Types of Succession
Primary Succession
Primary succession occurs on brand-new surfaces that have never supported life before. Think volcanic islands, freshly exposed glacial moraines, or cooled lava flows. These surfaces have no soil at all, so everything has to build from zero.
Pioneer species like lichens and mosses arrive first. They can survive on bare rock with almost no nutrients. Over time, these organisms break down rock surfaces and add organic matter as they die and decompose. This slowly creates a thin layer of soil.
As soil accumulates, slightly more demanding plants can take hold. The ecosystem progresses through a series of stages, each community modifying conditions enough to allow the next one in. This process is slow, often taking hundreds or thousands of years to reach a mature, stable climax community.
Secondary Succession
Secondary succession happens where an ecosystem has been disrupted but the soil remains intact. Common triggers include wildfires, hurricanes, abandoned farm fields, and clear-cut forests.
Because soil, seeds, and root systems are already present, recolonization happens much faster than in primary succession. Fast-growing grasses and herbaceous plants typically establish first, followed by shrubs and eventually trees.
The ecosystem may eventually reach a climax community that resembles what existed before the disturbance, though species composition often differs somewhat. A forest regrowing after a wildfire, for example, might favor different tree species than the original stand.
Sere
A sere is the complete sequence of successional stages from initial colonization to climax community, including every intermediate phase. Each stage within a sere has its own characteristic species and environmental conditions.
How long a sere takes and what stages it includes depend on climate, soil type, and the nature of the original disturbance. Two well-known examples:
- Old-field succession: An abandoned agricultural field progresses from weedy annuals to perennial grasses, then shrubs, then eventually forest.
- Volcanic primary succession: Bare lava is colonized by lichens, then mosses, then ferns and grasses, and eventually shrubs and trees over centuries.
Successional Stages and Species
Pioneer Species
Pioneer species are the first organisms to colonize a bare or disturbed area. In primary succession, these are typically lichens and mosses; in secondary succession, they're often fast-growing grasses and weeds.
What makes pioneer species successful in harsh environments:
- High reproductive rates and efficient dispersal (wind-blown seeds, lightweight spores)
- Tolerance for extreme conditions like temperature swings, drought, and nutrient-poor substrates
- Ability to modify the environment for future species. Lichens weather rock into mineral particles. Certain pioneer plants (like some legumes) fix atmospheric nitrogen into the soil, enriching it for later arrivals.
Their most important role is environmental modification. By breaking down rock, adding organic matter, and stabilizing surfaces, pioneers transform a hostile landscape into one that can support more complex communities.
Climax Community
The climax community is the final, relatively stable stage of succession. At this point, the dominant species can sustain their populations through self-replacement, and the ecosystem is roughly in equilibrium with local climate and soil conditions.
Examples of climax communities across different biomes:
- Temperate deciduous forest (oaks, maples, beeches in eastern North America)
- Tropical rainforest (high canopy diversity, multiple vertical layers)
- Tallgrass prairie (maintained by periodic fire and grazing)
- Tundra (low-growing shrubs, mosses, and lichens in arctic regions)
The exact composition of a climax community varies with climate, soil type, and disturbance regime. A region with frequent fires, for instance, may have a very different climax community than a nearby region without fire.
Facilitation
Facilitation is the mechanism that drives succession forward. Early-stage species change the environment in ways that benefit later-stage species, even if those changes eventually lead to the pioneers being outcompeted.
Here's how facilitation works in practice:
- Pioneer species colonize and begin modifying the substrate (breaking rock, adding organic matter).
- Soil conditions improve: nutrients increase, moisture retention gets better, and microhabitats form.
- New species that need better soil conditions can now establish and grow.
- These newer species further modify the environment, enabling yet another wave of colonization.
Specific examples include nitrogen-fixing legumes enriching soil nitrogen levels for non-fixing plants, and fallen trees creating nurse logs that provide a moist, nutrient-rich substrate where seedlings can germinate and grow.
Factors Influencing Succession
Disturbance
A disturbance is any event that disrupts ecosystem structure and function, often resetting or redirecting the course of succession. Disturbances vary along several dimensions, and each one affects successional outcomes differently:
- Type: Wildfires, hurricanes, floods, volcanic eruptions (natural) vs. deforestation, agriculture, urbanization (human-caused)
- Intensity: A low-intensity ground fire has very different effects than a high-intensity crown fire
- Frequency: How often disturbances recur shapes which species can persist
- Scale: A single treefall gap vs. a landscape-wide hurricane
Some ecosystems actually depend on regular disturbance. Fire-adapted ecosystems like chaparral and ponderosa pine forests rely on periodic burns to clear understory growth, release nutrients, and trigger germination of fire-dependent species. Without fire, these systems shift toward a different community structure.
Other disturbance-succession interactions to know:
- Clear-cutting resets forest succession to an early stage. The regrowing forest often has different species composition than the original because fast-colonizing species gain an initial advantage.
- Agricultural abandonment triggers secondary succession on former croplands. These old-field ecosystems progress through predictable stages of grasses, shrubs, and eventually trees over decades.
Understanding how disturbance shapes succession is central to conservation and restoration ecology. Predicting how ecosystems will respond to changing disturbance patterns (more frequent wildfires due to climate change, for example) requires a solid grasp of these successional dynamics.