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IB DP ESS — Topic 2: Ecology | SL / HL | Paper 1 & Paper 2A
🎯 Learning Intentions By the end of this note, you will be able to:
- Define zonation and transect using IB-aligned ESS terminology.
- Explain how abiotic gradients drive the formation of distinct zones within an ecosystem.
- Describe how a belt transect and a line transect are carried out in the field, including the role of quadrats and percentage cover estimation.
- Distinguish between a line transect and a belt transect, identifying when each method is most appropriate.
- Analyse transect data to identify patterns of species distribution along an environmental gradient, applying ESS analysis parameters including scale, reliability, and validity.
- Evaluate the strengths and limitations of transect sampling as a method for investigating zonation in real ecosystems.
- Discuss how human activities can disrupt natural zonation patterns and the implications for ecosystem function and biodiversity.
Real-World Context & Relevance
Walk from the high-tide mark down to the sea on any rocky shore in Cornwall, the Galápagos, or the coast of New South Wales, and something immediately striking happens: the organisms change. At the top of the shore, sun-bleached barnacles cling to bare rock, tolerating hours of desiccation and temperature extremes. A metre lower, dense mats of limpets and periwinkles appear. Lower still, the canopy of brown kelp begins, and in the subtidal zone, sea urchins, starfish, and anemones dominate. These are not random arrangements. Each band of life corresponds precisely to a band of physical conditions — primarily the duration and frequency of tidal immersion — and this banded pattern is one of the most visually striking examples of zonation in any ecosystem on Earth.
The same principle operates far from the coast. On the slopes of Mount Kilimanjaro in Tanzania, vegetation changes from tropical rainforest at the base, through montane forest, heath and moorland, alpine desert, and finally bare rock and permanent ice at the summit — each zone defined by declining temperature and changing precipitation with altitude. In a sand dune system at Studland Bay in Dorset, UK, plant communities shift from pioneer species like sea rocket ( Cakile maritima ) on the embryo dunes, through marram grass ( Ammophila arenaria ) on the yellow dunes, to dense scrub and eventually oak woodland on the fixed dunes inland. Understanding why these patterns exist, and how to measure them rigorously, is at the heart of ecological fieldwork in ESS.
🌍 Why This Matters Zonation patterns are sensitive indicators of environmental change. When abiotic gradients shift — due to sea-level rise, altered precipitation regimes, or pollution — zone boundaries move, species are displaced, and ecosystem function is compromised. Transect surveys provide the baseline data needed to detect and monitor these changes over time, making them a foundational tool in conservation biology, environmental impact assessment, and climate change monitoring.
Core Concepts & Definitions
To understand why organisms are distributed in distinct bands rather than spread uniformly across a habitat, we need to think about the relationship between abiotic conditions and species tolerance. Every species has a range of physical and chemical conditions within which it can survive and