The effects of manipulation
Quite often, one has to also control if manipulation
within the experimental areas has effected any changes other than what is
stipulated in the hypothesis. Besides the expected effect that the number
of supposed competitors decrease in numbers, other changes and disturbances
can occur when the mussels are removed from a sandbottom. Manipulative disturbances can
on their own affect survival within the experimental areas. Therefore, controls are needed
where mussels are first removed and then replaced so the same effect is
obtained as in the experimental areas, but without actually changing competition
between the competitors. Other controls could also be necessary. It is only
with the help of controls that we can see the effects of natural changes
and unwanted manipulation, and be able to separate these changes from those prescribed in the hypothesis.
Finally, an experiment must be repeated, or
with a scientific term replicated. Several experimental and control areas are needed over the area
that is to be studied. This is necessary as there can exist many natural
factors that effect the number of mussels on different areas of the beach.
During the period that the experiment is in progress, many factors can change
and can vary in different areas.
Suppose that we have only one experiment area and one control
area. When we conclude the experiment and see that the conditions between
the different species of mussels have changed, we will not know why. These
changes can either be due to changes in competition or changes in natural conditions within the experimental
area, compared to the control area. Such a natural change could for instance be when an influx of available food has occured at that particular part of the seabottom where the experimental
area is situated compared to the control area.
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