8.1 Primary and Secondary Succession

Primary and Secondary Succession

Ecological succession describes how ecosystems change over time as species gradually replace one another. Understanding the difference between primary and secondary succession is central to ecology because it explains both how brand-new ecosystems form and how damaged ones recover.

The core distinction is simple: primary succession starts from bare, lifeless substrate (no soil at all), while secondary succession starts on land that was disturbed but still has soil intact. Both processes move through a series of community changes toward a relatively stable climax community, but they differ dramatically in how long that takes and why.

Primary vs Secondary Succession

Starting Conditions and Initial Stages

Primary succession begins where no ecosystem existed before. Think of newly cooled lava flows, land exposed by a retreating glacier, or a newly formed volcanic island. There's no soil, no seed bank, no organic matter. The first organisms to arrive are pioneer species like lichens, mosses, and cyanobacteria that can survive on bare rock.

Secondary succession begins where an ecosystem has been disrupted but the soil remains. Wildfires, clear-cut logging, and abandoned farmland are classic examples. Because the soil is already there, complete with nutrients, seeds, and root fragments, the process skips the slow pioneer stage entirely. Grasses, herbs, and fast-growing weedy species colonize quickly.

Trajectory and Duration

The biggest practical difference between the two is speed:

• Primary succession often takes hundreds to thousands of years to reach a climax community. Soil has to form from scratch through rock weathering and the slow accumulation of organic matter. That bottleneck slows everything down.
• Secondary succession can progress in decades to a couple hundred years. The existing soil already contains nutrients, microorganisms, and a seed bank, so plant communities establish much faster.

Nutrient availability is the key factor driving this time difference. In primary succession, nutrients accumulate grain by grain. In secondary succession, they're already in place.

Ecosystem Development

During primary succession, ecosystem complexity increases slowly as soil deepens and nutrients build up. Each wave of organisms modifies the environment just enough to allow the next wave to move in.

During secondary succession, the existing seed bank and surviving root systems mean a relatively diverse plant community can appear almost immediately. The ecosystem doesn't have to rebuild its foundation; it just has to regrow its structure.

Both pathways ultimately converge on a climax community suited to local climate and soil conditions, though the route and timeline look very different.

Stages of Primary Succession

Stage 1: Pioneer Species and Initial Colonization

Pioneer species are the first organisms to colonize bare rock. They're tough, tolerant of extreme conditions, and don't need soil to survive.

• Crustose lichens are often the very first colonizers. They produce weak acids that slowly break down rock surfaces, beginning the process of soil formation.
• Mosses and cyanobacteria follow, trapping moisture and tiny particles of weathered rock.
• As these organisms die and decompose, they leave behind thin layers of organic matter. This is the earliest beginning of soil.

This stage is extremely slow. It can take decades just to produce a few millimeters of soil.

Stage 2: Soil Development and Early Plant Establishment

As weathering continues and organic matter accumulates, a thin but functional soil layer forms. This allows small herbaceous plants and grasses to take root.

• Early colonizers like fireweed and certain grasses add to soil development through root growth and leaf litter.
• Nitrogen-fixing plants are especially important here. Species that host nitrogen-fixing bacteria (like certain legumes) convert atmospheric nitrogen into forms other plants can use, boosting soil fertility for everyone.
• Each generation of plants lives, dies, and decomposes, deepening and enriching the soil further.

Stage 3: Shrub and Tree Establishment

With deeper soil, shrubs and small trees can establish. This is where the ecosystem starts to look more complex.

• Woody plants like alder shrubs and pine seedlings move in, stabilizing soil with their root systems and cycling nutrients more efficiently.
• The growing plant community creates new habitats: shade, leaf litter layers, and vertical structure that support insects, birds, and small mammals.
• Shade-tolerant species begin appearing in the understory beneath the first trees.

Stage 4: Climax Community Development

Eventually, soil depth and nutrient content support the growth of large, long-lived trees. The ecosystem reaches its most complex and stable state.

• A climax community has a well-developed structure: distinct canopy layers, diverse understory, and a rich soil ecosystem.
• Nutrient cycling and energy flow are roughly in balance. The community replaces itself rather than being replaced by something new.
• Examples include old-growth temperate forests (dominated by species like oak, beech, or hemlock) and mature tallgrass prairies.

Primary vs Secondary Succession Differences

Soil and Nutrient Availability

This is the most fundamental difference between the two types:

In secondary succession, the existing soil gives plants an enormous head start. Seeds already buried in the ground can germinate as soon as conditions allow.

Biological Foundation

Secondary succession doesn't start from zero biologically. Surviving root systems can resprout, soil microorganisms (bacteria, fungi) are still active, and animals that fled the disturbance can return quickly.

Primary succession has none of this. Every organism must arrive from outside the site, whether carried by wind, water, or animals. There are no surviving roots, no fungal networks, no soil invertebrates. The biological community builds entirely from scratch.

This is why secondary succession often skips the pioneer species stage. The biological infrastructure that pioneers spend centuries building is already in place.

Community Composition and Diversity

• Secondary succession typically shows higher species diversity from the very beginning, thanks to the seed bank and surviving organisms.
• Primary succession starts with just a handful of pioneer species and diversifies gradually as habitat conditions improve.
• Both converge on a climax community over time, but secondary succession gets there faster and with a more diverse cast of species along the way.

Factors Influencing Succession

Environmental Factors

Local environmental conditions shape which species can establish and how fast succession proceeds:

• Climate (temperature, precipitation, seasonality) determines the pool of species that can survive at a site.
• Soil characteristics like texture, pH, and nutrient content control which plants can establish. Acidic soils favor different species than alkaline ones.
• Topography matters more than you might expect. A north-facing slope stays cooler and moister than a south-facing slope at the same elevation, and the two can follow noticeably different successional paths.
• Elevation affects temperature and precipitation patterns, which is why succession on a mountaintop looks very different from succession in a valley.

Disturbance Regimes

Disturbances aren't just interruptions to succession; they're a normal part of how ecosystems function. What matters is their frequency, intensity, and scale.

• Frequent, low-intensity disturbances (like regular ground fires) can maintain an ecosystem in a mid-successional state rather than letting it reach climax.
• Some species are adapted to specific disturbance regimes. For example, lodgepole pines have serotinous cones that stay sealed shut until the heat of a fire melts the resin, releasing seeds onto freshly cleared ground.
• Flooding frequency shapes succession in riparian zones, favoring species that tolerate periodic inundation.

Biotic Interactions

Organisms don't just respond to succession; they drive it.

• Facilitation: Some species modify the environment in ways that help later species establish. Nitrogen-fixing alders improve soil fertility, making it possible for other trees to grow. Nurse plants in deserts provide shade and moisture that seedlings need to survive.
• Competition: As resources become limited, competitive species outcompete early colonizers, shifting community composition.
• Herbivory: Heavy grazing or browsing can slow succession by killing young plants, while reduced herbivory can speed it up.
• Symbioses: Mycorrhizal fungi form partnerships with plant roots, dramatically improving nutrient uptake. Plants with strong mycorrhizal networks often establish more successfully during succession.

Human Interventions

Humans are now one of the most powerful forces shaping succession.

• Prescribed burning maintains fire-dependent ecosystems like longleaf pine savannas, preventing succession from converting them to closed-canopy forest.
• Ecological restoration projects actively guide succession by planting native species, removing invasives, and restoring soil conditions.
• Invasive species can derail succession entirely. A fast-spreading invasive grass, for example, can dominate a site and prevent native shrubs and trees from establishing.
• Abandoned agricultural fields undergo secondary succession, but the direction depends heavily on what's in the soil. Fields with depleted, compacted soil and non-native seed banks may follow very different trajectories than fields abandoned with richer soil.