This is a legacy topic. View the most up to date content by selecting the exam board in the dropdown to the left.
Where do you even begin to count the number of organisms in a field, for example? How can quantitative data be obtained for a rocky shore? It’s not feasible to assess every single individual plant, count all the crabs you can possibly find, or estimate the abundance of all types of grass along a shingle ridge. Even if that was possible, the data would still not apply to the make up of a population, for example, at different times.
The answer is to obtain a sample. If a sample of crabs on a shore was to be taken, where would you look? In the spot where you can already see 3 of them, or in the spot where there are none? Well, in order for the sample to be representative, you must not involve yourself, as the experimenter, in the process of deciding the sample locations. That would incur experimenter bias and would render you hard-earned precious data invalid.
The sampling must be like those annoying, attention-seeking Facebook friends. It must be random. Random sampling can be carried out using quadrats. If you’re wondering what they are, look no further – they’re squares.
How would you make sure that your sampling is random? In a field, you could lay two long tapes perpendicularly to define the limits of the area where the samples will be taken from.
As you can see above, a tape is laid on one side of the sampling area. As you can’t see above, another tape is laid from one end of the first tape, across on the adjacent side of the sampling area (like a giant L). Then two random numbers are generated using a random numbers table. These numbers are used to determine the coordinates of the first quadrat placed on the field, by matching them on the two tapes. And voila! You have yourself a system for random sampling using quadrats.
Transects are tapes (like above) placed across an area which has some form of gradient caused by abiotic factors which directly determines the distribution and abundance of the organisms present. For example, a beach is not suited for random sampling because there are clear zones ranging from the low population zone near the sea, to the more densely inhabited areas further up the shore. In this case the best way of obtaining useful data is by systematic sampling.
After placing the tape across the shore, place quadrats at set intervals such as every 5 metres, then take your data down.
Depending on the size and type of organism, data can be collected in the form of numbers by counting the present organisms in each quadrat (frequency), or working out the percentage of area within a quadrat that a species occupies (percentage cover), then scale it up to the whole area investigated by multiplying.
Mobile species such as shrimps can’t be counted by the quadrat method. Instead, they are investigated using the mark-release-recapture method. This is something I personally did on my field trip for A level:
1. Capture shrimps using nets and count them.
2. Mark them by nipping half their tail diagonally (not proud :D)
3. Repeat, ensuring to account for the marked shrimps.
The more marked individuals you get, the smaller the total population is likely to be.