• 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.

When it comes to A2/A-level Biology, do you know to conduct random sampling?

• 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

In part 2, we will cover the factors affecting the population size of both animals and human.

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Drafted by Bonnie (Biology)