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Use of Random Sampling.

Comparing Species Diversity in Rainforest and Grassland.

In order to carry out this exercise we will use quadrats to carry out random samples within an area of mature rainforest and an area of grassland.

Rainforest trees are clearly different in size from grassland plants. Therefore while a standard 1m square quadrat works well in the grassland, our rainforest quadrat should be a little bigger.  One method is to use a tape measure to define a circular quadrat of 30m2 This has a radius of 3m.

Identifying random sites.  In both rainforest and grassland we can operate within an area 100 m long by 20 m wide to the right of the path. Using a random number generator to generate 2 sets of numbers, we can identify sample points. The first set of numbers, between 0 and 100 gives us the distance along the path, the second set of numbers between 1 and 20, gives us the perpendicular distance to the right of the path.

How many sites?

We have to survey enough sites to make the effect of random variations insignificant. Each group should survey 1 rainforest site in each habitat and 2 grassland sites. you need to establish

A)How many different species are present and

B) how many individual plants of each species.

Remember it is not necessary to identify species, but you will have to be careful, particularly in the rain forest, to avoid counting saplings and the same mature trees as different species, and to try to work out how many plants there really are when dealing with plants like vines which have multiple stems widely spread out.

To quantify diversity we can use the Simpson Index of Diversity.

There are various versions of the formula and of the index but we can use this one;


Where n is the number of organisms of a particular species and N is the total number of organisms of all species - 


The possible values are between 1 (infinite diversity) and 0 (only one species present)

We do not need to identify the species, just recognize different species of plants and count them.

Results table headings;

  • species (if unidentified use tree 1/ vine 6/ herbaceous plant 4/ epiphyte 2/ grass 3 etc.)

  • Number (n) (of that particular species)

  • n(n-1) for the formula



Use the random number generator to choose locations for your sampling like this;

  1. generate a random number between 1 and 50. This will be the distance from the marked start point along the path. You can measure it in PACES.

  2. generate a second random number between 1 and 20. This will be the distance in paces at right angles to the path.

  3. toss a coin to decide right or left off the path.

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