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understanding of the temporal variation both before and after. With this
improved temporal resolution, if significant interactions were found, the
interaction between before and after with respect to the control and impacted
areas should again be significant; the differences would not reflect just random
noise, which might have been the case if there had been only one data point
before and one after. In this case, sustained changes could be recognised but
it would still be impossible to attribute them directly to an anthropogenic effect
because they might have come about through some other process that had
nothing to do with the anthropogenic influences. Moreover, this approach was
insensitive to pulse-type events, which were typical of the kind of perturbation
expected. A pulse-type event involved a change followed by a slow return to
the level of the ambient community. However, this kind of statistical approach
would not easily recognise the interaction between before and after.
Moreover, even a noise event could influence a finding of significant
interaction.
Another approach, proposed by Tony Underwood in a series of papers
from 1991 to 1997 and recently summarised in his book on experimental
ecology 14 , called for using multiple controls to deal with this problem. While
the number of impacted sites could not be increased, the number of control
sites could be expanded. He suggested setting up an asymmetrical analysis of
variance (ANOVA) with a series of control sites instead of just one. Again, a
series of samples would be taken before and after, replicated at each station.
The idea would be to look for an interaction effect, which would appear as a
greater change in impacted sites than the average change in the controls. In
statistical terms, an analysis of variance table would show how to identify the
impact. The response of the impact locations to the anthropogenic influence
would be different from the response of the control sites, in a way that could
be separated from the effects of a natural event. The earlier design, with only
one sample before and one sample after, was no good because it could not
detect whether a spurious event had brought about the differences. Such an
event could be spatially located to make it look as if there had been an impact
or to remove the effect of an impact. The advantages of the new design were
that it had spatial replication, at least among the controls; it had temporal
replication with respect to both the controls and the impacted sites, and it
allowed the impact to be detected rather unequivocally.
Sampling design for impact assessment
Etter then offered his calculation of how such a sampling strategy could
be implemented in a claim area. He recommended that at least two control
sites be compared to one impacted site. The impacted site, which might be in
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