Random and Systematic Sampling Flashcards
Why do ecologists take small samples from habitats rather than identifying and counting every organism
Because its time consuming, virtually impossible and would destroy habitats
How can we ensure samples are representative? What must the number be
Take many samples to ensure it is representative
The number must be enough to minimise the risk of anomalies but not too large that it cannot be realistically carried out in the time available
Why is random sampling important
Because it is unbiased and means every area has an equal chance of being chosen
What can be counted in a quadrat
Frequency (Total number of individual)
Number of species
Percentage cover
How do we place quadrats to make it random
Use a grid or divide an area in to squares
Use a calculator/computer/random number generator to obtain random coordinates
What is recorded with a frame quadrat
Abundance of species within the quadrat
What is recorded with a point quadrat
Each individual species that touches the pin
What is recorded with a line transect
Organisms at the regular sample points
What is recorded with a belt transect? Where are the quadrats placed?
Quadrats are placed at regular sample points and record the abundance of organisms within each quadrat
Description of line transect
String/tape is stretched across the ground in a straight line
Description of belt transect, how wide is it usually
A strip,usually a metre wide, is marked by putting 2 lines parallel
Advantages of using a line transect
Quicker than belt
Any organism that passes over the belt gets recorded
Advantages of using a belt transect
There is more data on abundance
Data is more representative
Disadvantages of using a line transect
Sampling is limited to only organisms that touch the line, so data may not be representative
Disadvantages of using a belt transect
Time consuming
When is systematic sampling used
When we want to monitor changes in a community along an environmental gradient
Why do we use the mark, release, recapture technique
Because animals are too mobile to measure abundance using quadrats, so we use this technique to count how many there are
Steps to the mark, release, recapture technique
- Capture the 1st sample, count them, mark them harmlessly and release them
- Go back to the site a 2nd time, capture a return sample, count them, recognise those in the sample who have been caught twice, release
What can we do from comparing data from each mark, release, recapture sample
Estimate the size of the population
What is the equation for estimated population size
(Total number in 1st sample x Total number in 2nd sample) / Number of marked recaptured animals
What assumptions are made with the mark, release, recapture technique
That the proportion of marked to unmarked individuals in the 2nd/return sample is the same as the whole population
That marked individuals from the first sample distribute evenly amongst the remainder of the population after release
That no immigration or emigration occurs
That there are few deaths or births
That the method of marking the animals is non toxic
That the mark is not lost or rubbed off of the animals
Why could marks on animals affect their survival? What would this cause for the data
Marks on the organisms could affect their survival if they can no longer camouflage and hide from predators
This would cause overestimation
How can overestimation be caused
If survival is affected by marking animals
If the mark is washed off or removed by the animals
If unmarked organisms immigrate into the population
What are the two main types of trap
Pitfall trap
Sweep net
