Ecological Succession | Varsity Tutors

SECTION 1

Historical Context & Motivation

The concept of ecological succession emerged from careful observations that biological communities are not static — they change in composition and structure over time following disturbance or the creation of new habitat. Early naturalists noticed that abandoned farmland did not remain bare indefinitely but instead progressed through recognizable stages of vegetation, eventually developing into forest. Understanding these directional changes in community composition became one of the central goals of ecology during the late nineteenth and early twentieth centuries, and the resulting theories continue to shape conservation biology, restoration ecology, and environmental management today.

1899

Henry Cowles — Dune Succession

Henry Cowles studied plant communities on Lake Michigan sand dunes, documenting spatial sequences of vegetation that represented temporal stages. His work established succession as a legitimate field of ecological inquiry.

1916

Frederic Clements — Superorganism Model

Clements proposed that communities develop toward a single, stable endpoint called the climax community, likening ecosystem development to the growth of an organism. This deterministic view dominated ecology for decades.

1926

Henry Gleason — Individualistic Hypothesis

Gleason challenged Clements by arguing that species respond independently to environmental gradients, making community assembly more stochastic and less predictable than the superorganism model implied.

1977

Connell & Slatyer — Three Models of Succession

Connell and Slatyer formalized three mechanistic models — facilitation, tolerance, and inhibition — providing a framework that moved succession theory from descriptive pattern to testable process.

1980

Mount St. Helens Eruption

The catastrophic eruption provided a natural laboratory for studying primary succession in real time. Ongoing research there has revealed that succession is more complex and variable than classical models predicted.

The central question that succession theory addresses is deceptively simple: why do communities change over time, and can we predict the trajectory of that change? This question has profound practical implications — from predicting how a clear-cut forest will recover, to designing restoration projects for degraded wetlands, to understanding how climate change may redirect successional pathways in ecosystems worldwide.

SECTION 2

Core Principles & Definitions

Ecological succession describes the sequential replacement of species and communities in an ecosystem over time. Several foundational concepts underpin this process, and the AP Environmental Science exam expects you to distinguish between types of succession, identify characteristic species at different stages, and explain the mechanisms that drive community change.

Primary Succession

Community development on newly formed or exposed substrate with no pre-existing soil — for example, bare rock after a volcanic eruption, retreating glaciers, or new volcanic islands. Pioneer species such as lichens and mosses begin soil formation.

Secondary Succession

Community re-establishment on substrate where soil and a seed bank already exist — for example, after a wildfire, hurricane, logging, or agricultural abandonment. Proceeds faster than primary succession because soil nutrients and propagules remain.

Pioneer Species

The first colonizers of a disturbed area. Typically r-selected species — fast-growing, high reproductive output, and tolerant of harsh conditions. Examples include lichens, mosses, grasses, and weedy annuals.

Climax Community

The relatively stable, self-perpetuating endpoint of succession, characterized by K-selected species with high biomass and complex trophic structure. Modern ecologists recognize that disturbance often prevents a single fixed endpoint.

Disturbance & Resilience

Any event that disrupts community structure — fire, storms, human activity — can reset or redirect succession. Ecosystem resilience measures the speed and completeness with which a community recovers from disturbance.

✦ KEY TAKEAWAY

Think of ecological succession like renovating a building. Primary succession is constructing a building on an empty lot — you need to lay the foundation (soil) before anything else. Secondary succession is remodeling after storm damage — the foundation is already there, so rebuilding is faster. In both cases, the early work (pioneer species or demolition crews) creates conditions that allow later, more specialized occupants to move in.

SECTION 3

Visualizing Succession Stages

The following diagram illustrates the progression of primary succession from bare rock to a mature forest community. Notice how species diversity, soil depth, and biomass increase over time, while the dominant life forms shift from simple autotrophs to complex woody plants.

Primary succession begins on bare rock (left) and proceeds through lichens/mosses, grasses/herbs, shrubs, and ultimately a climax forest (right). Soil depth (brown layer) and biomass increase progressively. Timescales shown are approximate for temperate biomes.

Several key trends accompany the progression through successional stages. Species diversity generally increases in early and mid-succession before stabilizing or even declining slightly in late succession as dominant species competitively exclude others. Net primary productivity (NPP) peaks during intermediate stages when fast-growing species are abundant, whereas total biomass continues to accumulate and reaches its maximum in the climax community. Soil organic matter, nutrient cycling rates, and the complexity of food webs all increase along the successional gradient.

SECTION 4

Mechanisms Driving Succession

Three mechanistic models proposed by Connell and Slatyer in 1977 explain how early colonizers influence the establishment of later species. These models are not mutually exclusive — different mechanisms may operate simultaneously or at different stages within the same ecosystem.

Facilitation Model

In the facilitation model, early-arriving species modify the environment in ways that make it more suitable for later species, while simultaneously making conditions less favorable for themselves. A classic example is nitrogen-fixing lichens and alder trees that enrich nutrient-poor substrates, enabling the establishment of species with higher nutrient demands. This is the dominant mechanism during primary succession, where soil must be created from scratch.

Tolerance Model

The tolerance model posits that later-successional species can establish independently of the pioneer community because they tolerate low resource levels. Species replacement occurs because later species are superior competitors — particularly for light — and can grow to maturity under the canopy conditions created by earlier colonizers. Succession proceeds as a competitive hierarchy rather than through active facilitation.

Inhibition Model

In the inhibition model, early colonizers resist replacement by later species through allelopathy, space occupation, or resource monopolization. Succession advances only when pioneer individuals die, are damaged by disturbance, or are consumed by herbivores, thereby creating gaps. Rocky intertidal communities and certain chaparral shrublands exhibit strong inhibition dynamics, where established species suppress recruitment of potential successors for extended periods.

The three Connell–Slatyer models. In facilitation, pioneers improve conditions for later species. In tolerance, later species simply outcompete pioneers. In inhibition, pioneers block later species until they are removed by disturbance or senescence (dashed arrows).

SECTION 5

Primary vs. Secondary Succession in Detail

The distinction between primary and secondary succession is one of the most frequently tested concepts on the AP Environmental Science exam. The table below highlights the critical differences across multiple ecological dimensions.

Comparison of primary and secondary succession

Feature	Primary Succession	Secondary Succession
Starting conditions	No soil; bare rock, lava, sand, or glacial till	Soil intact; seed bank, root systems, and nutrients present
Pioneer species	Lichens, cyanobacteria, mosses	Grasses, annual herbs, wind-dispersed seeds
Timeframe	Hundreds to thousands of years	Decades to a few centuries
Rate-limiting factor	Soil formation (weathering + organic input)	Competition, seed dispersal, and light availability
Typical triggers	Volcanic eruption, glacial retreat, new island formation	Fire, logging, hurricane, agricultural abandonment
Real-world example	Surtsey Island (Iceland, 1963), Mount St. Helens blast zone	Yellowstone post-1988 fire, abandoned Dust Bowl farms

A common source of exam errors is misclassifying disturbance types. The critical question is: was the soil destroyed or not? A severe wildfire that burns the organic horizon but leaves mineral soil behind initiates secondary succession, whereas a lava flow that buries existing soil under meters of igneous rock triggers primary succession. Similarly, a flood that deposits thick layers of sediment — burying all previous soil — may create conditions more consistent with primary succession, even though water-based disturbances are typically associated with secondary succession in most AP exam contexts.

📝 AP Exam Tip

When an FRQ asks you to identify the type of succession, always justify your answer by explicitly stating whether soil is present or absent. Simply naming the type without explaining the soil condition will cost you the justification point.

SECTION 6

Worked Example: Identifying & Analyzing Succession

The following example mirrors the style of an AP Environmental Science FRQ. Practice identifying succession type, describing community changes, and connecting mechanisms to ecological outcomes.

Scenario: Abandoned Agricultural Field in the Southeastern United States

Step 1 — Identify the Type of SuccessionThe field was previously cultivated, meaning soil, nutrients, and a seed bank are already present. Because the substrate was not destroyed, this is secondary succession. The key evidence is that soil organic matter, microbial communities, and residual root systems persist from the agricultural phase.

Secondary succession — soil is intact.

Step 2 — Describe Pioneer Stage (Years 0–5)Pioneer species colonize the bare soil rapidly. In the southeastern US, these include annual grasses (e.g., crabgrass), ragweed, and horseweed. These r-selected species produce abundant seeds, grow quickly in full sunlight, and tolerate nutrient variability. Their root systems help stabilize the soil and add organic matter upon decomposition.

Pioneer community: annual grasses and herbaceous weeds.

Step 3 — Describe Intermediate Stages (Years 5–100)Perennial grasses and shrubs replace annuals within 5–15 years. By 15–30 years, shade-intolerant trees like loblolly pine (Pinus taeda) dominate, forming a closed canopy. The pine canopy reduces light at the soil surface, inhibiting the growth of sun-loving pioneer species and creating conditions that favor shade-tolerant hardwood seedlings growing beneath the pines.

Perennial grasses → shrubs → pine forest.

Step 4 — Describe Climax Stage (100+ Years)As the pines senesce and die, shade-tolerant hardwoods such as oaks (Quercus spp.) and hickories (Carya spp.) replace them, forming a climax community of deciduous hardwood forest. This community is relatively stable and self-replacing because oak and hickory seedlings can germinate and grow in their own shade. Species diversity, total biomass, and trophic complexity reach their highest levels at this stage.

Climax: mixed oak-hickory hardwood forest.

Step 5 — Identify the Dominant MechanismThis sequence is best explained by the facilitation model in early stages (grasses add organic matter, improving soil for shrubs) and the tolerance model in later stages (shade-tolerant hardwoods grow under the pine canopy and eventually replace pines). The pine stage could also exhibit some inhibition, as the dense pine canopy suppresses other shade-intolerant trees.

Early: facilitation; Late: tolerance (with some inhibition by pines).

SECTION 7

The Role of Disturbance & the Intermediate Disturbance Hypothesis

Classical succession theory often implies a march toward a single climax state, but ecologists now recognize that disturbance is a natural and often beneficial component of ecosystem dynamics. Fire-adapted ecosystems like grasslands and boreal forests depend on periodic disturbance to maintain their characteristic species composition. Without fire, these systems would undergo succession toward forest, fundamentally altering their biodiversity.

Disturbance frequency and its impact on succession and diversity

Disturbance Regime	Effect on Succession	Example
High frequency / intensity	Resets succession repeatedly; community trapped in early seral stages; low diversity dominated by pioneer species	Overly frequent prescribed burns; chronic overgrazing
Intermediate frequency	Creates mosaic of patches at different successional stages; maximizes landscape-level species diversity	Natural wildfire regime; periodic flooding in riparian zones
Low frequency / absent	Succession proceeds to climax; competitive exclusion reduces diversity; dominance by a few K-selected species	Fire suppression in pine forests; overprotected coral reefs

⚖️ INTERMEDIATE DISTURBANCE HYPOTHESIS

The intermediate disturbance hypothesis (Connell, 1978) predicts that species diversity peaks at moderate levels of disturbance. Think of it like a university campus: if renovations happen too frequently, no one can settle in; if they never happen, a few dominant groups monopolize all the space. At intermediate renovation rates, many different groups coexist because no single group can fully exclude the others. Similarly, intermediate disturbance maintains a mosaic of successional stages across the landscape, supporting both pioneer and climax species simultaneously.

SECTION 8

Human Impacts & Applications

Human activities can accelerate, arrest, or redirect ecological succession, often with significant consequences for biodiversity and ecosystem services. Understanding successional dynamics is essential for restoration ecology, land-use planning, and conservation policy.

Human impacts on successional trajectories

Human Activity	Impact on Succession
Agriculture	Arrests succession at a pioneer-like stage through continuous disturbance (plowing, herbicides). Abandonment initiates secondary succession (old-field succession).
Urbanization	Permanently removes ecosystems from successional pathways. Impervious surfaces prevent soil development and seed germination.
Invasive species	Can redirect succession by outcompeting native pioneer or mid-seral species, sometimes creating novel, persistent communities with no historical analog.
Fire suppression	Allows fuel accumulation and shifts fire-maintained ecosystems toward late-successional states, increasing catastrophic wildfire risk.
Restoration ecology	Uses succession principles to accelerate recovery — e.g., planting nurse species that facilitate later colonizers, reintroducing fire, or removing invasive species.

Climate change adds another layer of complexity by shifting temperature and precipitation regimes, potentially pushing ecosystems toward novel successional endpoints that differ from historical climax communities. For example, increased drought frequency in the western United States may prevent post-fire forests from regenerating, converting them to shrublands or grasslands — a phenomenon known as a type conversion. This underscores the modern ecological view that succession is not rigidly deterministic but is instead sensitive to environmental context and stochastic events.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

A retreating glacier in Alaska exposes bare rock that has been buried under ice for thousands of years. Lichens and mosses are the first organisms to colonize the exposed surface. Which type of succession is occurring, and why are lichens considered effective pioneer species in this environment? (A) Secondary succession, because the glacier altered but did not destroy the underlying soil (B) Primary succession, because lichens produce allelopathic chemicals that prevent competition (C) Primary succession, because no soil exists on the newly exposed rock and lichens can weather rock surfaces and begin soil formation (D) Secondary succession, because the seed bank from before glaciation can germinate once ice melts

PROBLEM 2 — BASIC CALCULATION

A researcher monitors species richness in a forest recovering from a clearcut. At year 5, she records 12 plant species. At year 25, she records 34 species. At year 80, she records 28 species. What is the average rate of change in species richness between year 5 and year 25, and what ecological explanation accounts for the decline from year 25 to year 80? (A) 1.1 species/year; the climax community has higher disturbance that reduces diversity (B) 1.1 species/year; competitive exclusion by dominant late-successional species reduces richness (C) 0.55 species/year; invasive species always reduce diversity in late succession (D) 22 species/year; nutrient depletion causes most species to die

PROBLEM 3 — INTERMEDIATE

In the rocky intertidal zone, mussels colonize open space quickly and resist displacement by barnacles and algae for years. Succession to a more diverse community occurs only when predatory sea stars remove mussels or storms create gaps. Which of Connell and Slatyer's succession models best describes this system? (A) Facilitation, because mussels improve substrate conditions for later species (B) Tolerance, because later species outcompete mussels under low-resource conditions (C) Inhibition, because mussels suppress other species and are replaced only when removed by disturbance or predation (D) Facilitation, because sea stars facilitate mussel recruitment

PROBLEM 4 — APPLIED

A restoration ecologist is tasked with accelerating the recovery of a former strip mine site where all topsoil has been removed, leaving behind compacted mineral substrate. Describe an experimental investigation to test whether planting nitrogen-fixing species (treatment) accelerates succession compared to natural colonization (control). In your response: (a) State a testable hypothesis. (b) Identify the independent and dependent variables. (c) Describe the experimental setup, including controls and replication. (d) Explain what data you would collect and how you would determine whether your hypothesis is supported.

PROBLEM 5 — CRITICAL THINKING

A national park manager presents data showing that fire suppression over the past 80 years has allowed a ponderosa pine savanna to succeed into a dense mixed-conifer forest. The dense forest has lower species diversity, higher fuel loads, and increased vulnerability to catastrophic wildfire and bark beetle outbreaks. (a) Explain how fire suppression altered the natural successional trajectory in this ecosystem. (b) Using the intermediate disturbance hypothesis, explain why the fire-maintained savanna likely had higher biodiversity than the current dense forest. (c) Propose a management strategy based on succession principles, and predict one potential short-term risk and one long-term benefit of your strategy. (d) Explain how climate change might affect the success of your proposed management strategy.

SUMMARY

Summary: Ecological Succession

Ecological succession is the directional change in community composition over time following disturbance or the creation of new habitat. Primary succession begins on bare substrate with no soil, proceeding from pioneer species like lichens and mosses through intermediate communities to a climax community over centuries to millennia. Secondary succession occurs where soil remains intact — after fire, logging, or agricultural abandonment — and proceeds much faster because nutrients, root systems, and seed banks persist.

Three mechanistic models explain species replacement: facilitation (pioneers improve conditions for later species), tolerance (later species outcompete pioneers under low resources), and inhibition (pioneers resist replacement until removed). The intermediate disturbance hypothesis predicts maximum biodiversity at moderate disturbance levels. Human activities — agriculture, urbanization, fire suppression, invasive species introduction, and climate change — can arrest, accelerate, or redirect successional pathways, making succession theory essential for restoration ecology and environmental management.