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IB DP ESS — Topic 2: Ecology | SL / HL | Paper 1 & Paper 2A/2B
🎯 Learning Intentions By the end of this note, you will be able to:
- Define ecological succession, primary succession, secondary succession, pioneer species, climax community, and seral stage using IB-aligned ESS terminology.
- Explain the mechanisms by which pioneer species modify abiotic conditions to enable colonisation by subsequent species during primary succession.
- Describe the sequence of seral stages from bare substrate to climax community in a named terrestrial and/or aquatic example.
- Distinguish between primary and secondary succession in terms of starting conditions, rate of change, and the role of the soil seed bank.
- Analyse how succession relates to changes in biodiversity, biomass, productivity, and ecosystem stability across seral stages.
- Evaluate the concept of the climax community, including the debate over whether a single stable endpoint is achievable in dynamic environments.
- Discuss how human activities can interrupt, accelerate, or redirect successional processes, and the implications for ecosystem management and restoration.
Real-World Context & Relevance
On 18 May 1980, Mount St. Helens in Washington State, USA, erupted with a force equivalent to 1,600 Hiroshima atomic bombs, stripping 600 km² of forest to bare volcanic rock and ash. Scientists expected the landscape to remain biologically dead for decades. Instead, within weeks, wind-blown spiders and insects arrived. Within two years, lupins — nitrogen-fixing pioneer plants — had colonised the ash fields. By the 1990s, shrubs and young conifers were establishing. Today, over four decades later, a mosaic of recovering forest, meadow, and wetland covers much of the blast zone, offering one of the most closely monitored natural experiments in ecological succession ever recorded (Dale et al., "Ecological Responses to the 1980 Eruption of Mount St. Helens," Springer, 2005).
This story is not unique. From the lava fields of Hawai'i Volcanoes National Park, where bare basalt is slowly colonised over centuries, to the abandoned agricultural fields of Eastern Europe rewilding after the collapse of Soviet collective farming, ecosystems are constantly in motion. Understanding succession — the directional, predictable change in community composition over time — is fundamental to understanding how ecosystems self-organise, recover from disturbance, and respond to human pressure. It also underpins practical decisions in conservation, ecological restoration, and land management worldwide.
🌍 Why This Matters Succession explains how life colonises new surfaces, how ecosystems recover after disturbance, and why biodiversity changes over time. For environmental managers, understanding successional dynamics is essential for designing effective restoration programmes, predicting how ecosystems will respond to climate change, and deciding when — and whether — to intervene in natural recovery processes.
Core Concepts & Definitions
To understand succession, it helps to first think about what makes a habitat habitable. Every organism requires a minimum set of abiotic conditions — temperature, moisture, nutrient availability, light — before it can establish. When a new surface is exposed, whether by volcanic eruption, glacial retreat, or a landslide, these conditions are often extreme and limiting. The first organisms to arrive are not the most competitive; they are the most tolerant of harsh