Getting Started
Ecological succession is the process that describes the gradual, predictable change in the species composition of a given area over time. This process occurs in all ecosystems, from a tiny pond to a vast forest, typically following a disturbance that clears away existing vegetation. Understanding succession is fundamental to seeing ecosystems not as static snapshots, but as dynamic, resilient systems constantly in a state of recovery and maturation.
What You Should Be Able to Do
After completing this section, you should be able to:
Explain the sequential process of ecological succession.
Compare and contrast the mechanisms and timelines of primary and secondary succession.
Describe how species richness, biomass, and net productivity change as an ecosystem matures.
Explain the distinct roles of pioneer, keystone, and indicator species within a community.
Key Concepts & Mechanisms
Ecological succession is best understood as a process of change and continuity over time, where an ecosystem develops from a simple, unstable state to a complex, more stable one.
Baseline Condition: Disturbance
Succession begins after a disturbance creates an opportunity for new colonization. The nature of this starting point determines the type of succession. In primary succession, the process begins in an environment devoid of vegetation and usually lacking topsoil, such as on a bare rock surface exposed by a retreating glacier or a new volcanic island. In secondary succession, the process starts in an area that previously supported life but has undergone a disturbance that reduced the existing population of organisms, such as a forest fire, a hurricane, or an abandoned agricultural field. The key difference is the presence of soil at the outset of secondary succession, which dramatically speeds up the process.
Key Changes: Stages of Development
As succession proceeds, the community undergoes a series of predictable changes.
Pioneer Stage: The first species to colonize the disturbed area are called pioneer species. In primary succession, these are typically organisms like lichens and mosses that can survive on bare rock. They chemically weather the rock, and as they die and decompose, they begin the slow process of soil formation. In secondary succession, pioneers are usually fast-growing herbaceous plants, like grasses and weeds, whose seeds were already in the soil or are easily dispersed by wind.
Intermediate Stage: As pioneer species alter the environment (e.g., by creating soil and adding nutrients), they make it more suitable for other species and less suitable for themselves. Grasses and wildflowers may be replaced by larger, more competitive plants like shrubs and small trees. This increase in plant diversity creates more habitats and food sources, leading to a rapid increase in species richness (the number of different species). During this stage, both total biomass (the total mass of all living organisms) and net primary productivity (the rate at which new biomass is produced) increase significantly as larger plants become established.
Climax Community: Eventually, the ecosystem reaches a relatively stable state known as the climax community. This community is characterized by large, slow-growing, and long-lived species, such as hardwood trees in a forest. While it is the most stable stage, it is not static; it exists in a state of dynamic equilibrium. In this mature stage, total biomass is at its peak, but the rate of net primary productivity may level off or even slightly decline as the energy captured by producers is balanced by the energy used for respiration by a large, dense community.
Key Continuities: The Driving Forces
Throughout all stages of succession, the fundamental driver is the modification of the environment by the organisms living within it. Each successive community paves the way for the next. The gradual accumulation of soil, nutrients, and biomass is a continuous trend that underpins the entire process, allowing for the transition from simple pioneer communities to complex climax ecosystems.
Key Models & Diagrams
The progression of primary and secondary succession can be visualized as a sequence of stages, highlighting their different starting points and timelines.
| Stage | Primary Succession (Starts with Bare Rock) | Secondary Succession (Starts with Soil) |
|---|---|---|
| Starting Point | Exposed rock, no soil (e.g., after volcanic eruption, glacial retreat). | Soil is present (e.g., after forest fire, abandoned farm). |
| Pioneer Species | Lichens and mosses. | Annual plants, grasses, and weeds. |
| Intermediate Species | Grasses, shrubs, and shade-intolerant trees like pine. | Larger grasses, shrubs, and fast-growing trees. |
| Climax Community | Shade-tolerant trees like oak and hickory. | Mature hardwood forest (often similar to the pre-disturbance community). |
| Timeline | Very slow (hundreds to thousands of years). | Relatively fast (decades to a few hundred years). |
Key Components & Evidence
Primary Succession: The colonization of new landforms. A classic example is the formation of new land by volcanic eruptions in Hawaii, where lichens are the first to colonize the barren lava rock.
Secondary Succession: The regrowth of an ecosystem after a disturbance. The regeneration of forests in the eastern United States on abandoned farmland is a well-documented example.
Pioneer Species: The first organisms to establish themselves in a successional sequence. Lichens are a symbiotic combination of fungus and algae, uniquely equipped to break down rock and create the first layer of organic soil.
Keystone Species: A species whose activities have a disproportionately significant role in determining community structure. For example, sea otters prey on sea urchins, preventing the urchins from overgrazing and destroying kelp forests, which are critical habitats for many other species.
Indicator Species: A plant or animal that, by its presence, abundance, or scarcity, demonstrates that a distinctive aspect of an ecosystem is present. For example, the presence of certain mayfly larvae indicates high water quality, as they are intolerant of pollution.
Species Richness: The number of different species in an ecosystem. It typically peaks during mid-succession when there is a wide variety of habitats available, before slightly declining in the climax stage as a few dominant species outcompete others.
Total Biomass: The total dry mass of organisms in a given area. This value steadily increases through the successional stages, reaching its maximum in the climax community.
Net Primary Productivity (NPP): The rate at which an ecosystem's producers convert solar energy into chemical energy as biomass, minus the rate at which they use some of that biomass for their own respiration. NPP is highest during the intermediate stages of rapid plant growth.
Skill Snapshots
Causation
Cause: The chemical weathering of rock by pioneer lichens Effect: initiates the formation of soil.
Cause: The growth of shrubs and trees during mid-succession Effect: increases habitat complexity, leading to higher animal species richness.
Cause: A forest fire clears the canopy but leaves the soil intact Effect: initiates the rapid process of secondary succession.
Comparison
Primary succession begins on a substrate without soil, whereas secondary succession begins on a substrate where soil is already present.
Pioneer species in primary succession (lichens) are soil creators, while pioneer species in secondary succession (grasses) are soil stabilizers and enrichers.
The timeline for primary succession is significantly longer (centuries to millennia) than for secondary succession (decades to centuries).
Change and Continuity Over Time (CCOT)
Baseline: A barren landscape with no soil after a volcanic eruption.
Change 1: Pioneer species like lichens colonize the rock, creating the first thin layer of soil.
Change 2: Grasses and shrubs establish themselves, leading to a rapid increase in biomass, productivity, and species richness.
Continuity: Throughout the process, the community structure continuously becomes more complex and stable, trending towards a climax forest.
Common Misconceptions & Clarifications
Misconception: A climax community is a permanent, unchanging final stage.
- Clarification: Climax communities are in a state of dynamic equilibrium. They still experience small-scale changes and disturbances (like a tree falling), and the overall species composition can shift over very long periods due to climate change.
Misconception: Succession is a linear process that always leads to a forest.
- Clarification: The nature of the climax community is determined by the region's climate and soil. In grasslands, the climax community is a stable prairie; in the tundra, it is low-growing shrubs and mosses. The path is a general pattern, not a rigid, universal blueprint.
Misconception: All disturbances are negative for an ecosystem.
- Clarification: Many ecosystems are adapted to and depend on periodic, natural disturbances. For example, some pine forests require regular, low-intensity fires to clear underbrush and open cones to release seeds, preventing larger, more destructive fires.
One-Paragraph Summary
Ecological succession describes the orderly and predictable replacement of communities in an ecosystem following a disturbance. The process begins with hardy pioneer species that colonize a barren or disturbed area. As each successive community alters the environment—for instance, by creating soil or providing shade—it creates conditions favorable for new species to move in. This progression, known as primary succession if starting without soil and secondary succession if soil is present, is marked by increases in total biomass, species richness, and ecosystem complexity over time. The process culminates in a stable, mature climax community, where keystone species may play a critical role in maintaining structure and indicator species reflect the overall health of the system.