Use of Quadrats: A* understanding for IGCSE Biology 4.2 4.3

This is going to be a dull post so I shall get my apologies in early…. Please do not read this if you are hoping to be inspired by the beauty of science nor if you want to learn about cutting edge technologies.  If you are struggling with insomnia or have a strange interest in square-shaped pieces of metal, this post might be suitable for you.

A quadrat is just a small square used in environmental biology to estimate populations of plants or to sample within an ecosystem.

1099-01_prd1347

This quadrat is 50cm by 50cm and divided into 25 smaller squares, just to make it slightly less dull….

There are two uses of quadrats.  The first is to make estimates of population, the second to investigate the distribution of organisms in an ecosystem.  Your school probably has playing fields and on the playing fields there will be plants such as dandelions.  But how many dandelions live on your playing field?  Well if you were feeling desperate, you might distract yourself by attempting to find this out.  You could cover every square inch of the field and keep an accurate tally count of the dandelion plants you find.  But this would take a long time and even plant ecologists have better things to do with their time.   So a quadrat can be used to randomly sample the field to save you the bother of counting every single plant.

How to use quadrats in random sampling?

Each quadrat has an area of 0.25m2 (50cm by 50cm).  So let’s imagine you randomly place your quadrat 10 times on the playing field and count the dandelion plants in it each time.   You get the following results:

1,1,3,4,0,3,0,1,0,2

This gives an average of 1.5 dandelions per quadrat.  If this is a representative sample of the total population, you can now estimate the total population of dandelion plants in the field.  You will need to know the total area of the field – let’s pretend it is 200m2.

So each quadrat contains an average of 1.5 dandelions.  How many quadrats in total represent the whole playing field?  You would need 800 quadrats to cover the whole field, so our estimate for the total population of dandelions is 1.5 x 800 = 1200 dandelions.

How do you get random samples?

The only way to sample randomly is to use truly random numbers to represent coordinates in the field.  Shutting your eyes and spinning round until you are disorientated before hurling the quadrat or perhaps alternatively dropping the quadrat from a great height both sound like random procedures but they are not….

Random numbers can be obtained from tables or from a website such as random.org

PB_fieldwork-using-your-school-playing-field-1

Sampling with a quadrat can only be done if the organisms you are trying to count do not move around, so basically it works for plants and barnacles.

Right I am feeling sleepy just writing about quadrats so am off for a lie down.

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