Zonation is a spatial change in a community along an environmental gradient. In IB ESS subtopic 2.5, it is measured using a transect that records species distribution and relevant abiotic factors. This is core content for SL and HL.
How Zonation Occurs
An environmental gradient changes abiotic factors, such as soil moisture, salinity, light intensity, exposure, pH or temperature. Species occupy different zones because of their tolerance limits and interactions such as competition.
A rocky shore, for example, may show bands of lichens, barnacles, mussels and seaweeds from upper to lower shore. Their distributions reflect immersion time, desiccation, wave exposure and competition.
A common misconception is that zonation and succession are identical. Zonation describes community change across space, whereas succession describes the replacement of one community by another over time.
Measuring Zonation With a Transect
| Stage | Method |
|---|---|
| Identify the gradient | Select a direction across expected environmental change. |
| Position the transect | Lay tape perpendicular to zones to create a line transect. |
| Sample organisms | Record species touching the line, or place quadrats at regular intervals for a belt transect. |
| Quantify distribution | Measure abundance, frequency or percentage cover. |
| Measure environment | Record salinity, moisture, pH or light at the same points. |
| Improve reliability | Repeat parallel transects from randomly selected starting positions and calculate means. |
| Analyse | Plot species abundance against distance and compare it with abiotic data. |
This systematic sampling reveals spatial patterns while random starting positions reduce sampling bias. Human disturbance, such as coastal trampling, should also be recorded because it may alter natural zones and influence conservation decisions.