Quadrats

Have you used quadrats before? You might have taken samples in your school playing field during one of your science subjects. If not, don't worry. Quadrats are (usually square) frames used for identifying a sample area and counting individuals that lie within it.

Although they're called quadrats (quad = four), they don't necessarily have to be square (i.e. have four equal sides). They can be any shape you like, as long as you can work out their area. But to make things easier, most quadrats are square. It saves you wasting time doing complex calculations when you could be out sampling the environment instead!

Quadrats: Definition

Quadrats are used to estimate the abundance and distribution of a species.

There are three types of quadrats: frame, grid, and point. We’ll go into more detail about these later.

Fig. 1 - A grid quadrat measuring vegetation in a sand dune habitat. Source: Wikimedia Commons

Quadrat Sampling

Quadrats are used for measuring biotic factors (living components of an ecosystem). There are four main biotic factors to focus on:

Measuring the biotic factors of an ecosystem is useful for research and monitoring. When combined with abiotic measurements, the data can help to draw a detailed picture of an ecosystem.

Quadrat Sampling Methods

Quadrats can be used for all three types of sampling: random, systematic, and stratified.

• Random sampling is truly unbiased; each member of the population has an equal chance of being selected. A random number generator is used to select coordinates within the study site where quadrat measurements will be taken.

• In systematic sampling, members of the population are chosen at uniform intervals. This method provides guaranteed coverage of the study site and is especially useful when the study site experiences an environmental gradient. Quadrat locations are predetermined using a map or a transect.

• In stratified sampling, the population is divided into known groups, then sampled randomly within these groups. The number sampled in each group is proportional to the size of the group. Quadrat locations are selected randomly within the groups.

Frame, Grid, and Point Quadrats

Let's study the three types of quadrats. Each has an associated calculation used to assess biotic factors.

Quadrats are used in passive sampling – i.e. sampling without removing the organisms from their natural habitat. Organisms are usually counted by hand. Alternatively, you can take a photograph for future analysis.

Quadrats and Transects

Transects are used for systematic sampling– where samples are taken at fixed, regular intervals.

There are two kinds of transects: continuous and interrupted.

• At continuous transects, you record vegetation along the whole length of the transect. They provide a high level of detail, but are very time-consuming to use. Thus, they're only suitable for short distances.

• At interrupted transects, you record vegetation at regular intervals along the transect. Using an interrupted transect is much quicker, but doesn't provide as much detail as a continuous transect.

When systematic sampling is combined with abiotic measurements, the data can be used to demonstrate ecological succession. The community composition varies depending on the physical conditions of the ecosystem.

Fig. 3 – Yellow dunes in Hayle, Cornwall. Source: unsplash.com

Kite Diagrams

Kite diagrams allow the comparison of frequencies along the same transect. The transect is shown on the x-axis. The frequency of each species is indicated by the width of the 'kite'.

Evaluating Plant Populations with Quadrats

When studying vegetation, it's important to consider your quadrat size. The size of your quadrat is usually related to the size of the vegetation being studied. For example:

• 10 cm x 10 cm quadrats: used for very small plants, e.g. algae or bryophytes

• 25 cm x 25 cm quadrats: used for short grassland or other low-growing vegetation

• 50 cm x 50cm quadrats: used for long grass or heathland

Furthermore, when planning Fieldwork, you need to consider the number of quadrat samples you will take. You must take enough samples to minimise the effect of anomalies and perform statistical tests, but not too many that you run out of time. It's recommended that 10 quadrat samples is the minimum for each study site.

Now, let's look at the calculations that are carried out using quadrat data. These calculations can be used to determine population size, species richness, species distribution, and biodiversity.

Percentage Frequency

You can calculate the percentage frequency of a species using data from your frame quadrat.

Percentage Frequency = (number of quadrats containing target species ÷ total number of quadrats) x 100

Percentage Cover

You can estimate the percentage cover of different species using a gridded quadrat. If an internal square is over half occupied by a species, it is assumed that the species covers the whole square. The results from multiple quadrats can be amalgamated and averaged to get an estimate for the entire study area.

Local Frequency

You can estimate local frequency using data from a point quadrat.

Local Frequency = (number of hits on a species/total number of pin drops) x 100

If 15-point quadrat measurements were taken, that's a total of 150 pin drops. Let's say that 40 pins hit thistles.

Local Frequency = (40 ÷ 150) x 100 = 26.7%

Using Quadrats

Pay close attention to this section – you may need it for planning practical work, and it's a typical exam question.

• Place tape measures at right angles to divide the study site into a grid.

• Use a random number generator to produce coordinates within the grid. Place a quadrat at each coordinate.

• Use a key to identify the species present in each quadrat.

• Then, count the number of individuals, or estimate percentage cover within each quadrat. If you're pushed for time, take photographs of each quadrat instead and do this part later.

• Record your results in a table.

• Calculate the average abundance of each species.

Advantages and Disadvantages of Quadrats

Let's weigh up the pros and cons of quadrat sampling.

I hope that this article has clarified quadrats for you. Remember that they are used to identify a sample area and study the sessile organisms within it. There are three types of quadrat: frame, grid, and point. Each has an associated calculation for assessing biotic factors.

^{1. Lorna Dawson, Sand Dunes, The James Hutton Institute, 2015}

^{2. Neil Chalmers, The OU Project Guide: Fieldwork and Statistics for Ecological Projects, 1989}