Organisation of an Ecosystem » Sampling with Quadrats and Transects

Why ecologists sample

An ecologist cannot count every daisy on a school field or every snail in a wood. There are far too many, and they move about. Instead, they look at a small part of the habitat and use it to work out what the whole habitat is like. This is called sampling.

A range of methods using quadrats and transects is used to find out two things about a species in an ecosystem:

• Distribution: where the species is found in the habitat.
• Abundance: how many individuals of the species there are.

Key terms:

• Quadrat: a square frame, often 1 m by 1 m, placed on the ground to mark out a small sample area.
• Transect: a line (such as a tape measure) laid across a habitat, along which samples are taken.
• Distribution: where a species is found.
• Abundance: how many of a species there are.

Using a quadrat

A quadrat is placed on the ground and you count how many individuals of your chosen species are inside the frame. Quadrats work best for plants and for slow-moving animals.

To get a fair result you must sample at random. This means every part of the habitat has an equal chance of being sampled. If you only pick the neat, easy places, your result is biased and will not represent the whole habitat.

Using a transect

A transect is used when conditions change across a habitat, for example from the edge of a hedge out into the open field, or from the shore up a beach. Because conditions change along the line, the species found there may change too.

• Lay a tape measure in a straight line across the habitat.
• Place a quadrat at regular intervals along the line, such as every 2 m.
• Count the species in each quadrat.

This shows how the distribution of a species changes along the line. A random sample cannot show this, because the quadrats are scattered with no pattern.

Estimating a population size

Once you have your counts, you can estimate how many individuals live in the whole area:

estimated population = mean number per quadrat × (total area of habitat ÷ area of one quadrat)

It is only an estimate. A bigger sample gives a more reliable one.

Mean, mode and median

These three are all kinds of average. Remember, the mean is the one you use to estimate a population.

• Mean: add up all the values, then divide by how many values there are.
• Mode: the value that appears most often.
• Median: the middle value when the values are put in order. With an even number of values, it is halfway between the two middle ones.

Key terms:

• Mode: the most common value in a set of results.
• Median: the middle value when results are put in order.

Plotting a graph of your results

Transect results are easiest to see on a graph. Follow these steps:

1. Put the distance along the transect on the x-axis (the thing you chose).
2. Put the number of organisms, or the mean number per quadrat, on the y-axis.
3. Choose a scale that uses most of the graph paper. If the biggest count is 18, go up to 20 in steps of 2, not 100.
4. Keep the steps even, label each axis with its units, and plot the points carefully.

For example, if the counts fall from 18 near the hedge to 2 at 10 m away, a line graph with a clear downward slope shows the species is more abundant near the hedge.

Exam-style question

A student investigated buttercups in a field of area 500 m². She placed ten 1 m² quadrats and counted the buttercups in each: 4, 6, 4, 5, 7, 3, 4, 6, 2, 4.

(a) Explain why she should place the quadrats at random. [2 marks]

(b) Calculate the mean number of buttercups per quadrat. [2 marks]

(c) Estimate the number of buttercups in the field. [2 marks]

(d) Give the mode of her results. [1 mark]