Succession is the process by which natural communities of organisms replace one another over time, gradually transforming a landscape from bare ground into a complex, mature ecosystem. A patch of volcanic rock becomes a meadow, then a shrubland, then a forest. A burned hillside regrows from weeds to pines to hardwoods. Each wave of species changes the environment in ways that allow the next wave to move in, until the ecosystem reaches a relatively stable state called a climax community.
Primary vs. Secondary Succession
The two main types of succession differ in their starting point. Primary succession begins on surfaces with no soil at all: bare rock exposed by a retreating glacier, a new volcanic island, a fresh lava flow. Because there’s no soil, no seed bank, and virtually no nutrients, primary succession is slow and can take centuries to reach maturity.
Secondary succession starts where soil already exists but the living community has been wiped out or severely damaged. A forest fire, a hurricane, an abandoned farm field. Because soil, seeds, and root systems are often still present, secondary succession moves much faster. Data from Duke University’s research forest in North Carolina maps out the timeline clearly: in the first year after a disturbance, fast-growing weeds like horseweed and crabgrass dominate. By year three, grasses take over. Pines begin establishing between years 3 and 18, forming a young pine forest by year 19. Hardwoods gradually grow in the understory beneath the pines, and by roughly 100 years or more, the area has returned to a climax oak-hickory forest.
How Pioneer Species Build Soil From Nothing
In primary succession, the first organisms to arrive are called pioneer species, and they share a set of traits that let them survive in harsh, barren environments. They reproduce quickly, produce large numbers of offspring, and spread through wind-dispersed spores or seeds that can survive long periods without moisture. They need plenty of light and tend to have short life spans.
Bacteria are typically the first to show up, followed by lichens, which are partnerships between fungi and algae. Lichens don’t need soil. They absorb water and minerals directly from rain, water vapor, and dust. As they grow, they release acids that slowly break down rock. When they die, their organic remains mix with rock fragments to create the first thin layer of soil. Mosses follow a similar pattern, producing their own acids and adding more organic material. Together, these organisms also enrich the new soil by fixing nitrogen and adding carbon, making it possible for the next round of plants, ones with roots, to establish themselves.
This process plays out over real timeframes. On Surtsey, a volcanic island that erupted off the coast of Iceland in 1963, botanical observations began while the eruption was still active. Researchers found plant material washing ashore by sea currents almost immediately. The first vascular plant seedlings appeared in 1965, just two years after the island formed, but those early colonizers were wiped out within weeks by volcanic ash and ocean waves. Persistent colonization didn’t begin until 1967 and beyond.
Three Forces That Drive Species Replacement
Ecologists have identified three mechanisms that explain why one group of species gives way to the next during succession. In practice, most ecosystems show elements of all three working simultaneously.
- Facilitation: Early species change the environment in ways that help later species establish. Lichens building soil for grasses is a classic example. The early arrivals essentially prepare the ground for their own replacements.
- Tolerance: Later species can survive on fewer resources than earlier ones. They move in alongside the pioneers and gradually outcompete them by thriving under conditions (lower light, less available nutrients) that the pioneers can’t handle.
- Inhibition: Established species actively resist being replaced and hold their ground until they die from old age, disease, or physical disturbance. In this model, succession moves toward dominance by whichever species lives the longest.
What a Climax Community Looks Like
A climax community is the relatively stable endpoint of succession for a given climate and geography. In the midwestern United States, that endpoint is typically a hardwood forest dominated by oaks and hickories. Along the Pacific coast of North America, redwood forests represent a climax state where species composition may change very little for decades or even centuries, with ancient trees dominating the canopy and infrequent disturbances offering few openings for new plants.
The climax community remains in relative equilibrium until a disturbance, like fire, disease, or a major storm, resets the process and succession begins again. That said, the original concept of a single, permanent climax for every region has been refined. Introduced pathogens have permanently removed once-dominant species like American chestnut from eastern forests, and invasive insects like emerald ash borer continue to reshape forest composition. Future mature forests in heavily affected areas will likely look different from anything that existed before, making it difficult to define a single “natural” endpoint.
Succession in Water
Succession doesn’t just happen on land. In aquatic environments, a process called hydrosere transforms open water into dry land over thousands of years. A shallow lake gradually fills with organic sediment, first supporting a marsh community of aquatic plants like sedges. As sediment continues to build, a fen (a wet, grassy wetland) forms. Over time, that fen may transition into a bog dominated by sphagnum mosses and shrubs, and eventually, if conditions allow, into a mature upland forest. One study traced this progression in a single site from an open lake around 8300 BC through a fen stage, arriving at a bog by roughly 630 AD, a process spanning nearly 9,000 years.
Succession Inside the Human Body
The same principles of succession apply at a microscopic scale. The human gut microbiome assembles through a predictable sequence of bacterial communities beginning at birth. Research published in the Proceedings of the National Academy of Sciences tracked this process in detail and found that it’s far from random. The diversity of gut bacteria increases gradually over time, with distinct phases triggered by life events.
In the earliest months, when an infant’s diet is exclusively milk, bacteria adapted to processing milk sugars (like Bifidobacteria) dominate. The introduction of solid foods triggers a major shift toward bacterial communities more typical of adults. The study identified four discrete phases of bacterial succession in one infant, punctuated by events like fevers, diet changes, and antibiotic treatments. By the final phase, the relative abundances of major bacterial groups remained constant over more than 400 days, a sign the gut microbiome had reached its own version of a stable state.
How Disturbance Shapes Diversity
Disturbance and succession are deeply linked, and the relationship between them affects how many species an ecosystem supports. The intermediate disturbance hypothesis predicts that biodiversity peaks when disturbances occur at a moderate frequency. Too few disturbances and a handful of dominant species crowd everything else out. Too many disturbances and only the hardiest pioneers can survive. At intermediate levels, both pioneer species and later-stage species coexist, producing the greatest variety.
Research in tropical forests confirms that pioneer species increase in number with more disturbance while shade-tolerant species decline, with overall diversity highest at intermediate levels. However, the effect is stronger in dry tropical forests than in wet ones, where other factors play a larger role in determining species richness. Pine savannas offer another example: these ecosystems, once thought to be temporary stages held in place by fire, appear to maintain themselves through their own internal feedback loops. Pine needles act as fuel that promotes the regular, low-intensity fires the savanna depends on, keeping the system stable without ever transitioning to a closed forest.