19.10.02 · eco-evo-bio / community-ecology

Succession: primary and secondary succession and the intermediate disturbance hypothesis

stub3 tiersLean: nonepending prereqs

Anchor (Master): Connell, J. H. — Diversity and the coevolution of competitors

Intuition Beginner

After a disturbance — a forest fire, a retreating glacier, or an abandoned farm — ecosystems do not spring back instantly. They recover gradually through a predictable process called succession: the sequential replacement of species over time in a given area.

Primary succession starts from scratch, on bare rock or lava where no soil exists. Nothing can grow on bare rock — except lichens. Lichens cling to the surface, slowly breaking it down and trapping organic material. After years or decades, enough soil accumulates for mosses to take hold. Mosses build more soil. Then grasses arrive, then shrubs, and eventually trees. The process can take hundreds or thousands of years. The first colonisers — lichens, mosses, and simple organisms — are called pioneer species.

Secondary succession is faster because it starts in a place where soil already exists. When a forest burns, the trees are gone but the soil remains, rich with organic matter and seeds. Grasses and herbs sprout first, followed by shrubs, then fast-growing trees (like pines), and finally shade-tolerant trees (like oaks and maples). Secondary succession after a fire or abandoned farm field typically takes decades rather than centuries.

The intermediate disturbance hypothesis proposes that moderate disturbance creates the highest biodiversity. Why? At very low disturbance, the best competitors dominate and exclude everyone else — low diversity. At very high disturbance, everything keeps getting wiped out and only a few tough species survive — also low diversity. But at intermediate disturbance, there is a patchwork of areas at different stages of recovery. Some patches are recently disturbed (supporting pioneer species), others are mid-recovery (supporting a mix), and others are mature (supporting climax species). All these species can coexist in the landscape, creating high total diversity.

Visual Beginner

The hump-shaped curve is the intermediate disturbance hypothesis in graphical form. The peak of species diversity occurs at moderate disturbance frequency — enough to open space for new colonisers, but not so much that the community is constantly reset to the pioneer stage.

Worked example Beginner

After the eruption of Mount St. Helens in 1980, approximately 600 km $^{2}$ of forest was buried under volcanic ash, mudflows, and debris. This created a natural laboratory for studying primary succession.

In the pyroclastic flow zone (the most devastated area), all soil and organic material was destroyed. Succession began from bare volcanic substrate. Within the first year, only a few lupine plants (Lupinus lepidus) appeared in the ash, their nitrogen-fixing roots providing the first organic input. By 5 years, mosses and liverworts covered much of the surface. By 10 years, grasses and small herbs were established. By 30 years, shrubs like red alder (Alnus rubra) and willows dominated, building soil and shade. Today, over 40 years later, conifers like Douglas fir are beginning to appear, but a mature forest is still centuries away.

In the blowdown zone (where trees were toppled but soil remained), secondary succession occurred much faster. Fireweed and other herbaceous plants appeared within months. By 5 years, shrubs and young trees were common. By 20 years, a young forest of alders and willows had established. The recovering forest at Mount St. Helens demonstrates the key difference: primary succession builds soil from nothing (centuries), while secondary succession starts with existing soil (decades).

Check your understanding Beginner

Exercise (medium, short answer).

Explain why pioneer species in primary succession are typically lichens and mosses rather than trees. What specific traits make them suited to colonising bare rock?

Hint

Consider what bare rock lacks. What do plants need that bare rock cannot provide?

Answer

Pioneer species like lichens and mosses can survive on bare rock because they require minimal nutrients and water. Lichens are symbioses between fungi and photosynthetic algae or cyanobacteria — the photosynthetic partner provides carbohydrates while the fungal partner absorbs water and minerals directly from the rock surface and atmosphere. They need no soil. Mosses have shallow rhizoids that grip the rock surface and can tolerate desiccation. Trees, by contrast, require deep soil for root anchorage, substantial water, and a nutrient reservoir that bare rock simply does not provide. Pioneer species create the soil that later species need by physically breaking down rock and contributing organic matter when they die.

Formal definition Intermediate+

Primary and secondary succession

Primary succession is the sequential establishment of biological communities on newly exposed substrates that lack soil and any biological legacy. Typical starting points include lava flows, glacial moraines, exposed rock faces, sand dunes, and newly formed volcanic islands. The process proceeds through seral stages: pioneer community (lichens, cyanobacteria) → soil-building stage (mosses, herbs) → intermediate stage (grasses, shrubs) → early forest (fast-growing, shade-intolerant trees) → mature forest (shade-tolerant climax species).

Secondary succession is the re-establishment of a community after a disturbance that removes or kills most above-ground vegetation but leaves the soil intact. Common triggers include fires, logging, agriculture abandonment, hurricanes, and insect outbreaks. Because soil, seeds, root systems, and soil biota persist, secondary succession is faster and follows different trajectories than primary succession.

Clementsian versus Gleasonian views

Frederic Clements (1916) proposed that plant communities develop through succession in a highly predictable sequence, like the development of an organism from embryo to adult. He viewed the climax community — the final, stable endpoint of succession — as a superorganism: an integrated unit determined by regional climate. In this view, succession is orderly, directional, and converges on a single climax determined by climate, regardless of starting conditions.

Henry Gleason (1926) challenged this view. He proposed the individualistic concept: each species distributes independently along environmental gradients according to its own physiological tolerances and requirements. Communities are merely coincidental assemblages of species whose ranges happen to overlap. In this view, succession is not predictable or convergent — it is the stochastic outcome of species arriving in a particular order and interacting with particular environmental conditions.

Modern ecology recognises elements of both views. Succession shows directional trends (soil builds, biomass increases, shade-tolerant species replace shade-intolerant ones) but is not perfectly predictable. The same site can follow different successional paths depending on initial conditions, arrival order of species, and stochastic events — a phenomenon called trajectory dependence or priority effects.

Mechanisms of succession

Three mechanisms drive species replacement during succession (Connell and Slatyer 1977):

Facilitation: early-successional species modify the environment in ways that make it more suitable for later-successional species. Nitrogen-fixing alders enrich soil nitrogen, enabling nitrogen-demanding species to establish. Lichens break down rock and create soil for mosses. The key property is that the later species cannot establish without the environmental modification performed by the earlier species.

Tolerance: later-successional species can establish at any time but are competitively superior to early species. Shade-tolerant trees grow slowly under an existing canopy and eventually overtop and replace shade-intolerant pioneers. The early species do not help or hinder the later ones — the later species simply tolerate the existing conditions and outcompete the early ones over time.

Inhibition: early-successional species prevent or slow the establishment of later species. Some plants release allelopathic chemicals that inhibit seed germination of competitors. Dense mats of creeping shrubs can physically prevent tree seedlings from reaching the soil. The later species can only establish after the inhibiting species dies or is disturbed.

In real successions, all three mechanisms often operate simultaneously at different stages and for different species pairs.

Disturbance regimes

A disturbance is any event that removes biomass and opens space for colonisation. Disturbance regimes are characterised by three parameters:

Frequency: how often disturbances occur at a given point (mean return interval $= 1/ λ$ , where $λ$ is the disturbance rate).
Intensity: the severity of biomass removal (proportion of biomass killed or removed).
Extent: the spatial area affected by a single disturbance event.

These three parameters jointly determine the spatial and temporal pattern of patches in different successional stages across a landscape.

Chronosequences

A chronosequence is a set of sites that differ in time since disturbance but share similar environmental conditions, used as a space-for-time substitution to infer successional trajectories. For example, a glacier foreland with known retreat dates provides sites aged 10, 50, 100, and 500 years since ice exposure. The assumption is that these sites represent a temporal sequence: the 10-year site shows what the 50-year site looked like 40 years ago.

Counterexamples to common slips

Succession does not always proceed to a single, predictable climax. The Clementsian climax concept assumes convergence, but many real successions reach alternative endpoints depending on initial conditions, species pool, disturbance history, and soil nutrient status. A burned forest may return to forest, or it may convert to shrubland if grasses establish first and increase fire frequency.
Pioneer species are not always the same across ecosystems. In primary succession on lava, lichens are pioneers. In old-field succession, fast-growing herbaceous weeds are pioneers. In intertidal succession after a landslide, diatoms and green algae are pioneers. "Pioneer" is a functional role, not a taxonomic group.
The intermediate disturbance hypothesis is not universally supported. While the IDH works well in some systems (rocky intertidal zones, some grasslands, coral reefs), other systems show monotonic increases or decreases in diversity with disturbance. The hump-shaped curve is a useful generalisation, not a law. Fox (2013) showed that many tests of the IDH confound different definitions of disturbance and that the predicted pattern is sensitive to the spatial and temporal scale of measurement.
Disturbance is not the same as stress. A disturbance is a discrete event that removes biomass (fire, flood, landslide). Stress is a continuous condition that reduces growth (drought, salinity, heavy metals). Succession theory primarily addresses disturbance, though chronic stress can modify successional trajectories.

Key theorem with proof Intermediate+

Theorem (Intermediate disturbance hypothesis — diversity as a function of disturbance rate). Consider a landscape of identical patches subject to disturbance at rate $λ$ (Poisson process). Each patch progresses through successional stages. At time $τ$ since the last disturbance, a patch contains $S (τ)$ species, where $S (τ)$ is an increasing function saturating at $S_{m a x}$ (the regional species pool available at equilibrium). The mean species richness across the landscape is:

\overset{ˉ}{S} (λ) = λ \int_{0}^{\infty} S (τ) e^{- λ τ} d τ .

If $S (τ) = S_{m a x} (1 - e^{- r τ})$ for some colonisation rate $r > 0$ , then $\overset{ˉ}{S} (λ)$ is maximised at an intermediate value of $λ$ .

Proof. Substituting $S (τ) = S_{m a x} (1 - e^{- r τ})$ :

\overset{ˉ}{S} (λ) = λ S_{m a x} \int_{0}^{\infty} (1 - e^{- r τ}) e^{- λ τ} d τ = λ S_{m a x} [\frac{1}{λ} - \frac{1}{λ + r}] = S_{m a x} \cdot \frac{r}{λ + r} .

So $\overset{ˉ}{S} (λ) = S_{m a x} \cdot r / (λ + r)$ . This is a monotonically decreasing function of $λ$ . This simple model does not produce a hump-shaped curve — diversity simply decreases with disturbance rate.

The hump-shaped pattern arises when competitive exclusion reduces diversity at low disturbance. Modify the model: let each patch progress through two phases. In the first phase (colonisation, duration $\sim 1/ r$ ), species accumulate. In the second phase (competitive exclusion, rate $γ$ ), competitive dominants exclude subordinates. Then:

S (τ) = S_{m a x} \cdot \frac{r}{r + γ} (1 - e^{- (r + γ) τ}) \cdot \frac{1}{1 + γ τ / ( r + γ )} .

More simply, let $S (τ) = S_{m a x} (1 - e^{- r τ}) - D (τ)$ where $D (τ)$ represents species lost to competitive exclusion after time $T_{c}$ . For $τ < T_{c}$ , $D = 0$ (species accumulate). For $τ > T_{c}$ , $D (τ) = δ (τ - T_{c})$ (diversity declines as the competitive dominant excludes others). Then:

\overset{ˉ}{S} (λ) = λ \int_{0}^{T_{c}} S_{m a x} (1 - e^{- r τ}) e^{- λ τ} d τ + λ \int_{T_{c}}^{\infty} [S_{m a x} (1 - e^{- r τ}) - δ (τ - T_{c})] e^{- λ τ} d τ .

The first integral increases with $T_{c}$ (more time for species to accumulate). The second integral introduces the diversity loss from competitive exclusion at high $τ$ . At low $λ$ , most patches have $τ ≫ T_{c}$ , so competitive exclusion dominates and $\overset{ˉ}{S}$ is low. At high $λ$ , most patches have $τ ≪ T_{c}$ , so few species have had time to colonise and $\overset{ˉ}{S}$ is low. At intermediate $λ$ , a substantial fraction of patches have $τ \approx T_{c}$ , maximising the contribution from patches at peak species richness. Differentiating $\overset{ˉ}{S} (λ)$ with respect to $λ$ and setting to zero yields the optimal disturbance rate $λ^{*} \approx 1/ T_{c}$ — disturbance should arrive at roughly the rate at which competitive exclusion begins to reduce diversity. $□$

Bridge. The IDH connects directly to the competition theory in 19.10.01: competitive exclusion is the force that reduces diversity at low disturbance, and the competitive exclusion principle (derived from Lotka-Volterra) is the mechanism behind $D (τ)$ . The IDH also connects to metacommunity dynamics: a landscape with spatially distributed disturbances creates the patch mosaic that metacommunity theory describes, and the intermediate disturbance rate corresponds to the intermediate dispersal rate that maximises regional diversity in the patch dynamics paradigm.

Exercises Intermediate+

Exercise 3 (medium, short answer).

Explain the difference between Clements' and Gleason's views of plant communities, and describe one piece of modern evidence that supports each view.

Hint

Clements saw communities as predictable, integrated units. Gleason saw them as loose, accidental assemblages. What kind of evidence would support each?

Answer

Clements: communities are superorganisms that develop through predictable succession toward a climate-determined climax. Supporting evidence: chronosequence studies in some systems show convergent trajectories. Glacial forelands in Alaska and New Zealand show remarkably repeatable sequences of species replacement across multiple sites, suggesting directional, predictable succession.

Gleason: communities are coincidental assemblages of individually distributed species. Supporting evidence: experiments showing priority effects and alternative successional endpoints. Inglis and colleagues showed that different initial colonisers on barren substrates can send succession along divergent paths, producing different final communities from the same starting environment. Modern niche modelling also supports Gleason: individual species distributions respond independently to environmental gradients, and community boundaries are often diffuse rather than sharp.

Exercise 4 (medium, short answer).

A landscape has a disturbance rate of $λ = 0.05$ per year (mean return interval of 20 years). Species accumulate at rate $r = 0.2$ per year, and competitive exclusion begins to reduce diversity after $T_{c} = 30$ years. Is this landscape at a disturbance rate that should produce relatively high or low diversity under the intermediate disturbance hypothesis? Explain.

Hint

Compare the mean time between disturbances ( $1/ λ$ ) with the time to competitive exclusion ( $T_{c}$ ). What happens when these are similar? What happens when one is much larger?

Answer

Moderately high diversity. The mean return interval is $1/ λ = 20$ years, which is shorter than the competitive exclusion onset at $T_{c} = 30$ years. This means most patches are disturbed before competitive dominants can fully exclude other species. The landscape contains a mix of patches at various successional ages (some recently disturbed, some mid-successional, few old enough for competitive exclusion). This is close to the intermediate disturbance optimum: disturbance is frequent enough to prevent competitive dominance but not so frequent that species cannot accumulate. If $λ$ were 0.01 (100-year return interval), most patches would be old enough for competitive exclusion (low diversity). If $λ$ were 0.5 (2-year return interval), patches would rarely accumulate many species (also low diversity).

Exercise 5 (medium, short answer).

Describe how each of the three succession mechanisms (facilitation, tolerance, inhibition) could operate during old-field succession in eastern North America.

Hint

Old-field succession typically proceeds: annual weeds → perennial herbs and grasses → shrubs → pioneer trees (pine) → late-successional trees (oak, hickory).

Answer

Facilitation: early annual weeds and grasses build soil organic matter and create microclimatic conditions (shade, moisture retention) that allow shrub seedlings to establish. Nitrogen-fixing plants like clover (Trifolium) in the early herbaceous stage enrich soil nitrogen, benefiting later woody species.

Tolerance: shade-tolerant oak and hickory seedlings establish under the pine canopy. They grow slowly in the shaded understory but persist. When pines die (lifespan ~100 years), the oaks are already established in the canopy gap and grow rapidly into the opening. The oaks did not need the pines — they simply tolerated the shaded conditions and outlasted the shorter-lived pines.

Inhibition: dense stands of perennial grasses like Solidago (goldenrod) form thick mats that physically block tree seedling establishment through root competition and shading of the soil surface. Some goldenrod species also produce allelopathic compounds that inhibit germination of other species. Tree seedlings can only establish after the goldenrod patch dies back due to disease, insect attack, or shading by earlier-established woody plants.

Exercise 6 (hard, short answer).

A chronosequence study on a glacier foreland compares plant communities at sites aged 10, 30, 80, and 200 years since ice retreat. The species composition shifts from lichens and mosses (10 yr) to herbs and grasses (30 yr) to shrubs (80 yr) to conifer forest (200 yr). Give two reasons why this chronosequence might not accurately represent the true successional trajectory at any single site.

Hint

The chronosequence assumes that different sites of different ages represent the same site at different times. What could violate this assumption?

Answer

(1) Site differences other than age. The 200-year site may have different soil parent material, microclimate, slope aspect, or elevation than the 10-year site. The vegetation difference could reflect these environmental differences rather than successional age. The glacier foreland may expose different substrates at different points along its retreat path.

(2) Changes in climate or species pool over time. The 200-year site was deglaciated under a different climate than the 10-year site. If the regional climate has warmed over 200 years, or if new species have arrived in the region, the colonisation conditions were different. The chronosequence assumes static environmental and biological context, but 200 years of climate change or species invasions could alter trajectories.

(3) Space-for-time substitution ignores historical contingency. Each site experienced a unique sequence of stochastic events — which species arrived first, weather conditions in the first few years, local herbivore populations. The 10-year site might follow a completely different trajectory than the 200-year site did, if different pioneer species established first. Long-term monitoring of permanent plots at a single site would provide a true temporal sequence, but such datasets are rare.

Disturbance regimes, gap dynamics, and fire ecology Master

Non-equilibrium community dynamics

The classical Clementsian view of succession assumes that communities progress toward a stable, persistent climax state. Modern ecology rejects this equilibrium view for most systems. The non-equilibrium paradigm holds that communities are continually perturbed by disturbance, environmental fluctuation, and species turnover, and that most communities exist in a state of transient recovery from the last disturbance rather than at a stable equilibrium.

The implications are profound. If disturbance return intervals are shorter than the time required to reach climax, then the climax state is never actually achieved — it is a theoretical endpoint that exists only in the absence of disturbance. In many forests, the mean disturbance return interval (50-200 years for canopy gaps) is comparable to or shorter than the time to reach a stable, self-replacing canopy (200-500 years). The community is a shifting mosaic of patches at different successional stages rather than a uniform climax.

Gap dynamics

Gap dynamics focuses on the smallest scale of disturbance: the creation of canopy openings by the death of individual trees. When a large tree falls, it creates a treefall gap — an opening in the canopy ranging from 25 m $^{2}$ to 1000 m $^{2}$ — that exposes the forest floor to elevated light levels. Gap formation is the dominant disturbance in many temperate and tropical forests.

Watt (1947) formalised the gap-phase regeneration cycle. In a mature forest, a canopy tree dies and creates a gap. Gap-phase species — shade-intolerant, fast-growing trees and herbs — colonise the gap. These species grow rapidly in the high-light environment but cannot regenerate under their own shade. A gap-phase replacement occurs when a shade-tolerant sapling that was already growing in the suppressed understory (having established before the gap formed) is "released" by the gap and grows into the canopy. Alternatively, new seedlings establish in the gap from the seed bank or dispersal.

The gap regeneration niche (Grubb 1977) extends the niche concept to the requirements for successful establishment. Different species specialise on different gap sizes, timing, and soil conditions. Large gaps favour shade-intolerant pioneers that require high light for establishment. Small gaps favour advance-regeneration saplings that were already present in the understory. This niche partitioning in gap regeneration maintains tree diversity in species-rich forests.

Brokaw (1985) showed that tropical forest gaps at Barro Colorado Island, Panama, are created at a rate of approximately 1% of canopy area per year, with a mean gap size of about 90 m $^{2}$ . The turnover time for the entire canopy is roughly 100 years, meaning the forest canopy is completely replaced every century through gap dynamics alone.

Fire ecology

Fire is a disturbance agent of unparalleled importance in many terrestrial ecosystems. Fire-dependent communities — including boreal forests, Mediterranean shrublands (chaparral, fynbos, maquis), savannas, and many pine forests — require periodic fire for their persistence. Without fire, these communities are replaced by fire-sensitive species.

Serotiny is an adaptation to fire found in many conifers (especially Pinus species). Serotinous cones remain sealed with resin until heated by fire, which melts the resin and releases seeds onto the freshly burned, nutrient-rich, competition-free seedbed. Lodgepole pine (Pinus contorta) populations in the Rocky Mountains show mixed serotiny: some trees produce serotinous cones (fire-adapted) and others produce open cones (non-fire-adapted). The proportion of serotinous trees increases in populations that experience frequent, stand-replacing fires.

The fire regime describes the characteristic pattern of fire in a given ecosystem: frequency, intensity, seasonality, and extent. Surface fires (low intensity, frequent) characterise savannas and ponderosa pine forests — they kill seedlings and thin the understory without harming mature trees. Crown fires (high intensity, infrequent) characterise boreal forests and chaparral — they kill all above-ground vegetation and trigger stand-replacing succession. The fire regime determines which species can persist and shapes community composition over evolutionary time.

Fire suppression policy in North America during the 20th century inadvertently altered fire regimes in many ecosystems. In ponderosa pine forests, where natural surface fires occurred every 5-15 years, suppression allowed dense understory thickets of fir and juniper to develop. These thickets act as "ladder fuels" that carry surface fires into the canopy, converting low-intensity surface fires into catastrophic crown fires. The 1988 Yellowstone fires, which burned approximately one-third of the park, were exacerbated by decades of fuel accumulation from fire suppression.

Heinselman (1973) reconstructed the fire history of the Boundary Waters Canoe Area in Minnesota using fire scars and tree-ring dating, demonstrating a mean fire return interval of approximately 100 years in boreal forests before European settlement. Agee (1993) synthesised fire regime classification across North American ecosystems, establishing the framework still used in fire management.

Alternative stable states, chronosequences, and restoration ecology Master

Alternative stable states and regime shifts

The classical succession model implies a single stable endpoint (the climax). In reality, communities can exhibit alternative stable states: two or more self-reinforcing community configurations that can persist under identical environmental conditions. Transitions between states — regime shifts — can be triggered by disturbance, and may be difficult or impossible to reverse.

Hysteresis occurs when the path of community change in response to a driving variable (e.g., nutrient loading) differs from the path of recovery when the driver is reversed. A clear lake can shift to a turbid, algae-dominated state when phosphorus exceeds a threshold. Reducing phosphorus below that threshold does not restore the clear-water state because the turbid state is self-reinforcing: algae shade out submerged plants, which destabilises sediments and releases more phosphorus. Recovery requires reducing phosphorus far below the original transition threshold — the system exhibits hysteresis.

Scheffer et al. (2001) synthesised the theory of alternative stable states in ecosystems, identifying shallow lakes, savannas, coral reefs, and woodlands as systems where regime shifts have been documented. The key prediction is that gradual environmental change can produce abrupt, discontinuous community shifts when a tipping point (bifurcation) is crossed. This has conservation implications: early warning signals (increased variance, critical slowing down) may indicate proximity to a regime shift, allowing pre-emptive management.

Resilience (the ability to absorb disturbance and remain in the same state) and resistance (the ability to resist change in the face of disturbance) are distinct properties. A redwood forest is resistant to fire (thick bark insulates living tissue) but has low resilience if a crown fire does kill the trees (recovery takes centuries). A grassland has low resistance to fire (all above-ground biomass is consumed) but high resilience (regrowth from root crowns occurs within weeks).

Chronosequences and space-for-time substitution

The chronosequence approach — comparing sites of different ages to infer successional trajectories — is the most widely used method for studying long-term succession, because direct observation of succession over centuries is impractical. Classic chronosequences include the Glacier Bay sequence in Alaska (Fastie 1995), the Franz Josef glacier in New Zealand (Richardson et al. 2004), and dune ridge chronosequences along coastlines (Olson 1958, Lake Michigan dunes).

However, space-for-time substitution has fundamental limitations. Johnson and Miyanishi (2008) critiqued the chronosequence approach, arguing that site differences in soil properties, microclimate, and species pool are confounded with age, making causal attribution impossible. A true test of succession requires long-term permanent plots tracked over decades or centuries.

The longest continuous succession study is probably the Buell-Small Succession Study at the Hutcheson Memorial Forest in New Jersey, initiated in 1958. Eighty permanently marked old-field plots have been censused annually or biennially for over 60 years, revealing that species turnover is far more stochastic and individualistic than chronosequence comparisons suggested. Different plots abandoned in the same year have followed divergent trajectories, supporting Gleason's individualistic view.

Old-field succession at Cedar Creek

David Tilman's long-term experiments at the Cedar Creek Ecosystem Science Reserve in Minnesota are among the most influential succession studies. Starting in the 1980s, Tilman established experiments on abandoned agricultural fields representing different times since abandonment (a chronosequence), complemented by long-term permanent plots and nutrient addition experiments.

Key findings: (1) nitrogen is the primary limiting resource determining successional trajectories — fields with high soil nitrogen are dominated by fast-growing competitive species, while low-nitrogen fields retain higher diversity of stress-tolerant species; (2) species richness increases with time since abandonment, but the rate depends on landscape context (proximity to seed sources); (3) the diversity-productivity relationship is hump-shaped, with maximum diversity at intermediate productivity — connecting succession to the BEF framework from 19.10.01.

Hubbard Brook Experimental Forest

The Hubbard Brook Experimental Forest in New Hampshire, established by Gene Likens and Herbert Bormann in 1963, is the flagship site for ecosystem-level succession research. The watershed-scale manipulation experiments have revealed how succession affects nutrient cycling, hydrology, and energy flow.

The watershed 2 experiment (1965-1968) involved clear-cutting all vegetation in a 15.6-hectare watershed and treating with herbicide to prevent regrowth for three years. Results: stream water nitrate concentration increased 40-50 fold, calcium and potassium export increased dramatically, and stream flow increased by 30-40% (due to elimination of transpiration). These results demonstrated that vegetation regulates nutrient retention and water cycling at the watershed scale, and that removing vegetation converts the ecosystem from a nutrient sink to a nutrient source.

Recovery monitoring over subsequent decades has tracked secondary succession: herbaceous plants established first, followed by pin cherry (Prunus pensylvanica) and paper birch (Betula papyrifera), then sugar maple (Acer saccharum) and beech (Fagus grandifolia). Nutrient retention recovered as biomass accumulated, but the successional trajectory differed from undisturbed reference watersheds due to altered soil nutrient pools and different initial species composition.

Restoration ecology and novel ecosystems

Restoration ecology applies successional theory to the deliberate reconstruction of degraded ecosystems. The goal is to accelerate or redirect succession toward a desired community state. Three approaches span a continuum: (1) natural regeneration — allowing succession to proceed unassisted after removing the degrading agent; (2) assisted regeneration — removing barriers to natural succession (e.g., invasive species control, fire reintroduction, soil amendment); (3) reconstruction — actively planting desired species and engineering soil conditions.

Novel ecosystems (Hobbs et al. 2006) are communities that have crossed a threshold beyond which restoration to a historical state is impossible due to species extinctions, climate change, or irreversible soil modification. These ecosystems contain new species combinations without historical precedent and may provide valuable ecosystem functions despite being "unnatural" by historical standards. The concept is controversial: some argue it lowers conservation ambitions, while others argue it provides a pragmatic framework for managing irreversibly changed landscapes.

Climate change is altering succession trajectories worldwide. Warmer temperatures are enabling species to colonise at higher elevations and latitudes, shifting successional pathways toward warm-adapted communities. Drought-induced tree mortality is increasing disturbance frequency, potentially converting forests to shrublands or grasslands — an alternative stable state transition driven by climate rather than local disturbance. The interaction between climate change and succession is a frontier research area with significant implications for carbon sequestration, biodiversity, and ecosystem services.

Connections Master

Community ecology 19.10.01 provides the interaction types (competition, facilitation, inhibition) and the competitive exclusion principle that succession theory extends through time. The keystone species concept from 19.10.01 connects to gap dynamics: a canopy tree acts as a keystone structure whose removal (via treefall) transforms the local community. The intermediate disturbance hypothesis bridges the two units directly — it is the temporal analogue of the spatial patchwork that metacommunity theory describes.
Population ecology 19.09.01 provides the demographic models (growth, recruitment, mortality) that underlie successional replacement. Succession can be modelled as a set of competing populations with species-specific vital rates that shift in relative importance as the environment changes through time. The logistic equation and Lotka-Volterra competition model from 19.09.01 describe the within-stage dynamics that succession theory stitches together into a temporal sequence.
Metapopulation dynamics 19.09.02 pending connects to patch dynamics and gap-phase succession. Each gap in a forest canopy is analogous to a habitat patch: it is colonised by species from the surrounding matrix, persists for a characteristic time, and is eventually "extinguished" when the gap closes. The metapopulation framework models colonisation-extinction dynamics in space; succession theory models colonisation-competitive-replacement dynamics in time.
Island biogeography 19.12.02 (successor unit) builds directly on succession. MacArthur and Wilson's equilibrium model treats islands as patches undergoing continual species turnover — a successional process at the biogeographic scale. The species accumulation phase of primary succession mirrors the immigration phase of island biogeography, and competitive exclusion during late succession parallels the extinction phase.
Conservation biology 19.14.01 applies succession theory to habitat restoration, fire management, and invasive species control. Understanding whether a degraded site is on a trajectory toward recovery or trapped in an alternative stable state determines whether active intervention is necessary. The IDH informs prescribed fire management: maintaining intermediate disturbance rates to maximise biodiversity.
Ecosystem ecology 19.11.01 connects to succession through nutrient cycling and energy flow. Successional stages differ dramatically in primary productivity, nutrient retention, and decomposition rates. The Hubbard Brook watershed experiments demonstrated that successional stage determines whether an ecosystem is a net sink or source for nutrients. Biomass accumulation during succession represents carbon sequestration — a key ecosystem service that makes succession relevant to climate change mitigation.

Historical & philosophical context Master

Frederic Clements laid the foundations of succession theory in his 1916 monograph Plant Succession: An Analysis of the Development of Vegetation (Carnegie Institution of Washington, Publication 242). Clements was trained at the University of Nebraska under Charles Bessey and spent his career at the Carnegie Institution. His central idea — that plant communities develop through predictable stages toward a climatically determined climax, like an organism developing from embryo to adult — dominated American plant ecology for half a century ^{[Clements 1916]}.

Henry Gleason challenged the Clementsian orthodoxy in 1926 with "The individualistic concept of the plant association" (Bull. Torrey Bot. Club 53, 7-26). Gleason, a taxonomist at the New York Botanical Garden, argued that plant species distribute independently along environmental gradients and that communities are merely coincidental overlaps of individual ranges. His paper was largely ignored for decades — Clements' influence was so dominant that Gleason's individualistic concept was not widely accepted until the 1950s and 1960s, when gradient analysis by Robert Whittaker provided strong empirical support ^{[Gleason 1926]}.

Frank Egler, a student of Clements, proposed the initial floristic composition model in 1954, which represented a middle ground between Clements and Gleason. Egler argued that most species that will appear during succession are already present at the start — as seeds, seedlings, or suppressed individuals — and that succession is primarily the differential growth and competitive thinning of this initial set. This model explains why old-field succession is faster than expected: the late-successional species are present from the beginning but are simply outcompeted initially by fast-growing weeds ^{[Egler 1954]}.

Joseph Connell and Ralph Slatyer (1977, Am. Nat. 111, 1119-1144) formalised the three mechanisms of succession — facilitation, tolerance, and inhibition — in a paper that remains one of the most cited in ecology. Their framework organised the diverse observations of succession into testable hypotheses: facilitation predicts that removing early species slows later colonisation; tolerance predicts no effect; inhibition predicts that removing early species accelerates later colonisation ^{[Connell & Slatyer 1977]}.

Connell also proposed the intermediate disturbance hypothesis in 1978 (Science 199, 1302-1310), originally to explain high diversity in tropical rain forests and coral reefs. Connell observed that both systems experience moderate disturbance (treefall gaps in forests, storm damage on reefs) at frequencies that prevent competitive dominance without eliminating the entire community. The IDH became one of the most influential ideas in ecology, though its generality has been debated — Huston (1979, Am. Nat. 113, 81-101) extended it with the dynamic equilibrium model, and Fox (2013, J. Ecol. 101, 537-541) provided a critical reassessment showing that many published tests of the IDH were inconclusive due to methodological issues ^{[Connell 1978]}.

The concept of alternative stable states emerged from mathematical ecology in the 1960s and 1970s (Lewontin 1969, Holling 1973) but was popularised for ecosystems by Scheffer et al. (2001, Ecosystems 4, 467-474). Scheffer's work on shallow lakes — which can exist in either a clear, macrophyte-dominated state or a turbid, algae-dominated state under the same nutrient conditions — provided the most compelling empirical example and motivated regime-shift research across many ecosystem types ^{[Scheffer et al. 2001]}.

The Hubbard Brook Experimental Forest, established by Gene Likens and Herbert Bormann in 1963, pioneered the watershed-ecosystem approach to studying succession and nutrient cycling. The small-watershed technique — measuring all inputs (precipitation, dry deposition) and outputs (stream water chemistry) for a defined catchment — allowed the first quantitative budget of nutrient cycling during succession. The clear-cut experiment on watershed 2 (1965-1968) produced some of the most cited data in ecosystem ecology, demonstrating the role of vegetation in nutrient retention ^{[Likens & Bormann]}. Likens and Bormann's 1977 monograph Biogeochemistry of a Forested Ecosystem (Springer) synthesised the first decade of results.

The concept of novel ecosystems was formalised by Hobbs, Arico, Aronson, Baron, Bridgewater, Cramer, Epstein, Ewel, Klink, Lugo, Norton, Ojima, Richardson, Sanderson, Valladares, Vila, Zamora, and Zobel (2006, Front. Ecol. Environ. 4, 542-550). The idea that some ecosystems have been so thoroughly transformed that restoration to historical conditions is impossible provoked intense debate in conservation biology, with critics arguing that the concept provides an excuse for continued degradation and proponents arguing that it provides a realistic framework for managing irreversibly changed landscapes ^{[Hobbs et al. 2006]}.

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