Biology · Grade 12 · Population Dynamics and Ecology · Term 4

Ecological Succession and Disturbances

Students examine the process of ecological succession (primary and secondary) and the role of natural and anthropogenic disturbances in shaping ecosystems.

• Ontario Curriculum Expectations
• HS-LS2-6

About This Topic

Ecological succession traces the staged replacement of species in an ecosystem after a disturbance, leading toward a stable climax community. Primary succession starts on bare substrates like glacial till or lava flows, where pioneer species such as lichens and mosses initiate soil formation. Secondary succession follows disruptions on sites with intact soil, like abandoned fields or burned forests, allowing faster recolonization from seed banks and root sprouts. In Ontario, students connect this to familiar landscapes, such as Algonquin Park's post-fire recovery or Niagara Escarpment pioneer communities.

Disturbances, both natural like wildfires, floods, and insect outbreaks, and anthropogenic such as logging, mining, or urbanization, reset succession trajectories and maintain biodiversity through patch dynamics. This topic integrates with population dynamics by examining species interactions, competition, and facilitation across seral stages. Students analyze how disturbance frequency and intensity influence community structure and predict long-term shifts, building skills in evidence-based forecasting.

Active learning excels for this topic since succession spans decades, beyond classroom timelines. Model-building, simulations, and data analysis from real case studies let students manipulate disturbance variables, track community changes, and debate predictions collaboratively. These approaches make abstract, long-term processes concrete and reveal ecosystem resilience patterns.

Key Questions

1. Differentiate between primary and secondary succession.
2. Analyze how disturbances, such as fires or floods, can influence community structure.
3. Predict the long-term ecological changes following a major environmental disturbance.

Learning Objectives

• Compare and contrast the initial conditions and processes of primary and secondary ecological succession.
• Analyze the impact of specific natural disturbances (e.g., wildfire, flood) on the species composition and structure of an ecosystem.
• Evaluate the long-term consequences of anthropogenic disturbances (e.g., deforestation, urbanization) on ecosystem resilience and biodiversity.
• Predict the trajectory of ecological change over time following a significant environmental disturbance, citing supporting evidence.

Before You Start

Community Ecology and Species Interactions

Why: Students need to understand concepts like competition, facilitation, and predation to analyze how species composition changes during succession.

Biotic and Abiotic Factors

Why: Understanding the roles of both living organisms and environmental conditions is crucial for explaining how ecosystems respond to disturbances and change over time.

Key Vocabulary

Ecological Succession	The gradual process by which ecosystems change and develop over time, involving the replacement of one community of organisms by another.
Primary Succession	Ecological succession that begins in an environment devoid of life and soil, such as on newly formed volcanic rock or glacial till.
Secondary Succession	Ecological succession that occurs in an area where a previous community existed but was disturbed, leaving soil intact, such as after a forest fire or abandoned farmland.
Disturbance	An event, natural or human-caused, that disrupts an ecosystem, altering its structure, composition, or function.
Pioneer Species	The first species, typically hardy and fast-growing, to colonize a barren or disturbed environment during primary succession.

Watch Out for These Misconceptions

Common Misconception

Ecological succession always reaches a single, unchanging climax community.

What to Teach Instead

Frequent disturbances create shifting mosaics of seral stages, preventing uniform climax. Mapping activities with real aerial photos help students visualize patch dynamics and appreciate ongoing change through peer comparisons.

Common Misconception

Primary and secondary succession follow identical processes and timelines.

What to Teach Instead

Primary starts slower without soil or propagules, while secondary advances quicker on disturbed but fertile ground. Tray models let students time both directly, noting differences in pioneer establishment and fostering discussions on starting conditions.

Common Misconception

Anthropogenic disturbances always harm ecosystems more than natural ones.

What to Teach Instead

Human actions can mimic natural regimes, like controlled burns promoting diversity. Debate simulations reveal context-dependent effects, helping students weigh evidence from case studies collaboratively.

Active Learning Ideas

Tray Simulation: Primary Succession

Provide trays with bare rocks, lichens (moss substitute), soil builders, and seeds. Small groups add layers weekly, watering and recording species establishment over 4-6 weeks. Compare growth rates and discuss pioneer roles. Accelerate with heat lamps if time is short.

45 min·Small Groups

Jigsaw: Disturbance Analysis

Assign groups one disturbance type (fire, flood, logging) with Ontario examples and data sets. Each group analyzes impacts on succession stages, then rotates to teach peers. Synthesize findings in a class chart on community changes.

50 min·Small Groups

Prediction Cards: Post-Disturbance Timeline

Distribute species cards with traits (pioneer, intermediate, climax). Pairs sequence them after a simulated disturbance, justifying choices based on tolerances. Class votes and revises timelines using real recovery data from Parks Canada reports.

35 min·Pairs

Patch Dynamics Map: Whole Class Model

On a large poster, map an ecosystem grid. Students add disturbance events and track succession patches over simulated years. Discuss how spatial variation creates mosaics, using markers for species shifts.

40 min·Whole Class

Real-World Connections

• Forestry professionals in British Columbia use their understanding of secondary succession to manage post-wildfire recovery, deciding whether to replant native species or allow natural regeneration to occur.
• Environmental consultants assess the ecological impact of proposed developments, like new housing projects near the Oak Ridges Moraine, by predicting how urbanization will alter natural succession patterns and biodiversity.
• Restoration ecologists work in areas like the Canadian Prairies to re-establish native grasslands after agricultural abandonment, guiding secondary succession to promote biodiversity and ecosystem services.

Assessment Ideas

Exit Ticket

Provide students with two scenarios: Scenario A describes a lava flow, and Scenario B describes a clear-cut forest. Ask students to write one sentence identifying the type of succession each scenario will undergo and one key difference in the initial colonizing species.

Discussion Prompt

Pose the question: 'How can a large wildfire, while destructive, actually benefit a forest ecosystem in the long term?' Guide students to discuss concepts like seed germination, nutrient cycling, and the creation of habitat diversity.

Quick Check

Present students with images of different stages of succession (e.g., bare rock with lichen, young trees in a field, mature forest). Ask them to label each image with the correct stage of succession (primary, secondary, climax) and briefly explain their reasoning.

Frequently Asked Questions

What differentiates primary from secondary ecological succession?

Primary succession begins on lifeless substrates like bare rock, requiring soil-building pioneers and taking centuries. Secondary follows soil-present disturbances like fires, using remnant seeds for quicker recovery, often decades. Ontario examples include post-glacial sites versus logged areas; hands-on models clarify these paths through direct observation of establishment rates.

How do disturbances like wildfires shape succession in Canadian ecosystems?

Wildfires clear competitors, exposing soil for secondary succession while releasing seeds from serotinous cones in species like jack pine. They maintain boreal forest diversity by creating early-successional habitats for wildlife. Students analyze fire return intervals from data to predict community resilience and biodiversity hotspots.

How can active learning improve understanding of ecological succession?

Simulations like tray models or card sorts let students actively build and disrupt successional sequences, mirroring real dynamics. Collaborative predictions from disturbance data build forecasting skills and reveal misconceptions through group debate. These methods transform abstract timelines into tangible experiences, boosting retention and systems thinking over lectures alone.

What long-term changes follow major disturbances in ecosystems?

Post-disturbance, pioneer species dominate initially, yielding to competitors and climax forms over time, though ongoing disturbances sustain diversity. Predictions consider scale: small floods enhance edges, large ones reset widely. Case studies from Ontario floods or fires guide students to model trajectories using species interaction webs.