Choice of quadrat size depends to a large extent on the type of survey
being conducted. For instance, it would be difficult to gain any meaningful results using
a 0.5m2 quadrat in
a study of a woodland canopy! Small quadrats are much quicker to survey, but are likely to
yield somewhat less reliable data than large ones. However, larger quadrats require more
time and effort to examine properly. A balance is therefore necessary between what is
ideal and what is practical. As a general guideline, 0.5 – 1.0m2 quadrats would be suggested for
short grassland or dwarf heath, taller grasslands and shrubby habitats might require 2m
quadrats, while quadrats of 20m2 or larger, would be needed for woodland habitats. At the other end of the
scale, if you are sampling moss on a bank covered with a very diverse range of moss
species, you might choose to use a 0.25m2 quadrat.

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To record percentage cover of species in a
quadrat, look down on the quadrat from above and estimate the percentage cover occupied by
each species (e.g. species A – D left). Species often overlap and there may be several different vertical layers.
Percentage cover may therefore add up to well over 100% for an individual quadrat.
The estimation can be
improved by dividing the quadrat into a grid of 100 squares each representing 1% cover.
This can either be done mentally by imagining 10 longitudinal and 10 horizontal lines of
equal size superimposed on the quadrat, or physically by actually dividing the quadrat by
means of string or wire attached to the frame at standard intervals. This is only
practical if the vegetation in the area to be sampled is very short, otherwise the
string/wire will impede the laying down of the quadrat over the vegetation.

Quadrats are most often used for sampling, but are
not the only type of sampling units. It depends what you are sampling. If you are sampling
aquatic microorganisms or studying water chemistry, then you will most likely collect
water samples in standard sized bottles or containers. If you are looking at parasites on
fish, then an individual fish will most likely be your sampling unit. Similarly, studies
of leaf miners would probably involve collecting individual leaves as sampling units. In
these last two cases, the sampling units will not be of standard size. This problem can be
overcome by using a weighted mean, which takes into account different sizes of sampling
unit, to arrive at the mean number of organisms per sampling unit.

There are three main ways of taking samples.

1. Random Sampling.

2. Systematic Sampling
(includes line transect and belt transect methods).

3. Stratified Sampling.

Which method to
use?

1. RANDOM SAMPLING

Random sampling is usually carried out when the area under study is
fairly uniform, very large, and or there is limited time available. When using random
sampling techniques, large numbers of samples/records are taken from different positions
within the habitat. A quadrat frame is most often used for this type of sampling. The
frame is placed on the ground (or on whatever is being investigated) and the animals, and/
or plants inside it counted, measured, or collected, depending on what the survey is for.
This is done many times at different points within the habitat to give a large number of
different samples.

In the simplest form of random sampling, the quadrat is thrown to
fall at ‘random’ within the site. However, this is usually unsatisfactory
because a personal element inevitably enters into the throwing and it is not truly random.
True randomness is an important element in ecology, because statistics are widely used to
process the results of sampling. Many of the common statistical techniques used are only
valid on data that is truly randomly collected. This technique is also only possible if
quadrats of small size are being used. It would be impossible to throw anything larger
than a 1m2 quadrat and even this might pose problems.
Within habitats such as woodlands or scrub areas, it is also often not possible to
physically lay quadrat frames down, because tree trunks and shrubs get in the way. In this
case, an area the same size as the quadrat has to be measured out instead and the corners
marked to indicate the quadrat area to be sampled.

A better method of random sampling is to map the area and then to lay a numbered grid
over the map. A (computer generated) random number table is then used to select which
squares to sample in. (Random number Table). For example,
if we have mapped our habitat , and have then laid a numbered grid over it as shown
(Figure – below) , we could then choose which squares we should sample in by using the
random number table.

If we look at the top of the first column in the random number table, our first number is 20. Moving downwards, the next two
numbers in the random number table would be 74 and 94, but our highest numbered square on
our grid is only 29 (Figure above). We would therefore ignore 74 and 94 and move on to the
next number which is 22. We would then sample in Square 22. Continuing down the figures in
this column, we would soon come across the number 20 again. As we have already selected
this grid for sampling we would similarly ignore this number and continue on to the next.
We would continue in this fashion until we had obtained enough samples to be
representative of the habitat. There are other methods for selecting numbers from a random
number table, but this is the simplest.

In some habitats it
may be difficult to set up numbered grids (e.g. in woodland) and in these a ‘random
walk’ may be used. In this method, each sample point is located by taking a random
number between 0 and 360, to give a compass bearing, followed by another random number
which indicates the number of paces which should be taken in that direction.

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