IB Environmental systems and societies 2.5A Zonation Notes

Zonation: Communities Arranged in Bands Across Space

1. Zonation is the arrangement of species into distinct zones as abiotic and biotic conditions change across space.
2. It is a spatial pattern, which is what separates it from succession, the change in a community over time.
3. You see it wherever there is a clear environmental gradient: rocky shores, sand dunes, mountains, and lake edges.

Abiotic Gradients Set the Template, Biotic Factors Refine It

Abiotic gradients

1. Altitude: temperature and oxygen fall with height, so vegetation changes from forest at the base to alpine tundra higher up.
2. Latitude: sunlight and temperature change from the equator to the poles, setting the pattern of global biomes.
3. Tidal level: on a shore, exposure to air, salinity, and wave action change between high and low tide.
4. Soil type: pH, salinity, and nutrient levels differ from place to place, shaping which plants grow where.
5. Distance from water: moisture falls as you move inland, so lush riverside vegetation gives way to dry scrub.

Biotic interactions

1. Competition: species compete for light, space, or nutrients, which sharpens the boundary between zones.
2. Predation: predators can confine prey to particular zones.
3. Mutualism: some plants grow only where a partner species, such as a pollinator or mycorrhizal fungus, is present.

Worked Examples: Reading Zonation on the Ground

Rocky shore zonation

1. The rocky intertidal is the classic example, because conditions change sharply over a few metres.
2. The gradient runs from the upper shore (exposed, dry, salty) to the lower shore (submerged, stable, oxygen-rich).
3. Moving up the shore, four bands appear:
   1. Spray zone: splashed but rarely submerged; lichens and periwinkles.
   2. High-tide zone: alternately wet and dry; barnacles and limpets.