(2A) Surveying Methods 1,  Surveying using quadrats

A quadrat is defined as a frame, traditionally square, used in ecology and geography to isolate a standard unit of area for study of the distribution of an item over a large area.

You can measure how common an organism is in two or more sampled areas of a habitat using a small quadrat and comparing the distribution numbers of species of plants or animals in each location in a much larger area.

For a plant in the same habitat (e.g. same field) you might choose dry/damp areas or bright light/shaded areas or any permutation of conditions (here 4 possibilities, yes?).

Suppose you are surveying a field, you can place the 1 m² quadrat in specific locations or choose some places at random over a wide area.

The frame of the quadrat can be made of wood or metal. Illustrated is 1 m x 1 m quadrat and wire strung across at 10 cm intervals. In this case there 100 10x10 cm square possibilities for sampling, each has x,y coordinates of 1-10,1-10. You do NOT count all 100 mini-squares, instead you can use a random number function on your calculator to select e.g. 10 of them. The square with x,y co-ordinates of 7,4 is shown on the quadrat diagram. This 'mesh' size is ok for very small organisms e.g.  tiny flowers.

I wrote myself a quick computer programme in BBC basic (above left) to generate 10 random  x,y coordinates (above right). Link to the above programme (it might work on Microsoft platforms after querying it, probably won't work on other platforms?)

After placing the quadrat at selected locations you e.g. count the flowers in each 10 x 10 cm² square or the total in the whole1 m² of the quadrat - the whole quadrat is 1 m x 1m.

Here the yellow flowers are quite large and best counted per 1 m², giving you quantitative data e.g. species of flower/m².

To count the population using 10 x 10 cm squares it needs to be a very small flower or insect.

Photos from the Cornfield wild flower project Hutton-le-Hole - Ryedale Folk Museum

Again, I've superimposed a 1 m² quadrat, sub-divided into 20 cm x 20 cm smaller quadrats.

Here you could count each species of flower per m² or choose a smaller are of 20 x 20 cm² (0.04 m² quadrat) or 40 x 40 cm² (0.16 m² quadrat) areas - you just have to make a sensible decision.

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(2B) Examples of quadrat calculations based on sampling data

Example 1. Calculating a population density

Suppose you did a count of some very small species of flower in 10 of 10 cm² mini-quadrats (10 cm x 10 cm) of a 1 m² quadrat placed in a sunny location. The mini-quadrats can be selected using the random number generator.

Data counts 1-10: 7, 8, 12, 9, 9, 10, 11, 10, 9, and 8 flowers

Total count = 93 flowers

Average per 10 cm² = 93/10 = 9.3 flowers/mini-quadrat

Now there are 100 10 cm² squares in the full 1 m² quadrat.

Therefore total in 1 m² quadrat = 9.3 x 100 = 930 flowers.

The 'flower density' = 930 per m²

If you repeated the measurements in a more shaded spot, you might find a much lower population density of the same flower.

 

Estimating a population size from the population density

Using the above data from small sampling areas (see also example 2).

If you know the total area, call it A in m², you just multiply the 930 x A = total population in that area (see next example).

This is just a scaling up exercise from several small sample areas chose at random.

 

Example 2. Calculating a population size (abundance)

Suppose you counted the abundance of a relatively rare flower using a 1 m² quadrat placed 8 times at random across a piece of land (its habitat) measuring 80 m x 120 m.

Flower data counts 1-8: 2, 5, 0, 1, 2, 0, 1 and 4

(a) Calculate the average density of the rare flower per metre²

Total flower count = 15

Flower density = 15/8 = 1.875/m²  (no need to round up at this stage)

(b) Calculate the whole population size of the flower in this particular habitat

Total area of habitat = 80 x 120 = 9600 m²

Total population = density x total area

Population size = 1.875 x 9600 = 18,000 flowers

(maybe its not that rare in this made-up calculation!)

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Key points - ecological surveying using quadrats

Based on the syllabus-specifications for students taking the UK AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10 biology).

Revision notes tailored for biology students on Surveying Using Quadrats - a key method in ecology for studying the distribution and abundance of organisms in a habitat.

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What Is a Quadrat?

A quadrat is a square frame, typically 0.25 m² or 1 m² in size, used for sampling organisms in a specific area.

It helps estimate population size, distribution, and biodiversity without having to count every organism in a large area.

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When and Why Use a Quadrat?

Quadrats are used when:

• Investigating how organisms are distributed in a habitat.

• Comparing biodiversity between two areas.

• Estimating population density of stationary or slow-moving organisms (e.g., plants, lichens, moss, small insects).

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Types of Quadrats

• Frame quadrat: A square frame with or without a grid. Most common type.

• Point quadrat: A frame with a horizontal bar and pins dropped vertically; record what species each pin touches.

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How to Use Quadrats – Step by Step

1. Mark out a sampling area using tape measures (e.g. 10m x 10m).

2. Use a random method to decide where to place quadrats. (e.g., use random number generators or coordinates to avoid bias).

3. Place the quadrat on the ground at each selected coordinate.

4. Record your data:

   • Count the number of each species (abundance).

   • Record the presence/absence (frequency).

5. Repeat in multiple spots for reliable results.

6. Calculate mean values and estimate total populations:

7. To calculate the estimated total population size from a quadrat survey, you need to know the area of your study area, the area of your quadrat, and the number of organisms counted within the quadrats.

8. The formula is:

9. Estimated Total Population = (Total Study Area) / {(Quadrat Area) x (Average number of organisms per quadrat)}

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Transect Lines (for Distribution Studies)

• A transect is a straight line across a habitat.

• Place quadrats at regular intervals along the line (e.g., every 1 metre).

• Useful for observing how abiotic factors (e.g., light, moisture) affect species distribution.

  See section (3) Surveying using transects

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What You Might Measure

• Species richness: Number of different species.

• Species frequency: How often a species appears.

• Percentage cover: Estimating how much of the quadrat is covered by a particular organism.

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Tips for Exams

• Always justify use of quadrats (“to reduce bias”, “to allow for random sampling”).

• Be clear about why repeated samples increase accuracy.

• Know how to interpret results in relation to environmental factors.

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Keywords, phrases and learning objectives for this part on ecological surveying and biodiversity

Be able to describe methods of data collection for surveying the abundance of plant and animal species using quadrats.

Be able to do quadrat calculations based on sampling data as a measure of ecology biodiversity.

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