To study a habitat, it is often necessary to measure the abundance of a species in a given space.
Random and systematic sampling techniques are used to obtain a representative sample, therefore conclusions will be valid.
- There are three factors to consider when using quadrats.
(1) Size of the quadrat - This will depend upon the size of the plants or animals being counted and how they are distributed within the area.
(2) The number of quadrats to record - The larger the number of sample quadrats, the more reliable the results will be. However, as the recording of a species within a quadrat is time consuming, a balance must be struck between the validity of the results and the time available.
(3) The position of each quadrat - To produce statistically significant results, random sampling must be used in order to avoid bias.
- Lay out two long tape measures at right angles along two sides of the study area.
- Obtain a series of coordinates by using random numbers taken from a table or generated by a computer.
- Place a quadrat in the intersection of each pair of coordinates and record the species within it.
Issues may arise where organisms are only partly within the quadrat.
It must be decided beforehand how these will be counted and the decision reached must be carried out consistently throughout the fieldwork.
It is sometimes more logical to measure abundance and distribution using a systematic method, particularly where some form of transition in the community takes place.
A line transect comprises a string or tape stretched across the ground in a straight line.
Any organism over which the line passes is recorded, or alternatively, the species present are recorded at set intervals on the line.
Belt transects may also be used.
It is a strip, usually a metre wide.
Species within the belt are recorded or quadrats are placed within it at set intervals (interrupted belt transect).
This method is more time consuming than the line transect but is more representative.
The measure of abundance within these two sampling techniques can either be measured by finding the frequency (more time consuming) or the percentage cover (subjective).
Since most animals are mobile, it can be difficult to estimate their abundance using the above methods, we therefore use the mark-release-recapture technique.
A known number of animals are caught, marked in some way, and hen released back into the community.
A set amount of time later (around a day), the same number of individuals are collected and the proportion of those that are marked is recorded.
Estimated population size = (no. of individuals in 1st sample x no. of individuals in 2nd sample) /
(no. of marked individuals recaptured)
This technique relies on a number of assumptions.
- The proportion of marked to unmarked individuals in the second sample is the same as the proportion of marked to unmarked individuals in the population as a whole.
- The marked individuals released from the first sample distribute themselves evenly among the population and have sufficient time to do so.
- No immigration or emigration into and out of the population.
- There are few, if any, births and deaths within the population.
- The method of marking is not toxic, nor does it compromise camouflage and make the individual more liable to predation.
- The mark is not lost or rubbed off.
- Where possible, the organisms should be studies in situ.
- If it is necessary to remove them, the numbers taken should be kept to a minimum.
- Any organisms removed should be returned to their original habitat, even if they’re dead.
- A sufficient period of time should elapse before the site is used for fieldwork again.
- Disturbance or damage should be avoided
Drafted by Bonnie (Biology)