Table of Contents
Succession: How Ecosystems Develop Over Time
Ecosystems are not static. After a disturbance—or on a newly formed surface—they tend to change in relatively regular ways. The long‑term, directional change in species composition, structure, and functioning of an ecosystem is called ecological succession.
Primary vs. Secondary Succession
Primary succession
Primary succession starts on surfaces where there was previously no soil and essentially no life.
Typical starting conditions:
- No developed soil layer
- Extreme conditions (strong radiation, temperature fluctuations, little water retention)
- Few or no nutrients available in usable form
Examples:
- Newly exposed rock after glacier retreat
- Fresh lava flows and volcanic islands
- Newly formed sand dunes, mine dumps, or construction rubble without soil
Typical sequence (schematic, not universal):
- Pioneer stage
- Colonized by hardy pioneer species such as lichens, cyanobacteria, algae, and some mosses.
- These organisms:
- Fix or trap small amounts of nutrients
- Break down rock physically and chemically
- Accumulate organic matter when they die
- First, a very thin, poor “proto‑soil” forms.
- Early soil development and herb stage
- As more organic matter accumulates, simple soil horizons develop.
- Grasses, herbs, and small annual plants can now root.
- Litter and root activity improve soil structure and water retention.
- Shrub stage
- Deeper and more structured soil allows shrubs and small woody plants.
- Roots stabilize the substrate.
- Nitrogen‑fixing plants (e.g., some legumes) may improve soil fertility.
- Young forest stage
- Fast‑growing, light‑demanding trees (pioneer tree species) establish.
- Example groups (depending on region): birches, poplars, some pines.
- A simple forest structure forms, with an increasingly shaded forest floor.
- Mature community
- Shade‑tolerant, often longer‑lived tree species establish beneath pioneers.
- Over time, they may dominate as the forest closes and structures become more complex.
- A multi‑layered community with trees, shrubs, herbs, mosses, and rich soil biota develops.
Primary succession is generally slow because soil must first be created. Timescales can range from hundreds to thousands of years.
Secondary succession
Secondary succession starts on a site where an ecosystem existed before but was disturbed or partially destroyed.
Starting conditions:
- Soil (including seed bank, spores, roots, rhizomes) is largely present
- Nutrients and organic matter already accumulated
- Some organisms may have survived in protected microhabitats
Typical triggers:
- Forest fire
- Storm damage, windthrow
- Abandonment of agricultural fields (fallow land)
- Flooding, landslides that do not remove all soil
- Human clearcutting or construction that leaves soil intact
Typical sequence:
- Bare or disturbed ground
- Rapid germination of seeds already in the soil or brought by wind/animals.
- Often dominated by fast‑growing herbs and grasses.
- Pioneer weed and grass stage
- Annual and biennial plants proliferate.
- Many are r‑strategists (numerous offspring, short life cycles).
- They stabilize the soil and add new organic matter.
- Shrub and young woodland stage
- Shrubs and light‑demanding tree seedlings appear.
- Remnant individuals (e.g., surviving stumps, root suckers) regrow quickly.
- Maturing vegetation
- Depending on climate and land use, this may progress toward forest, savanna, shrubland, or another regional vegetation type.
Secondary succession is often much faster than primary succession, because soil and propagules are already available. Substantial vegetation can recover within decades.
Mechanisms Driving Succession
Succession is not just a random sequence of species; it is shaped by interactions among species and by changes in the physical environment that organisms themselves cause.
Three classical mechanisms:
- Facilitation
- Early species improve conditions for later species.
- Examples:
- Lichens and mosses help form soil and retain water.
- Nitrogen‑fixing plants increase nitrogen availability.
- Later species often cannot establish without this preparation.
- Inhibition
- Early species hinder the establishment of later species.
- Examples:
- Dense ground cover shading the soil, preventing seedlings of other species.
- Production of allelopathic substances (chemicals that suppress competitors).
- Succession proceeds when inhibiting species die or are disturbed.
- Tolerance
- Later species are neither facilitated nor strongly inhibited but can tolerate existing conditions.
- They gradually outcompete early species due to:
- Better use of resources under changed conditions (e.g., shade tolerance)
- Greater longevity or higher efficiency at low resource levels
In real ecosystems, all three mechanisms often act together, and their relative importance can change over time.
Changes in Ecosystem Structure During Succession
As succession proceeds, several typical structural changes occur:
- Species composition
- Early: few species, dominated by generalist, stress‑tolerant pioneers.
- Later: more species, including specialists with narrower niches.
- Vegetation structure
- Early: simple, usually one or few layers (e.g., low herb layer).
- Later: more vertical layering (canopy, understory, shrub and herb layers, moss layer).
- Soil development
- Organic matter increases.
- Soil horizons become more differentiated.
- Water‑holding capacity and cation exchange capacity generally improve.
- Soil organisms (bacteria, fungi, animals) increase in diversity and biomass.
- Microhabitat diversity
- Creation of shade, litter, coarse woody debris, and differing moisture regimes.
- More niches → more species can coexist.
Changes in Ecosystem Functioning During Succession
Ecosystem processes also change:
- Biomass and productivity
- Total living biomass typically increases with succession.
- Primary production is often high in early/mid‑stages, may level off or slightly decline later as respiration and maintenance costs rise.
- Nutrient cycles
- Early: high nutrient losses (erosion, leaching), cycles are “leaky”.
- Later: stronger internal recycling, closed nutrient cycles:
- More litterfall
- More decomposers
- Nutrients are retained longer within the system.
- Energy flow
- Early: relatively simple food webs, few trophic levels.
- Later: more complex food webs, more trophic levels and detritus pathways.
- Microclimate regulation
- Vegetation buffers temperature extremes, reduces wind speed, and alters humidity and light conditions.
Climax Communities and Dynamic Equilibria
Earlier ecology often described succession as moving toward a climax community—a relatively stable, self‑maintaining vegetation type determined mainly by climate and regional conditions.
Key points and modern view:
- Climax as an idealized endpoint
- Represents a community relatively stable over long periods, given no major disturbance.
- In a temperate region, this may be a mature, mixed forest; in another, a grassland.
- Dynamic equilibrium
- Even “climax” systems are not truly static:
- Individuals die, regenerate, and shift in abundance.
- Small disturbances (gap formation, local erosion) constantly create new micro‑succession events.
- Multiple stable states
- Under similar climate, different stable community types can exist depending on history and disturbance regime (e.g., forest vs. savanna maintained by frequent fires).
- Patch dynamics
- Mature landscapes often consist of a mosaic of patches in different successional stages.
- Example: forest with treefall gaps, young regrowth, mature stands, and old‑growth patches.
Disturbance Regimes and Their Role
Disturbances are discrete events that change ecosystem structure and resource availability (e.g., fire, storm, flood, insect outbreak, human logging).
Key aspects:
- Frequency: how often disturbances occur
- Intensity: how severe they are (e.g., complete canopy removal vs. few trees lost)
- Extent: area affected
Consequences for ecosystem development:
- Very rare, mild disturbances:
- Allow communities to approach late successional or “climax‑like” states.
- Moderate, regular disturbances:
- Sustain a mix of successional stages (high habitat diversity).
- Can increase landscape‑level biodiversity.
- Frequent, severe disturbances:
- Keep systems in early successional stages.
- Can reduce long‑term biomass and habitat complexity.
Many ecosystems have natural disturbance regimes:
- Mediterranean shrublands maintained by fire.
- River floodplains shaped by periodic inundations and channel shifts.
- Coastal dunes formed and reworked by windstorms.
Human‑induced changes (fire suppression, clearcutting, altered flooding by dams) often modify these regimes and thereby the typical developmental pathways of ecosystems.
Retrogression and Degradation
Not all changes are progressive toward more biomass or diversity. Ecosystems can also retrogress or become degraded:
- Retrogression
- Long‑term shift to lower productivity and simplified vegetation under nutrient depletion or harsh conditions.
- Example: old, heavily leached soils in wet tropics where nutrients become locked in insoluble forms.
- Degradation
- Loss of structure, functioning, and biodiversity, often due to human activities:
- Overgrazing → loss of vegetation cover, erosion.
- Deforestation → soil erosion, loss of nutrients, altered hydrology.
- Pollution or salinization → loss of sensitive species.
A degraded system:
- May follow a new successional trajectory different from the original.
- Can become trapped in an alternative stable state (e.g., shrub‑encroached grassland, or algal‑dominated lake) that is difficult to reverse.
Ecological Resilience and Stability
Resilience refers to an ecosystem’s ability to absorb disturbance and recover its structure and function.
Different components:
- Resistance
- How little an ecosystem changes when disturbed.
- Example: deep‑rooted grasslands resisting drought better than shallow‑rooted crops.
- Recovery (engineering resilience)
- Speed and extent of return to pre‑disturbance conditions.
- Example: rapid regrowth of fire‑adapted vegetation after a low‑severity fire.
- Ecological resilience (regime shift concept)
- Capacity of a system to remain in the same overall “regime” (e.g., clear‑water vs. turbid lake) despite disturbances.
- If thresholds are crossed (e.g., nutrient loading, loss of key species), the system may shift to a fundamentally different configuration.
Resilience influences:
- How succession unfolds after disturbances.
- Whether ecosystems return to their previous state or reorganize into a new community type.
Human Influence on Ecosystem Development
Human activities increasingly shape how ecosystems develop and change:
- Land use
- Agriculture, forestry, urbanization create new disturbance patterns.
- Abandoned fields show secondary succession, sometimes leading to semi‑natural habitats.
- Biological invasions
- Introduced species can alter successional pathways by outcompeting natives or changing nutrient cycles.
- Climate change
- Alters temperature, precipitation, and disturbance regimes (e.g., more fires, storms, droughts).
- Can shift which species are competitive at different successional stages.
- Restoration ecology
- Attempts to steer or accelerate ecosystem development toward desired states:
- Reforestation or afforestation
- Re‑wetting drained wetlands
- Reintroduction of key species (e.g., large herbivores, predators)
- Uses knowledge of natural successional processes to plan interventions.
Understanding the development and change of ecosystems is essential for managing landscapes, conserving biodiversity, and restoring damaged environments in a rapidly changing world.