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In order to capture the complete range of environmental variation
in a designated survey area, sampling units, or strata, need to
be identified. This is also known as stratifying your sampling
procedure (i.e. identifying 'strata' within the area being sampled
and including them into your sampling plan). Stratified sampling
differs from random sampling, in that you are making some presumptions
about the presence of sub-units (strata) within the study area.
However, you will then sample randomly within those strata. (Note:
'Strata' is plural, and 'stratum' is singular.)
The strata chosen will depend on the scale of investigation and
type of organisms being sampled. A number of different types of
strata may be used. The choice of units will depend on the objectives
and scope of the survey. The main types of sampling strata are as
follows:
- geological units and topographical landforms
- soil types
- vegetation types
- drainage catchments
- climatic zones
- bioregions
Some of the information can be derived from sources described
above. However, there is limited information existing for some areas,
or the information may exist but at an inappropriate scale. In this
situation it is necessary to assess the survey area prior to the
main sampling effort to identify the main sampling units that will
be used.
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Please Read
pp. 99-104 of your text book and then consider the following
entry in your e-journal.
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4.2
Stratifying is an important concept to grasp. Let's consider
it a bit further. Please look at the image provided.
This aerial photograph shows areas of savanna woodland adjacent
to a monsoonal floodplain.
What strata do you identify?
How would you stratify your sampling strategy in this terrain
if you were sampling vegetation.
Please email your answers to your lecturer. These questions
will form the basis of our next online discussion.
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