The size of a population can be estimated using: randomly placed quadrats,…

The idea

For non-motile or slow-moving organisms, count in quadrats. Where the habitat looks uniform, place quadrats randomly: lay two tapes at right angles as axes and generate coordinates with random numbers, which removes bias. Record the number of individuals, the percentage cover where individual plants cannot be counted, or the frequency. Scale up with the mean number per quadrat divided by the quadrat area and multiplied by the total area. Where there is an environmental gradient, random placement would hide the pattern, so use a belt transect: a tape along the gradient with quadrats at regular intervals. For motile organisms use mark-release-recapture. Capture a first sample, count and mark the individuals harmlessly, release them, allow time for mixing, then capture a second sample.

What earns the marks

• Random placement of quadrats using coordinates generated from random numbers, to avoid bias.
• A belt transect with quadrats at regular intervals where an environmental gradient is present.
• Scaling up correctly, using the mean number per quadrat, the quadrat area and the total area of the habitat.
• The correct mark-release-recapture expression, first sample multiplied by second sample divided by the number marked in the second sample.
• Any two valid assumptions, such as marked individuals mixing randomly, marking not affecting survival or behaviour, marks not being lost, and no births, deaths or migration between samples.

Common mistakes and how to put them right

• Throwing a quadrat over the shoulder and calling the placement random.

• Using random quadrats where there is a clear gradient, or a transect where there is none, so the pattern is missed or invented.

• Forgetting to divide by the quadrat area when scaling up, so the estimate is wrong by a whole factor.

• Inverting the mark-release-recapture formula and dividing by the first sample instead of the number recaptured carrying marks.

• Giving only the assumption that population size does not change, and missing that marks must persist and must not affect predation.

Exam tips

• Write the mark-release-recapture formula before substituting; a correct expression can be credited even if the arithmetic then fails, and round to a whole organism.
• If asked why the estimate is too high, look for marks being lost or marked animals dying, since both reduce the recapture count.
• Say what you would record in each quadrat, such as percentage cover, rather than just saying use a quadrat.