Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
It is rare to try to catalogue everything. In an area of
grassland or woodland, you might choose to concentrate
on just one or two groups, such as flowering plants
and insects. On a rocky shore, a study may involve the
most obvious organisms such as seaweeds and molluscs
(Worked example 3, page 432).
There are now two questions to ask: how are the
different species spread throughout the ecosystem and how
many individuals of each species are there? The answers to
these two questions describe what we call distribution
and abundance.
sample is random is to mark out an area with measuring
tapes and use a random number generator, such as an app
on a mobile phone. The random numbers give you the
coordinates of the sampling points in relation to the two
tapes you have used to mark out the area (Figure 18.8).
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QUESTION
18.3 Some students surveyed the species diversity of an
area of woodland and some grassland nearby. They
used a 20-minute timed search.
a Explain why they used the same technique for the
two areas.
b They found 56 species in the woodland and 12 in
the grassland. What other data could they have
collected to compare the biodiversity of the two
ecosystems?
Sampling
To find out which species are present in an ecosystem,
and the size of the population of each of them, the ideal
method would be to find, identify and count every single
organism that lives there. We can sometimes do this if the
area is very small or the species are very large. But it is only
rarely possible. Instead, we take samples from the area we
are interested in, and use these to make an estimate of the
total numbers in the area.
Sampling can be random or systematic. If an area
looks reasonably uniform, or if there is no clear pattern
to the way species are distributed, then it is best to use
random sampling.
Random sampling using quadrats
A quadrat is a square frame that marks off an area of
ground, or water, where you can identify the different
species present and/or take a measurement of their
abundance. You need to decide on a suitable size for the
quadrat and how many samples you will take.
Samples must be taken randomly to avoid any bias.
For example, you might choose to take all of your samples
from the place with fewest species simply because it is the
easiest to do. This would not be representative of the whole
area you are surveying. The usual way to ensure that a
Figure 18.8 In random sampling, quadrats are positioned
randomly in an area marked off by measuring tapes. This
reduces the chances of bias in sampling the ecosystem.
You can use your results in two different ways: to
calculate species frequency and species density. Species
frequency is a measure of the chance of a particular species
being found within any one quadrat. You simply record
whether the species was present in each quadrat that you
analyse. For example, if you placed your quadrat 50 times,
and found daisy plants in 22 of your samples, then the
species frequency for daisies is:
22 × 100 = 44%
50
Species density is a measure of how many individuals
there are per unit area – for example, per square metre.
The number of individuals that you have counted is
divided by the total area of all your quadrats.
It is not always possible to count individual plants and
animals because of the way that they grow. For example,
many animals and plants grow over surfaces forming a
covering and it is almost impossible to count individuals.
How do you decide how many grass plants there are in a
quadrat that you have placed on a lawn? In this case, you
can estimate the percentage cover of the species within
your quadrat (Figure 18.9). To help with this, you can use
a 100 cm × 100 cm quadrat with wires running across it at
10 cm intervals in each direction, dividing the quadrat into