Monitoring to detect impacts
Before/After, Control/Impact sampling design
Because they are easily studied, rocky shores provide a means of assessing changes
which may reflect effects in other marine communities. This principle has been used by
OPRU to monitor the impacts of operations at the Sullom Voe oil terminal in Shetland. More
recently several publications have appeared which have developed a solid scientific
framework for impact assessment (e.g. Stewart-Oaten et al., 1986; Underwood
1991; 1992; 1995). When new activities begin within or close to SACs, any potential
impacts on conservation features should be assessed.
The abundance of almost any species will fluctuate over time.
Abundances will also fluctuate from place to place. Perturbations of human origin may be
judged to have had an impact when they alter the scale of these fluctuations over space
and time. In many cases an impact results in changes in the mean abundance of species. For
example, the average number of dogwhelks close to an effluent outflow might be low
compared with other areas and to the same area before the installation of the outflow. It
is also possible that an impact could alter the magnitude (variance) of fluctuations
without affecting the mean abundance (Underwood, 1991). In order to identify the presence
of an impact with a high degree of statistical certainty, it is necessary to establish a
link between the presence of the factor causing the impact and the observed fluctuations
in abundance of target species. The abundance should have changed after the appearance of
the supposed impact, in a way which is not predicted by changes at other similar sites.
Control/Impact sampling design
The BACI (Before/After, Control/Impact) design was proposed by Bernstein and Zalinski
(1983) and Stewart-Oaten et al (1986) as a means of assessing impacts. Measures of
species abundance would be taken at two sites (the site of the putative impact and a
similar, control, site where no impact was expected) on several occasions both before and
after the onset of the putative impact. Repeated measurements allow a determination of
whether observed changes at the site of the putative impact are part of the pre-existing
cycle of change. The control site is intended to show whether any observed disruption in
this cycle is part of a wider effect not due to the putative impact. The use of a single
control led to criticism of this method (Underwood, 1992). A natural change at the control
site which was coincidentally similar to that caused by the impact at the other site could
lead to the impact going undetected. Alternatively, a change at the control site from
before to after the onset of the putative impact while the other site remained unchanged,
could result in an impact being diagnosed where there was none. The solution, proposed by
Underwood (1992), is to use several randomly-selected control sites. Thus the effects of
global trends can be separated from those of natural fluctuations within individual sites.
Further details on the design and analysis of this type of sampling programme can be found
in Underwood (1991; 1992).