Zonation at Higher Level: The same ideas as SL, with different examples: the Wadden Sea salt marsh, Kilimanjaro, Florida's mangroves and the Okavango Delta. At HL, expect to explain the edges of each zone: which limits come from tolerance, which from competition.
Practise this as you read
- Trace one abiotic factor along each gradient.
- Contrast zonation (space) with succession (time).
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Change across space: Ecosystems change across space as well as over time. The change across space is zonation: it happens along an environmental gradient. Each species lives only where conditions stay within its tolerance limits.
The points to remember
- Zonation: the community changes along an environmental gradient.
- Along the gradient, abiotic factors (temperature, water, salt, exposure) change bit by bit.
- Each species has tolerance limits, so it can only live where conditions suit its niche.
- Competition sets the edges: a stronger species pushes a weaker one out of part of the gradient.
- So species form bands (zones) that change over distance, across space.
Remember it as: Gradient, tolerance, competition: three words, three reasons for bands.
Real example: on the salt marshes of the Wadden Sea (Netherlands, Germany, Denmark), glasswort grows on the lowest mud, flooded twice a day. Sea lavender grows in the middle marsh. Sea couch grass grows on the highest marsh, flooded only a few times a year, and crowds out plants that cannot keep up.
Name the gradient: Always say WHAT changes along the gradient (temperature, time out of water) and HOW that changes which species live there.
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Climb a mountain and you pass through bands of different vegetation: altitudinal zonation. The number of species usually falls as you go up.
Why the vegetation changes with altitude
- Higher up it is colder (about 6.5 °C less per 1,000 m), so the growing season is shorter.
- Plants that survive cold grow higher up: conifers photosynthesise as soon as it is above 0 °C.
- Plants that need a long growing season (deciduous trees) grow lower down.
- Soils get thinner and poorer, so tree roots cannot grow: alpine grassland replaces forest.
- Wind gets stronger, so only low-growing plants survive; more UV favours waxy, compact plants.
- Less oxygen and less water higher up; on south-facing slopes (north of the equator) zones sit higher.
Real example: on Kilimanjaro, Tanzania, farms and montane rainforest give way to giant heather at about 2,800 m, to an alpine desert above 4,000 m and to rock and ice near the 5,895 m summit.
Two answers that fail: 'There is less vegetation higher up' does not count: say the TYPE of vegetation changes. And 'the vegetation changes because the temperature changes' is not enough on its own: say how the cold changes which plants can grow.
Reading two maps?: Pair a vegetation type with its height: 'alpine grassland grows at high altitude'. A range of numbers alone is not a relationship.
On a coast the gradient is tidal level. The sea covers the lowest shore for most of the day and the highest shore for almost none of it.
The points to remember
- Tidal level is the gradient: the higher up the shore, the longer it is out of water.
- Low shore: under water most of the time: stable conditions, the most species.
- Middle shore: wet and dry every tide.
- Upper shore: out of water most of the time: drying out, heat and salt, few species.
- Upper limits are often set by drying out; lower limits by competition and predators.
Real example: in the Florida Everglades, USA, mangroves form bands. Red mangroves stand on prop roots at the sea's edge; black mangroves send up breathing roots from airless mud; white mangroves and buttonwood grow on higher ground that floods less.
Why the low shore is richest: Link it to stability: under water most of the time, so temperature and moisture hardly change, and many species can live there. Higher up, fewer species can tolerate drying out.
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Mountains and shores are the classic cases, but any steady change across space can make zones: latitude, soil depth or distance from a river or lake.
The points to remember
- Latitude: less solar energy towards the poles, so biomes change from the equator to the poles.
- Soil horizons: organisms change with depth, as oxygen, water and organic matter change.
- Distance from a water source: water in the soil falls, so plants change from wet-loving to dry-tolerant.
- Local scale: a shore or lake edge (metres). Global scale: biomes by latitude (thousands of km).
- Abiotic factors form zones too: water temperature changes with depth, distance from land, latitude and ocean currents.
Local scale
- A rocky shore, a lake edge, a forest edge
- Metres to a few kilometres
- Gradients such as tide or soil moisture
Global scale
- Biomes from the equator to the poles
- Thousands of kilometres
- Gradients such as solar energy and rainfall
Real example: in the Okavango Delta, Botswana, papyrus swamp lines the permanent channels, then grassland that floods for part of the year, then dry woodland on ground the floods never reach.
A map of water temperature: Asked why sea or lake water forms bands of temperature? Give one reason with its link: shallow water near land warms faster; the water is cooler further from the equator; a cold current brings cold water. 'Climate change' or 'salinity' do not explain the pattern.
Zonation is easy to confuse with succession. Both are about communities changing, but along different axes: one across space, the other through time.
The points to remember
- Zonation: a gradient of abiotic factors; each species settles where the conditions suit its niche.
- So the community changes over distance: zonation is spatial.
- Succession: pioneer species colonise bare ground and change the abiotic conditions.
- Then other species can colonise and outcompete them, on towards a climax community.
- So the community changes over time: succession is temporal.
Remember it as: Zonation: walk across it. Succession: wait for it.
Zonation
- Changes over distance (space)
- Caused by an abiotic gradient
- e.g. a mountainside, a rocky shore
Succession
- Changes over time
- Caused by each community changing conditions
- e.g. after a fire, on new volcanic rock
Real example: climbing Kilimanjaro you cross five zones in a day: zonation. Surtsey, a volcanic island that rose off Iceland in 1963, was bare rock; mosses, then grasses, then shrubs arrived over sixty years: succession.
End on the contrast: In a [7] on both ideas, finish with the key difference: zonation changes over distance, succession over time. Leaving that out caps the answer at 6. Writing about only one of them caps it at 4.
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How this comes up: Paper 1, a case study: a mountain-zone diagram. Outline why the vegetation changes with altitude [2].
Figure 1 shows the zones of vegetation on Mount Kilimanjaro, Tanzania.
With reference to Figure 1, outline why the vegetation changes with altitude.
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