Practical: quadrats and transects revision notes

Edexcel International GCSE Biology (4BI1), chapter 14: Ecosystems, specification point 4.2, 4.4B. Free lesson.

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Quadrats help you estimate the size of a population by sampling an area fairly. Transects show how species change across a gradient, such as a change in light or soil moisture.

How to sample randomly with quadrats

Random sampling helps you estimate how many organisms are in an area without counting every one. Random placement matters because choosing sites by eye could favour places where organisms are easier to see or more common. Lay two tape measures at right angles along the edges of the area to make a grid. Use a random number generator to choose pairs of coordinates. Do not choose the positions by eye. Place a quadrat at each coordinate pair and count the organisms inside it. For plants, you can count individuals or estimate percentage cover. Repeat for at least ten quadrats. Taking more samples…

How to estimate the total population

First find the mean number of organisms per quadrat. Then scale this up to the whole area using the ratio of the total area to the area of one quadrat.

How to compare two areas

If asked to compare two areas, sample both using the same method. Choose areas that differ in one way, then compare their means and suggest a reason for any difference. For example, compare a mown area with an unmown area, or a shaded area with an open area. Use the difference between the mean counts to describe how the populations differ.

When and how to use a transect

A transect answers a different question from random quadrat sampling. It shows how the distribution of species changes across a gradient, rather than estimating the total population in an area. A transect is not random, and is not meant to be, because its purpose is to follow change across the gradient. Lay a tape measure in a straight line across the gradient, such as from the water's edge up a shore. Place quadrats at regular intervals along the line, for example every 2 m. At each point, record which species are present and their abundance or percentage cover. Also record relevant…

How to interpret a transect graph

Distance goes along the bottom axis and percentage cover goes up the side. The lines show how species abundance changes along the transect. A species common near the start may disappear further along, while another may become more common. The point where two species' lines cross shows where their percentage cover is equal. You can describe a correlation between a species and an abiotic factor, but the data alone does not prove that the factor caused the change. Another condition may also vary along the gradient.