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IB DP Biology — Unit C: Interaction and Interdependence | SL | C4 Ecology
📋 What You Will Learn
- Identify the difference between primary and secondary succession and the conditions that initiate each
- Explain the sequential stages of succession from pioneer communities to climax communities, including the mechanisms that drive species replacement
- Analyse how disturbance events affect ecosystem stability and the rate of recovery
- Apply the concepts of resilience and resistance to evaluate how ecosystems respond to natural and human-caused disturbances
- Interpret data from ecological studies showing changes in species diversity and biomass across successional stages
🔍 Guiding Question How do ecosystems change over time, and what factors determine whether a disturbed ecosystem can recover to its original state?
🌍 Why This Matters When Mount St. Helens erupted in 1980, it obliterated over 600 km² of forest in minutes — leaving bare volcanic rock where ancient woodland had stood. Yet within decades, plants, insects, and eventually large mammals had returned, rebuilding an ecosystem from nothing. This is succession in action: the predictable, directional process by which life reclaims disturbed land.
The Problem of Bare Ground
Here is the problem every ecosystem must solve: bare ground is hostile. No soil, no shade, no organic matter, no stable moisture. And yet, given enough time, even the most barren landscape — a lava field, a retreating glacier's wake, an abandoned car park — will eventually become a forest. How? The answer is succession: a process so consistent and directional that ecologists can predict, with reasonable accuracy, what a patch of bare rock will look like in 500 years.
Before we can follow that story, we need the vocabulary to tell it. Each term below is introduced at the moment it becomes necessary — not as a list to memorise, but as a tool to understand what is happening and why.
📖 Key Definition — Ecological Succession Ecological succession is the directional, sequential change in the species composition of a community over time, driven by the modification of the physical environment by organisms already present.
📖 Key Definition — Pioneer Species Pioneer species are the first organisms to colonise a bare or severely disturbed habitat. They are typically stress-tolerant, capable of fixing nitrogen or tolerating low-nutrient conditions, and their presence modifies the environment in ways that allow subsequent species to establish.
📖 Key Definition — Climax Community A climax community is the relatively stable, self-sustaining assemblage of species that develops at the end of a successional sequence under a given set of climatic and edaphic (soil) conditions. It represents the endpoint of succession in a particular environment.
Primary Succession: Life from Nothing
Imagine standing on a fresh lava field in Iceland. The rock is black, glassy, and utterly lifeless. There is no soil — just mineral substrate. No seeds are waiting in the ground. No root fragments survive. This is where primary succession begins: on surfaces that have never previously supported life, or from which all biological material has been completely removed.
📖 Key Definition — Primary Succession Primary succession is the sequential colonisation and community development that occurs on a substrate that has never previously supported life, or from which all living material and soil has been removed — such as bare rock, sand dunes, or newly formed volcanic islands.
Stage 1 — Colonisation by Pioneer Species
The first colonisers of bare rock are typically cyanobacteria and lichens. Lichens are not single organisms — they are a mutualistic partnership between a fungus and a photosynthetic partner (alga or cyanobacterium). The fungal partner secretes weak acids that slowly weather the rock surface, releasing mineral ions. Cyanobacteria fix atmospheric nitrogen, beginning the slow accumulation of organic nitrogen compounds in what will eventually become soil.
These pioneers are extraordinary stress-tolerators: they survive desiccation,