Sample along the change: To see zonation, you sample across it. A transect records the living things AND the abiotic factors at the same points, so you can find what controls where each species lives.
The points to remember
- A transect is laid along the gradient, from one end to the other.
- Samples are taken at points along it: biotic data (which species, how many).
- Abiotic factors are measured at the same points.
- Comparing the two shows which variables affect the distribution of each species.
- A line transect records what touches the line; a belt transect uses quadrats along it.
Remember it as: Lay the line, sample the points, measure the conditions, match them up.
Real example: at Wembury Bay in Devon, England, a rocky shore protected as a marine conservation area, school groups lay a tape from the low-tide mark up to the cliff to see how the seaweeds change up the shore.
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Along the tape you sample at fixed steps, usually with a quadrat. The steps must be close enough to catch every zone: short steps on a steep shore, longer ones across wide dunes.
The points to remember
- Mark sampling points at regular intervals along the tape (systematic sampling).
- Place a quadrat at each point; name each species with an identification key.
- Record percentage cover (seaweeds, plants) or the number of individuals (limpets, snails).
- Or percentage frequency: the share of the quadrat's small squares that hold the species.
- Repeat with parallel transects and work out the mean for each point: more reliable data.
Percentage cover
- How much of the quadrat the species covers
- Best for seaweeds, grasses, lichens
- e.g. bladder wrack covers 60% of the square
Percentage frequency
- How many of the 100 small squares hold it
- Quick for plants that are hard to count
- e.g. marram grass is in 36 of 100 squares: 36%
Real example: at Wembury, a class places a 0.5 m quadrat every 2 m up the shore, repeats this on three transects 5 m apart, and takes the mean cover at each distance.
Say why each step matters: Regular intervals: no zone is missed. Parallel transects and means: one odd quadrat does not decide the result. The same quadrat size everywhere: the samples can be compared.
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The point of a transect is to explain the zones, so at every sampling point you also measure the abiotic factors that could control them, with the same care: the same time of day, repeated readings.
The points to remember
- Choose factors that change along the gradient and could limit the species.
- Shore: height above low tide, time out of water, rock temperature, salinity, wave exposure.
- Dunes: wind speed, soil moisture, soil pH, organic matter, light, temperature.
- Mountain: altitude, air temperature, soil depth, wind speed; data loggers record all day.
- Where a species' kite begins or ends at the same place a factor changes, that factor may limit it.
Real example: on the Wembury transect, serrated wrack is most common at 2 m, where the rock is out of the water for only about 3 hours a day. Channelled wrack is most common at 14 m, out of the water for about 17 hours a day.
A link is not proof: Several factors change together along a gradient, so a match between a kite and one factor shows the factor MAY limit the species, not that it does. Competition can set an edge too.
Transect data are shown in a kite diagram. Each species gets a band along the transect: wide where it is common, a thin line where it is absent.
The points to remember
- The horizontal axis is the transect: distance along it, or each sampling point.
- Each species has its own kite, drawn around its own line.
- The width at a point shows how common the species is there; read it with the scale.
- To draw it, plot each value both above and below the line, half on each side.
- Join the points above, then the points below, and shade between the two lines.
To draw one yourself, work one species at a time, from a table of the mean values at each point:
Describe a kite with numbers: Give where it starts, where it peaks and where it ends: 'bladder wrack is found from 2 to 14 m and is widest, about 70% cover, at 8 m.'
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How this comes up: Paper 2, Section B (b): explain how transect data are collected and turned into a kite diagram [7].
A class wants to show how bladder wrack is distributed up a rocky shore at Wembury Bay, Devon.
Explain how data can be collected and manipulated to produce a kite diagram of the distribution of bladder wrack along the shore.
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