10.8 - Random sampling Flashcards
(13 cards)
What is sampling a method of?
a method of investigating the abundance and distribution of species and populations
What is within random sampling?
In random sampling the positions of the sampling points are completely random or due to chance
This method is beneficial because it means there will be no bias by the person that is carrying out the sampling that may affect the results
What is within systematic sampling
In systematic sampling the positions of the sampling points are chosen by the person carrying out the sampling
There is a possibility that the person choosing could show bias towards or against certain areas
Individuals may deliberately place the quadrats in areas with the least species as these will be easier and quicker to count
This is unrepresentative of the whole area
When is random sampling the best?
When a sampling area is reasonably uniform or has no clear pattern to the way the species are distributed then random sampling is the best choice
Method of choosing co-ordinates/area of examination
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)
Method of calculating population density
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
Calculating the abundance of a species using percentage cover
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
Calculating the abundance of a species using frequency
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%
What is species frequency?
Species frequency is the probability that the species will be found within any quadrat in the sample area
What is species density? Use an example
Species density 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
What are the limitations of using qudrats and transects?
Quadrats and transects can only be used for sessile and immobile species (eg. plants and slow-moving animals)
What is the limitation of using frequency?
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
How can % cover and frequency be used together?
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
