Ecological succession is the natural process of change that occurs in all ecosystems. This predictable sequence involves the replacement of one community of organisms with another over a period that can span decades to millennia. The process ultimately moves toward a relatively stable and self-sustaining assemblage of species known as a climax community. Understanding the causes of succession requires looking at the forces that initiate the change, the biological mechanisms that sustain the progression, and the large-scale environmental conditions that dictate the final outcome.
Immediate Triggers: Environmental Disturbances
The starting point for ecological change is often a sudden event that clears the existing life, creating an open space for new organisms to colonize. These events are broadly categorized based on whether they destroy all life and soil in the area, or if they merely disrupt an existing community.
Primary Succession
Primary succession is initiated by disturbances that result in a lifeless substrate, such as bare rock or newly deposited sediment, where no soil or organic matter remains. Examples include the formation of new land following a volcanic eruption, the exposure of rock after a massive landslide, or the retreat of a glacier leaving behind scoured bedrock and glacial till. Pioneer species, like lichens and mosses, must begin the slow process of breaking down rock and creating the first rudimentary soil layers from scratch.
Secondary Succession
Secondary succession occurs after an event disturbs an existing ecosystem but leaves the underlying soil, seed bank, and some organic matter intact. This type of succession is far more common and proceeds much faster because the foundation for life already exists. Triggers include major wildfires, large-scale logging, severe floods, and the abandonment of agricultural fields. These events remove the dominant vegetation but allow for rapid regrowth from surviving roots or quickly germinating seeds in the fertile soil.
Internal Drivers: Biological Interactions Between Species
Once a disturbance has cleared the way, the progression of succession is driven by the organisms themselves as they interact with and modify their environment. These biological interactions determine the rate and path by which early-stage communities are replaced by later-stage ones.
Facilitation
One key mechanism is facilitation, where early pioneer species alter the physical environment in ways that make it more favorable for later, more complex species. For instance, nitrogen-fixing bacteria living in the roots of pioneer plants like alder enrich nutrient-poor soil, allowing larger, nitrogen-demanding trees to establish later. Similarly, lichens colonizing bare rock secrete acids that help weather the rock surface, creating the first shallow pockets of soil suitable for mosses and small grasses.
Inhibition
Inhibition occurs when early successional species actively suppress the establishment or growth of later species, often through competition or chemical means. Some plants produce allelochemicals, compounds released into the soil that inhibit the germination or growth of potential competitors. Succession is stalled until the inhibitory species dies or is removed by disturbance, releasing the monopolized resources or space.
Tolerance
Tolerance suggests that later-stage species are neither helped nor hindered by early species; instead, they tolerate the initial conditions and outcompete the early colonists over time. These species often possess traits, such as greater shade tolerance or longer lifespan, that allow them to persist while earlier, fast-growing species decline. For example, shade-tolerant tree seedlings establish slowly beneath the canopy of pioneer trees, eventually replacing them as the older canopy dies off.
Macro-Scale Factors: Long-Term Climate and Geological Shifts
While immediate triggers start the process and biological drivers sustain it, the ultimate direction and composition of the final community are shaped by environmental forces operating over vast timescales. These macro-scale factors determine the potential endpoint of succession.
Long-Term Climate Change
Long-term climate change, spanning centuries or millennia, dictates the overall conditions that a mature ecosystem must be adapted to, such as average temperature and annual precipitation. Shifts in global climate, like the transition from a glacial to an interglacial period, fundamentally change which plant and animal species can survive in a region. These changes influence the selection of species that will form the climax community, making the endpoint of succession a dynamic, moving target.
Geological Processes
Geological processes also exert a slow but profound influence by altering the physical landscape and resource availability. Tectonic uplift can change the elevation and slope of a region, affecting drainage patterns and soil stability over millions of years. The ongoing process of soil formation, where bedrock weathers into deeper layers, determines the water-holding capacity and nutrient base for long-lived species. Changes in ocean currents or wind patterns can shift moisture delivery and seasonal temperatures, impacting the reproductive success and geographical range of regional species.