Walk from the sea up a rocky shore and you do not get a random mess of living things. You get bands. Seaweed low down, mussels and limpets in the middle, lichens up top. That banding is called zonation, and it happens because the conditions change steadily as you walk — and every species can only cope with part of that change.
📘 What you need to know
Zonation is a change in the species you find as you move across space, following a gradient in an abiotic factor.
Common gradients: tidal height, altitude, latitude, soil depth, distance from water, temperature, light and moisture.
Bands form for two reasons: each species can only tolerate part of the gradient, and where conditions get easier, competition pushes the weaker competitors out.
Zonation is change over space. Succession is change over time. These are different ideas and examiners love to test the difference.
You sample a gradient with a transect (line or belt), placing quadrats at set intervals, and you record an abiotic factor at each one.
A kite diagram shows distribution and abundance of several species on one graph.
What zonation actually is
Zonation is the pattern you see when the community changes gradually as you move from one place to another. The thing driving it is an environmental gradient: some abiotic factor that gets steadily stronger or weaker as you travel in one direction.
Think about a mountainside. At the bottom it is warm, sheltered and the soil is deep, so you get broadleaved trees. Climb higher and it gets colder and windier, the soil gets thinner, and the trees give way to conifers, then to low shrubs, then to grass and moss, then to bare rock. Nobody planted those bands. They appear because temperature, wind and soil depth all change together with height.
The same thing happens with distance from a pond, with depth in a lake, with how salty the soil is behind a beach, and — the classic exam example — with height up a rocky shore.
Definition
Zonation = the arrangement of species into bands in response to a gradient in one or more abiotic factors
Why the bands form (the bit most notes skip)
It is tempting to say “each species just lives where it can survive”. That is only half the story, and the better half is worth marks.
On a shore, the top edge of a species’ band is usually set by abiotic stress. Go too high and the animal is out of water for too long, dries out, overheats, and dies. Physical conditions set the upper limit.
The bottom edge is usually set by other organisms. Lower down the shore, conditions are easy, so lots of species can live there. That means more competition for space and more predators. A tough, slow-growing species like a barnacle gets crowded out or eaten. So biotic factors set the lower limit.
If a question asks you to explain a zonation pattern, give both halves: what stops the species living higher up (drying out, heat, wave splash) and what stops it living lower down (competition for space, predators). Two reasons, two different types of factor.
The shading on the right shows how long each band spends under water. That single factor sets up almost everything else you see on the shore.
Measuring a gradient: transects
You cannot describe zonation properly by eye. You need numbers, and you get them with a transect: a tape measure laid out along the direction the conditions change in.
🧩 How to run a belt transect
Find the gradient. Lay the tape so it runs along the direction of change, for example straight up the shore from the low tide mark.
Choose an interval. Place a quadrat every set distance, for example every 5 m. Equal gaps means systematic sampling.
Record the biotic data. In each quadrat count the individuals, or estimate percentage cover for plants and seaweeds.
Record the abiotic data. At the same spot measure the factor you think is driving the pattern — height above the low tide mark, soil pH, light, moisture.
Repeat. Run two or three more transects nearby and average them, so one odd patch of rock does not decide your result.
Plot and describe. Draw a kite diagram, or plot the abiotic factor against abundance, and describe the pattern in words.
Two versions come up:
Line transect — you record only the species that actually touch the tape at each marked point. Quick, but it gives you presence rather than proper abundance.
Belt transect — you place a quadrat at each point, so you get abundance as well as presence. Slower, much more useful.
Why not random sampling? Random quadrats are the right choice when a habitat looks fairly uniform and you want an unbiased average. Along a gradient, random points would scatter all over the shore and blur the very pattern you are trying to show. You want systematic sampling here, and saying so earns the mark.
Showing the results: kite diagrams
A kite diagram puts several species on one graph so you can see where each one lives and how much of it there is.
The x-axis is distance along the transect.
Each species gets its own strip with a central horizontal line.
The shape is drawn out an equal distance above and below that line, so it looks like a kite. The width shows abundance.
Extra strips can be added for abiotic factors, such as height above sea level, so you can line up cause and effect.
Three sand dune plants along the same 30 m transect. The overlap between the strips is the interesting part: that is where two species are competing for the same ground.
Worked examples
WORKED EXAMPLE 1
A belt transect up a rocky shore gave these limpet counts per quadrat: 0.5 m above low tide = 21, 1.5 m = 18, 2.5 m = 11, 3.5 m = 4, 4.5 m = 0. Describe and explain the pattern.
Describe first, using the numbersLimpet numbers fall as height up the shore increases, from 21 down to 0.Quote the top and bottom values. “It goes down” on its own is a weak answer.Then explain, using an abiotic factor
Higher up the shore the rock is out of the water for longer, so limpets are exposed to air, heat and drying for more of the day.
Finish with the biological consequence
They lose water and cannot feed on algae while exposed, so fewer can survive.
Numbers fall with height because exposure time increases
WORKED EXAMPLE 2
Using the kite diagram above, state where marram grass is most abundant and suggest why creeping willow is absent from the first 15 m.
Read the widest point of the marram strip
The marram kite is widest at 15 m along the transect.
Now think about conditions in the first 15 m
That end is closest to the sea: loose blowing sand, salt spray and very little soil or fresh water.
Link that to the plant
Creeping willow cannot cope with salt spray and unstable sand, and it needs a soil that holds water, so it only appears further inland.
Marram peaks at 15 m; willow needs stable, less salty groundMarram survives there because its long roots reach deep water and it tolerates salt.
WORKED EXAMPLE 3
A student used a 100-square quadrat. Bladderwrack filled 37 squares fully and about half of 10 more. Calculate the percentage cover.
Turn the part-filled squares into whole ones10 squares half full = 10 × 0.5 = 5 whole squaresAdd them up37 + 5 = 42 squares out of 100Percentage cover = 42%Percentage cover is used for plants and seaweeds because you cannot sensibly count individuals.
💡 Exam tip
If the command word is describe, only say what the data shows and quote figures. Save the reasons for explain.
Always name the abiotic factor you think is driving the gradient. “Conditions change” scores nothing.
Say systematic sampling for a transect and random sampling for a uniform habitat. Knowing which one and why is a common two-mark question.
Mention repeats. One transect is one line across one bit of shore; two or three let you average out odd patches.
On a kite diagram, check the scale before reading a value, and remember the shape is drawn both sides of the centre line.
Correlation is not cause. A transect shows a species and a factor changing together; to claim one causes the other you need to rule out the factors that change alongside it.
⚠ Common mix-up
Zonation is not succession. Zonation is bands across space at one moment. Succession is one place changing over time.
Reading a kite the wrong way. Measuring the full width and calling it the abundance doubles your answer.
Only giving abiotic reasons. The lower edge of a band on a shore is usually set by competition or predation, not by physical stress.
Confusing a line and a belt transect. Only the belt transect uses quadrats and gives you abundance.
Saying “the tide” as if it were the factor. Be specific: exposure time to air, drying out, wave action, salinity.
Forgetting the abiotic readings. A transect with only species counts cannot explain anything; you need both columns of data.
Up next: Succession and How Communities Change — the same idea, but the change happens over time instead of across a hillside.
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