2.5 Zonation, succession and change in ecosystems

Walk from the waterline of a rocky shore up to the cliffs above it, and you pass through a series of visibly distinct bands, each dominated by different species. That pattern is zonation. The community changes not because time has passed, but because the physical and biological conditions change across space.

Zonation is a spatial phenomenon. The key distinction from succession (covered later) is that zonation describes differences across a gradient at one moment in time, while succession describes change at one place across time.

The gradients that drive zonation include:

• Elevation (altitude): temperature and oxygen availability fall with height, shifting vegetation from tropical forest at low altitude to alpine tundra higher up
• Latitude: solar radiation and mean temperature decrease from equator to poles, producing the global sequence of biomes
• Tidal level: on coasts, exposure to air, wave action, and salinity varies predictably from the spray zone down to permanently submerged rock
• Soil horizons: soil chemistry, texture, and moisture change with depth and with distance from the parent material
• Distance from a water source: moisture availability declines away from rivers or lakes, shifting vegetation from dense riparian growth to open scrub or desert

Both abiotic and biotic factors shape which species occupy each zone. Abiotic factors set the physical limits of tolerance; biotic interactions (competition, predation, mutualism) then refine the boundaries within those limits.

To document and quantify zonation, ecologists use transects. The logic is simple: by sampling at fixed points along a gradient, you can map how both species and abiotic conditions change together, which allows you to identify the variables that most strongly influence species distribution.

Three types of transect are commonly used:

• Line transect: a tape measure is stretched along the gradient and all organisms touching the line are recorded. Fast, but captures only what falls exactly on the line.
• Belt transect: a strip of fixed width (commonly 0.5 m or 1 m) is sampled continuously or at intervals along the line. More data-rich than a line transect.
• Interrupted transect: sampling occurs only at fixed intervals (e.g., every 2 m) rather than continuously. Practical when gradients are long.

Abiotic variables to measure at each sampling point (selected to match the gradient being studied):

Biotic variables to record at each point:

• Species richness (number of species present)
• Percentage cover per species (using a quadrat placed at the sampling point)
• Abundance of each species

Procedure for a belt transect investigation:

1. Identify a clear environmental gradient (rocky shore, sand dune system, forest edge).
2. Lay a measuring tape perpendicular to the gradient, from one end to the other.
3. Mark sampling intervals (e.g., every 1 or 2 m).
4. At each interval, place a quadrat and record all species and their percentage cover.
5. Simultaneously record the relevant abiotic measurements.
6. Enter data into a systematic table for analysis and graphing.