Succession

Succession definition

Succession is not just “change over time” — it is the emergence of ecosystem structure through feedbacks between biota and abiotic conditions.

Key properties:

• Directional but not strictly deterministic

• Path-dependent (initial conditions matter)

• Non-linear (thresholds, feedback loops)

Succession represents a dynamic trajectory shaped by feedbacks, disturbance regimes, and resource constraints rather than a fixed pathway toward equilibrium.

Autogenic succession

Driven by ecosystem engineering by organisms

Mechanisms:

• Soil formation (pedogenesis)

• Nutrient accumulation (especially N and P)

• Light attenuation (canopy closure)

• Microclimate buffering (humidity, ताप, wind)

Feedback loop:
Vegetation → soil → nutrients → more vegetation

Allogenic succession

Driven by external forcing

Examples:

• Climate change (post-glacial warming)

• Sedimentation (estuaries, dunes)

• Sea-level change

• Integrated Case: Fal Estuary

• Sediment accretion raises elevation

• Changes inundation frequency

• Allows zonation:

  • Pioneer → salt marsh → woodland

This is physical forcing overriding biological control

Primary succession

• No nitrogen (key limiting nutrient)

• No organic carbon

• Poor water retention

• No soil present

• only organic matter is wind-blown

• Begins on newly formed substrates not occupied by any organisms

• E.g., lava flows, newly exposed rock faces, alluvial deposits, glacial moraines

Primary succession case study: Glacier Bay

Chronosequence approach:

Space substituted for time

1. 0–10 years

   • Bare substrate

   • Microbial crusts begin

2. 10–50 years

   • Dryas establishes

   • Symbiotic nitrogen fixation (Frankia bacteria)

   • Soil nitrogen increases significantly

3. 50–100 years

   • Alder invasion:

     • Strong nitrogen fixation

     • Soil acidification

     • Competitive exclusion of pioneers

4. 100–300 years

   • Conifer establishment:

     • Sitka spruce

     • Western hemlock

• Early facilitation → later inhibition

• Alder eventually suppresses earlier species

This shows succession is not purely facilitative

Secondary succession: Case: Old-field succession (North America)

Mechanistic detail:

• Seed bank already present

• Rapid colonisation via:

  • Wind-dispersed seeds

  • Dormant seeds triggered by disturbance

Sequence:

1. Ruderals (r-selected)

   • Ambrosia artemisiifolia

   • High fecundity, low competitive ability

2. Winter annuals

   • Exploit early germination advantage

3. Perennials

   • Root competition increases

4. Woody species

   • Light becomes limiting factor

Key transition driver: Shift from below-ground competition (nutrients)
to above-ground competition (light)

Secondary succession: Case: Germination of Ambrosia artemisiifolia triggered by disturbance

•Unfiltered light, fluctuating temperature, reduced CO₂ concentration

•Summer annuals are replaced  by winter annuals

•Head-start in competition

Degradative succession

•Autogenic, primary succession

•Colonization and subsequent decomposition of dead organic matter

•E.g., pine needles

•Fall in august ð fungi colonise

•Other fungi and mites penetrate

•After ~2 years, invasion by soil microfauna

•After ~7 years, complete decomposition (acidic humus formed)

Allogenic succession case study: Post-glacial Europe

• Climate warming after last Ice Age (~10,000 yrs)

• Species shifts:

  • Pioneer: birch, pine

  • Later: oak, ash

  • Cold stages: tundra vegetation

Evidence: pollen cores

Allogenic succession case study: Fal Estuary

• Salt marsh expands seaward

• Woodland invades inland

Key species:

• Spartina anglica

Controlled by sediment deposition + tidal height

Successional mechanisms

1. Facilitation

• Early species improve environment

• Example: nitrogen-fixing plants in Glacier Bay

2. Inhibition

• Early species prevent later colonisers

• Only removed by disturbance

3. Tolerance

• Late species tolerate low resources and outcompete

Succession is not governed by a single mechanism but by shifting dominance of facilitation, inhibition, and tolerance through time

Theoretical Models of Succession

1. Competitive exclusion (no disturbance)

• Dominant species exclude others

• Low diversity

2. Intermediate Disturbance Hypothesis (IDH)

• Occasional disturbance → highest diversity

3. Constant disturbance (colonial model)

• High turnover

• Dominated by r-selected species

Tilman resource ratio hypothesis

Succession is governed by:

• Nutrient availability ↑ over time

• Light availability ↓ due to shading

Early stage:

• High light, low nutrients

• Species adapted to rapid uptake

Late stage:

• Low light, high nutrients

• Shade-tolerant, efficient species dominate

This explains predictable species replacement

Diversity patterns

Intermediate Disturbance Hypothesis

ecosystems experiencing moderate levels of disturbance support greater biodiversity than those subject to rare or frequent disturbance. Formulated by ecologist Joseph H. Connell in 1979

• disturbances create new niches

• rare or mild - dominant species can monopolize resources, reducing diversity

• frequent/severe - few species can persist

• intermediate levels, both pioneer and climax species coexist: the environment remains dynamic enough to prevent exclusion yet stable enough to allow regeneration and species turnover. This yields maximum species richness across scale

Disturbance theory

1. Low disturbance

• Competitive exclusion

• Low diversity

2. Intermediate disturbance

• Prevents dominance

• Maintains diversity

3. High disturbance

• Only r-selected species survive

Coral Reef Succession

Mechanisms:

• Space = limiting resource

• Corals compete intensely

Disturbance effects:

• Hurricanes remove dominant corals

• Opens substrate for colonisation

Outcomes:

• Stable reef → low diversity

• Moderate disturbance → peak diversity

• Constant disturbance → low diversity

Coral reef succession: No disturbance:(competitive exclusion model)

•As the reef becomes complex, organisms compete for space

•Dominant organism outcompetes

•Occurs in stable environments

•Low species diversity

•E.g., highly protected patch reefs within lagoons or protected bays; deeper water

Coral reef succession: •Occasional strong disturbance (intermediate disturbance model)

•Storms and hurricanes allow for other species to move in

•Dominant species cannot reach competitive exclusion

•Recovery period after disturbance

•Area of high diversity

Coral reef succession: Constant strong disturbance (colonial model)

•Constant exposure

•Shallow environment

•High turnover of species

•r-selected species

human trampling

r/K selection

Fundamental trade-off:

• Colonisation ability vs competitive ability

Early succession:

• r-selected:

  • Fast growth

  • High dispersal

  • Short lifespan

Late succession:

• K-selected:

  • Slow growth

  • High efficiency

  • Strong competitors

Link to Grime’s CSR model:

• R → disturbed

• C → mid-succession

• S → late, resource-limited

Climax community

• Dynamic equilibrium

• Multiple possible states

•No community is stable for long because of natural disturbances, e.g., storms, fires

•Forest climax may take 300 years to develop – may never reach this stage

Reasons:

• Disturbance resets succession

• Environmental heterogeneity

• Biological interactions

Cyclic succession

Occurs at small spatial scales

Example:

• •E.g., Forest tree dies and falls, light increases

  •Pioneer species germinate in the gap, etc.

Prevents system-wide climax

Retrogression

Decline in ecosystem productivity over time without disturbance

Case:Coolooladune systemProcess:

• Long-term weathering removes phosphorus

• P becomes limiting

Effects:

• Decline in:

  • Biomass

  • Productivity

  • Microbial activity

Succession is not always progressive

e.g.:

• Glacier Bay, Alaska

• Hawaiian Islands

• Cooloola, Australia

Non-linearity and alternative states

• Systems can shift abruptly

• Thresholds exist

Example:

• Coral reef → algal-dominated system

Driven by:

• Disturbance

• Nutrient loading

• Loss of herbivores