Ecological Succession Stages

Why This Matters

Ecological succession is one of the most testable concepts in AP Biology because it connects so many big ideas: energy flow through ecosystems, community interactions, and biodiversity. When you understand succession, you're really understanding how ecosystems recover from disturbance, how species interactions drive community change, and why some ecosystems are more resilient than others. The College Board loves asking about succession because it lets them test your grasp of facilitation, competition, niche partitioning, and trophic dynamics all at once.

Here's the key insight: you're being tested on the mechanisms that drive ecosystem change, not just the vocabulary. Don't just memorize that lichens are pioneer species—know why they can colonize bare rock (they break it down, accumulate organic matter, and modify the environment for later arrivals). Every stage of succession illustrates principles of community ecology and energy flow that appear throughout Unit 8. Master the "why" behind each concept, and you'll be ready for any FRQ they throw at you.

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Types of Succession: Where It All Begins

The distinction between primary and secondary succession comes down to one critical factor: whether soil is present. This determines the timeline, the pioneer species involved, and the trajectory of community development.

Primary Succession

• Starts on lifeless substrates without soil—occurs after volcanic eruptions, glacial retreat, or on newly exposed rock surfaces
• Takes centuries to millennia because organisms must first create soil through weathering and organic matter accumulation
• Pioneer species like lichens and mosses initiate the process by breaking down rock and enabling later colonizers

Secondary Succession

• Occurs where soil remains intact—follows disturbances like forest fires, floods, or agricultural abandonment
• Proceeds much faster than primary succession because seeds, roots, and nutrients already exist in the soil (seed bank)
• Often returns to a community resembling the original—demonstrates ecosystem resilience and the importance of soil as a nutrient reservoir

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Drivers of Change: What Pushes Succession Forward

Succession doesn't happen randomly—it's driven by specific mechanisms. Understanding facilitation, inhibition, and tolerance helps you predict which species will dominate at each stage and why communities change over time.

Facilitation

• Early species modify the environment to benefit later arrivals—this is the classic model of succession where each stage "prepares" for the next
• Includes providing shade, adding nutrients, or stabilizing soil—nitrogen-fixing bacteria and decomposers play critical roles here
• Connects directly to community ecology concepts like mutualism and ecosystem engineering (think beaver dams)

Inhibition

• Established species prevent or slow colonization by others—occurs through resource competition or chemical warfare (allelopathy)
• Challenges the simple "facilitation-only" model of succession—real ecosystems show mixed dynamics
• Influences species composition by determining which organisms can successfully establish despite resistance

Tolerance

• Some species succeed regardless of what's already present—they're adapted to a wide range of conditions
• Tolerant species often dominate later stages because they can establish under the shade or resource limitations created by pioneers
• Key concept for understanding climax communities—the species that persist are those tolerant of stable, competitive conditions

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The Succession Sequence: From Bare Ground to Stability

These terms describe the actual steps organisms go through as they colonize and transform an environment. Think of this as the life cycle of an ecosystem.

Nudation

• Creation of a bare substrate available for colonization—the "blank slate" that starts succession
• Results from disturbances like volcanic activity, landslides, fire, or human clearing
• Marks time zero for tracking ecosystem development and recovery

Colonization

• Arrival and initial establishment of organisms in the disturbed area
• Depends on dispersal mechanisms—wind-blown seeds, spores, or mobile organisms reaching the site
• Critical bottleneck because only species with effective dispersal and stress tolerance can be pioneers

Ecesis

• Successful survival and reproduction of colonizing species—not just arrival, but establishment
• Requires matching between organism traits and environmental conditions—many colonizers fail at this stage
• Determines which species actually drive early succession forward

Competition

• Struggle for limited resources like light, water, nutrients, and space intensifies as species accumulate
• Shapes community structure by favoring species with competitive advantages at each stage
• Connects to niche partitioning and competitive exclusion—core community ecology concepts from Topic 8.5

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Endpoints and Equilibrium: Where Succession Leads

These concepts address what happens when succession "finishes"—though modern ecology recognizes that ecosystems are rarely static.

Climax Community

• The stable, mature endpoint of succession where species composition changes little over time
• Characterized by high biodiversity and complex trophic interactions—food webs are most developed here
• Represents dynamic equilibrium, not a frozen state—minor fluctuations occur, but the community self-maintains

Sere

• The complete sequence of stages from pioneer community to climax community
• Each seral stage has characteristic species and environmental conditions—useful for identifying where an ecosystem is in succession
• Provides a framework for predicting ecosystem development and planning restoration efforts

Stabilization

• The process of reaching equilibrium after succession completes
• Marked by balanced energy flow and nutrient cycling—inputs roughly equal outputs
• Indicates ecosystem resilience—the community can recover from minor disturbances without restarting succession

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What Drives the Driver: Internal vs. External Forces

Succession can be pushed forward by forces within the ecosystem or by outside environmental changes. This distinction matters for predicting how ecosystems will respond to climate change and human impacts.

Autogenic Succession

• Driven by internal biological processes—species interactions, growth, death, and decomposition
• Reflects the community shaping its own environment over time through facilitation and competition
• The "default" model of succession when external conditions remain stable

Allogenic Succession

• Driven by external environmental changes—climate shifts, flooding, fire regimes, or human disturbance
• Can accelerate, reverse, or redirect succession depending on the nature of the external force
• Increasingly relevant as climate change alters disturbance patterns and growing conditions worldwide

Reaction

• The cumulative effect of organisms on their environment—how the community changes abiotic conditions
• Includes both facilitative and inhibitory effects—soil enrichment, shading, water retention changes
• Central mechanism explaining why early-stage species eventually get replaced by later-stage species

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The Foundation: Pioneer Species

Pioneer species deserve special attention because they determine whether succession can proceed at all.

Pioneer Species

• First colonizers of barren or disturbed environments—lichens, mosses, and certain bacteria in primary succession; fast-growing plants in secondary
• Possess key adaptations: stress tolerance, efficient dispersal, rapid reproduction, and often nitrogen fixation
• Transform the environment by weathering rock, building soil organic matter, and altering microclimate—classic facilitation in action

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Quick Reference Table

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Self-Check Questions

1. Compare and contrast primary and secondary succession. What single factor most determines which type occurs, and how does this affect the timeline of ecosystem recovery?

2. Which two mechanisms of succession (facilitation, inhibition, or tolerance) would best explain why a forest understory remains dominated by shade-tolerant species even after canopy trees die?

3. A volcanic island emerges from the ocean. List the sequence of processes (nudation → stabilization) that would occur, and identify which step represents the greatest bottleneck for ecosystem development.

4. FRQ-style prompt: An ecologist observes that a recently burned grassland is recovering more slowly than expected. Using the concepts of inhibition and allelopathy, explain one mechanism that could account for this observation.

5. How would you distinguish between autogenic and allogenic succession if you were studying a wetland ecosystem over 50 years? What evidence would indicate each type is occurring?