Practical Random sampling

Question 1

Apparatus

Answer 1

Quadrat
Random number generator

Question 2

Method

Answer 2

Mark up a grid on a map or a to-scale drawing of the area being studied and label the grid with coordinates

It is important that the area is big enough to get a representative estimate for the specific habitat/ecosystem

Use a random number generator to choose a set of coordinates

To avoid sampling bias which could lead to over or under-estimation (either subconsciously choosing areas that are easier to access or that look like they contain individuals)

Question 3

Calculating Population Density (number of individuals per m­­­2

Answer 3

Place a quadrat at the generated coordinate

Count the number of individuals in each quadrat

Use a running mean to determine the number of quadrats required to get a representative sample:

Calculate the mean number of individuals per quadrat for the first two quadrats found, then the mean of the first three, then the mean of the first four –there is no change in mean

Multiply the mean number of individuals per quadrat by the whole area to calculate the estimated population size

Question 4

Calculating the Abundance of a Species using Percentage Cover (%):

Answer 4

Usually used for plants
Place a grid quadrat at this coordinate
Each square of a grid quadrat is equivalent to 1% cover

Count the number of squares in each quadrat within which the species occupies over half the square

If 30 squares contain the species, the percentage cover is 30%

This method is subjective and therefore the same person should make the estimate for all samples to control this variable

Question 5

Calculating the Abundance of a Species using Frequency (%

Answer 5

Place a frame quadrat at multiple coordinates generated

Count the number of quadrats that contain the species

If 3 out of 10 quadrats contain the species the frequency is 30%

Question 6

Results from quadrats species frequency is

and how would you calculate

Answer 6

Species frequency is the probability that the species will be found within any quadrat in the sample area

The number of quadrats that the species was present in is divided by the total number of quadrats and then multiplied by 100

For example, if bluebells were found in 18 out of 50 quadrats the species frequency would be (18/50) x 100 = 36%

Question 7

Species density what is it and how to calculate

Answer 7

indicates how many individuals of that species there are per unit area
The number of individuals counted across all quadrats is divided by the total area of all the quadrats
For example, if 107 bluebells were found across 50 quadrats that are 1m2 each the species density would be 107/50 = 2.14 individuals per m2
.

Question 8

It can sometimes be difficult to count individual plants or organisms. When this is the case ………

Answer 8

percentage cover of the species within the quadrat can be estimated instead
The quadrat is divided into 100 smaller squares. T

he number of squares the species is found in is equivalent to its percentage cover in that quadrat
For example, if grass is found in 89 out of 100 squares in the quadrat then it has a percentage cover of 89%

Question 9

Limitations

Answer 9

Quadrats and transects can only be used for sessile and immobile species (eg. plants and slow-moving animals)

Some species can be counted to find their abundance but others that are very small or in high numbers require abundance to be calculated using percentage cover or frequency techniques

The frequency technique shows how common a species is but it does not give information on the estimated number of individuals or the size of the population
Percentage cover and frequency, when used together, give a good picture of the distribution of a species

If a species had a high mean percentage cover but a low frequency it would suggest the species lives in groups in preferred areas of the habitat

This can be used to answer questions such as: does the species prefer light or dark/wet or dry/crowded or sparse/low or high pH / exposed or shaded environments?

Question 10

In the exam, you could be asked to design an appropriate experiment to investigate the distribution of a species or to note how the overall characteristics of organisms differ across a habitat. For example, calculating whether the size of leaves differs between different environments within a habitat or whether data from random sampling suggests whether a species grows in groups or individually\