Cambridge International A Level Biology Revision Guide

Chapter 18: Biodiversity, classification and conservation
QUESTION
18.6 a Calculate D for shore B in Worked example 18.2.
Show all your working as in the table. The easiest
way to do this is to use a spreadsheet. Once you
have set up a spreadsheet you can use it for
calculating this index of diversity for other data.
b Compare the diversity of the two shores.
a very low species diversity. A value near 1 represents a
very high species diversity.
One advantage of this method is that you do not need
to identify all, or even any, of the organisms present to the
level of species. You can, for example, just decide to call
the species of anemone that has short tentacles ‘anemone
A’, and the species that has few long tentacles ‘anemone B’.
So long as you can recognise that they are different species,
you do not need to find their scientific names. But beware:
some species have many phenotypic forms (Figure 18.5).
The higher the number we get for D, the greater the
diversity. You can probably see that the diversity depends
on the number of different species there are, and also the
abundance of each of those species. A community with
10 species, but where only one species is present in large
numbers and the other 9 are very rare, is less diverse than
one with the same number of species but where several
different species have a similar abundance.
Comparisons using this diversity index should be on a
‘like for like’ basis, so the communities should be similar
and the organisms chosen should also be similar. For
example, it should not be used to compare the diversity of
fish in a lake with the diversity of moths in a forest.
Systematic sampling
Random sampling is not suitable for every place that
you may wish to survey. You might want to investigate
how species are distributed in an area where the physical
conditions, such as altitude, soil moisture content, soil
type, soil pH, exposure or light intensity change. For
example, suppose you want to investigate the change at
the edge of a field where it becomes very marshy. In this
case, you should randomly select a starting point in the
field and lay out a measuring tape in a straight line to
the marshy area. You then sample the organisms that
are present along the line, which is called a transect. The
simplest way to do this is to record the identity of the
organisms that touch the line at set set distances – for
example, every two metres. This line transect will give you
qualitative data that can be presented as in Figure 18.11.
You can also use the belt transect technique by placing a
quadrat at regular intervals along the line and recording
the abundance of each species within the quadrat. Data
from a line transect can be shown as a drawing. Data from
a belt transect can be plotted as a set of bar charts or as a
kite diagram (Worked example 3).
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Line transect – a line across one or more habitats
The organisms found at regular points along a line are noted. Transects are used to detect changes in community composition
along a line across one or more habitats.
Interrupted belt transect
The abundance of organisms within quadrats placed at regular points along a line is noted.
Figure 18.11 Systematic sampling using transects: a a line transect, and b an interrupted belt transect.