as the community matures, especially for light, as taller, longer-lived species begin to dominate. Shade-intolerant pioneer species are eventually excluded by canopy-forming trees. This continuous cycle of environmental modification and competitive exclusion ensures the process moves systematically through the seral stages until the rate of species change slows.
Trajectory Toward Stability
The goal of both primary and secondary succession is the development of a stable, self-perpetuating ecological community known as the climax community. This final stage represents a state of dynamic equilibrium with the regional climate and local environmental conditions.
Climax communities are marked by a maximum accumulation of biomass and high species diversity compared to earlier seral stages. The organisms present are long-lived, large, and possess a complex structural organization, such as a mature forest with multiple canopy layers. The defining metabolic characteristic of this final state is that the net ecosystem productivity approaches zero.
This near-zero net productivity signifies a balanced budget where the total energy captured by producers (gross primary production) is nearly equal to the energy consumed by the entire community through respiration. The ecosystem is no longer rapidly accumulating new biomass, but efficiently cycles nutrients and maintains its existing structure.