UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 23/05/2023 [SEARCH]

Surveying ecology : 2. Surveying Methods 1 surveying using quadrats and examples of quadrat calculations based on sampling data

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There are various sections to work through,

after 1 they can be read and studied in any order.

INDEX of biology notes on ecological surveying


(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.

(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!)



INDEX of biology notes on ecological surveying


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