Ecological Succession: How Ecosystems Change Over Time

Primary Succession

Primary succession occurs on newly exposed surfaces where no soil or biological community previously existed. Examples include land exposed by retreating glaciers, newly formed volcanic islands, bare rock surfaces created by landslides, and sand dunes. Because there is no pre-existing soil, primary succession begins with the colonization of rock surfaces by organisms capable of surviving without soil, including lichens, mosses, and certain bacteria and algae.

Lichens, symbiotic partnerships between fungi and photosynthetic algae or cyanobacteria, are among the most important pioneer organisms in primary succession. They produce acids that slowly dissolve rock surfaces, and their death and decomposition contribute organic matter that begins the process of soil formation. Over decades to centuries, this thin layer of soil accumulates enough to support mosses, then herbaceous plants, then shrubs, and eventually trees. The entire process of primary succession from bare rock to mature forest can take hundreds to thousands of years, depending on climate and other conditions.

The primary succession observed on the volcanic island of Surtsey, which emerged from the ocean off the coast of Iceland in 1963, has provided ecologists with a rare opportunity to study the process from its very beginning. The first vascular plants appeared within a few years, carried by ocean currents and birds. Seabirds that colonized the island brought nutrients in their droppings that enriched the soil and accelerated plant colonization. By 2008, over 60 plant species and numerous arthropod, bird, and seal species had colonized the island.

Secondary Succession

Secondary succession occurs on surfaces where a biological community has been disturbed or removed but where soil and a seed bank remain intact. Abandoned agricultural fields, burned forests, logged areas, and storm-damaged habitats all undergo secondary succession. Because soil and seeds are already present, secondary succession proceeds much faster than primary succession, typically reaching a mature stage within decades to a century rather than centuries to millennia.

The classic study of secondary succession comes from abandoned agricultural fields in the Piedmont region of the southeastern United States. When farming ceases, fields are first colonized by annual weeds, followed within a few years by perennial grasses and wildflowers. Within 5 to 15 years, fast-growing, sun-loving pine trees shade out the grassland species. Over the next several decades, shade-tolerant hardwood trees such as oaks and hickories gradually replace the pines, eventually forming a mature deciduous forest. Each stage modifies environmental conditions, particularly light availability and soil characteristics, in ways that favor the species of the next stage while disadvantaging the current occupants.

The speed and trajectory of secondary succession depend on the severity of the disturbance, the proximity of seed sources, the condition of the soil, and the climate. Light disturbances that leave much of the soil seed bank and root systems intact allow rapid recovery. Severe disturbances that strip away topsoil or compact the ground can slow succession dramatically. The availability of mycorrhizal fungi in the soil is particularly important, as many tree species depend on fungal partners for nutrient uptake and cannot establish without them.

Mechanisms of Succession

Ecologists have proposed three models to explain the mechanisms driving species replacement during succession. The facilitation model, proposed by Connell and Slatyer in 1977, states that early-arriving species modify the environment in ways that make it more suitable for later species. Nitrogen-fixing plants may enrich the soil, enabling species that require higher nutrient levels to establish. This model best describes primary succession, where each stage actively creates conditions necessary for the next.

The tolerance model states that later species succeed because they can tolerate the lower resource levels created by early colonizers, not because early species facilitate their arrival. Late-successional tree species, for example, can germinate and grow in the shade cast by earlier species, while the earlier species cannot regenerate under their own canopy. Over time, the shade-tolerant species replace the shade-intolerant ones simply by outlasting them.

The inhibition model states that early colonizers actually resist replacement by later species, and succession proceeds only when early colonizers are damaged or killed by disturbance, herbivory, disease, or senescence. In this model, the order of species arrival and the longevity of established species play important roles in determining community composition. In reality, all three mechanisms operate simultaneously in most successional sequences, with their relative importance varying depending on the specific species, site conditions, and stage of succession.

Climax Communities and Modern Views

The concept of the climax community, a stable, self-perpetuating community that represents the endpoint of succession for a given climate, was central to early ecological thinking. Frederic Clements proposed that every region has a single climax community determined by its climate, and that all succession in that region converges toward that climax regardless of starting conditions. This deterministic view dominated ecology for decades but has been largely replaced by more nuanced perspectives.

Modern ecology recognizes that succession is more variable, less predictable, and less endpoint-oriented than Clements envisioned. Multiple stable states are possible for the same site, depending on the history of disturbance, the order of species arrival, and chance events. Disturbances of various types and intensities are natural features of all ecosystems, and most communities are a mosaic of patches at different successional stages rather than a uniform climax. The concept of shifting mosaic steady state describes landscapes where the overall composition remains relatively constant even though individual patches are constantly changing through disturbance and succession.

Succession in Aquatic Ecosystems

Succession also occurs in aquatic ecosystems, though the process differs from terrestrial systems. Lakes undergo a form of succession called eutrophication, in which nutrient accumulation over time causes a gradual transition from oligotrophic (low-nutrient, clear water, low productivity) to mesotrophic to eutrophic (high-nutrient, turbid, highly productive) conditions. In natural succession, this process takes thousands of years, but human nutrient loading has accelerated eutrophication in many lakes from millennia to decades.

Freshwater wetlands, stream channels, and marine habitats all undergo successional processes following disturbance. Coral reefs damaged by storms or bleaching events undergo succession from bare substrate through algal colonization to eventual coral recovery, a process that can take decades if conditions are favorable. Kelp forests stripped by sea urchin overgrazing (urchin barrens) can recover rapidly when urchin populations are controlled by the return of predators such as sea otters, illustrating how trophic interactions can drive or arrest successional processes in marine systems.

Disturbance and Succession in Management

Understanding succession has practical applications in forestry, agriculture, conservation, and restoration. Forest managers use knowledge of successional dynamics to plan timber harvests that mimic natural disturbance patterns, maintaining a diversity of forest age classes across the landscape. Prairie managers use prescribed fire to reset succession and prevent woody plant encroachment in grassland ecosystems that depend on periodic burning.

Restoration ecologists use successional principles to guide the recovery of degraded sites, selecting species appropriate to the current successional stage rather than immediately planting late-successional species that cannot survive without the soil conditions and microclimate created by earlier stages. Understanding how succession interacts with invasive species is also critical, as disturbed sites in early succession are often most vulnerable to invasion by aggressive non-native plants. Managing the successional process, sometimes by accelerating it through planting and sometimes by arresting it through disturbance, is a fundamental tool in ecosystem management.

Climate change adds another layer of complexity to succession. As temperature and precipitation patterns shift, the species that historically dominated late-successional communities may no longer be well-suited to local conditions. Forests recovering from disturbance may develop into novel communities unlike anything in the historical record, with species assemblages determined by which organisms can tolerate the new climate rather than by traditional successional trajectories. Managing for resilience rather than for specific community compositions is becoming a central theme in contemporary ecology.

Long-term ecological research sites around the world provide invaluable data on successional processes by tracking the same plots over decades. The Hubbard Brook Experimental Forest in New Hampshire, monitored continuously since 1963, has documented forest recovery after logging and revealed how nutrient cycling changes as forests mature. Similarly, permanent plots in tropical forests have shown that successional trajectories can be highly variable depending on the species that colonize first and the distance to intact forest seed sources. These long-term datasets demonstrate that succession is both more complex and more fascinating than simplified textbook diagrams suggest.