When an abiotic factor changes gradually across an area, random sampling may miss the pattern. Examples include the change from low tide to high tide on a rocky shore, from the sea to inland on a sand dune, or from the shade of a wood into an open field. Here, ecologists use systematic sampling along a transect: samples are taken at regular intervals along a line.Stretch a tape measure across the area in the direction of the change. Then use one of two methods.Line transect
Record every species that touches the line at regular intervals, for example every metre. It is quick but gives no measure of abundance.Belt transect
Place a quadrat at regular intervals along the line and record the density or percentage cover of each species. In a continuous belt transect the quadrats are placed next to each other along the whole line; in an interrupted belt transect they are spaced at intervals. Belt transects give more information but take longer.At each sampling point, the relevant abiotic factors are also measured, such as light intensity, soil moisture, pH or height above low tide. This lets you look for a correlation between the distribution of species and the abiotic factor.Results are often shown as a kite diagram, with the width of each kite showing the abundance of a species at each distance.
Worked example
An interrupted belt transect is 30 m long, with a quadrat every 3 m starting at 0 m. The number of quadrats is 30 ÷ 3 + 1 = 11.Several transects can be used across the same area to make the results more reliable.
Record every species that touches the line at regular intervals, for example every metre. It is quick but gives no measure of abundance.Belt transect
Place a quadrat at regular intervals along the line and record the density or percentage cover of each species. In a continuous belt transect the quadrats are placed next to each other along the whole line; in an interrupted belt transect they are spaced at intervals. Belt transects give more information but take longer.At each sampling point, the relevant abiotic factors are also measured, such as light intensity, soil moisture, pH or height above low tide. This lets you look for a correlation between the distribution of species and the abiotic factor.Results are often shown as a kite diagram, with the width of each kite showing the abundance of a species at each distance.
Worked exampleAn interrupted belt transect is 30 m long, with a quadrat every 3 m starting at 0 m. The number of quadrats is 30 ÷ 3 + 1 = 11.Several transects can be used across the same area to make the results more reliable.