Zonation at HL: Zonation works the same for SL and HL. You'll reuse it at HL in fieldwork and when analysing how ecosystems change across space.
No HL-only content here — just understand what causes zonation.
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Big picture: Ecosystems are dynamic systems that change over time and across space as environmental conditions vary.
What is zonation?
Zonation It occurs because conditions such as temperature, moisture, light, or exposure change gradually across space.
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Different conditions favour different species, so distinct zones form.
Why zonation occurs
- Abiotic factors change gradually across space
- Species have tolerance limits for conditions
- Competition excludes less adapted species from some zones
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Common environmental gradients
- Altitude – temperature decreases and growing seasons shorten with height
- Latitude – solar energy decreases from the equator towards the poles
- Tidal level – exposure to air and water changes with the tides
- Soil depth – nutrients, oxygen, and moisture vary between soil layers
Altitudinal zonation (mountains)
As altitude increases, temperature and oxygen levels decrease while wind exposure increases. This leads to changes in vegetation from forests at lower altitudes to shrubs, grasses, alpine plants, and eventually bare rock or ice.
Biodiversity generally decreases with increasing altitude.
Tidal zonation (rocky shores)
Tidal zonation is caused by changing sea levels due to tides, creating zones that differ in how often they are submerged.
- High tide zone – exposed to air most of the time
- Middle tide zone – alternates between wet and dry
- Low tide zone – submerged most of the time
The lowest tidal zone usually has the highest biodiversity because conditions are more stable.
Studying zonation using transects
Transect Transects allow scientists to record how species presence and abundance change across different conditions.
- A line is placed across the gradient
- Species are recorded at regular intervals
- Patterns of distribution are analysed
Kite diagrams
Kite diagram The width of the kite represents the number of organisms, while its length shows where the species occurs.
Kite diagrams are commonly used in ESS exam questions to interpret zonation patterns.
Zonation at different scales
- Local scale – rocky shores, forests, lake edges
- Global scale – climate zones and biome distribution
Key exam takeaways
- Zonation is caused by changing abiotic conditions
- Species distribution reflects tolerance limits
- Transects and kite diagrams are used to study zonation
- Zonation occurs at both local and global scales
IB-style question — Altitudinal zonation and the transect method
A student investigates vegetation zonation on Mount Ruapehu, New Zealand. Outline why vegetation changes with altitude and describe the transect method used to collect the data. [5]
How to answer it, step by step
- Why vegetation changes with altitude
• Temperature drops ~6.5 °C per 1 000 m → shorter growing season, fewer species survive.
• Soils become thinner and less nutrient-rich; wind exposure increases → each altitude band has distinct abiotic conditions → different plant communities. - Transect method
• Lay a tape from base to high altitude, perpendicular to slope contours; sample at systematic intervals (e.g. every 50 m elevation).
• Place 1 m² quadrats at each point, record % cover per species; repeat (e.g. ×5) and average for reliability.
Final answer
Separate marks for (a) naming a changing abiotic factor and linking it to vegetation, and (b) describing the method in enough detail. 'Use quadrats' alone is not enough — include 'systematic intervals', 'percent cover', and 'repeat sampling'.