Site-Specific Ecological Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 706

15.4.4 Weighting of Effect Values
Besides the issue of scaling, the risk assessor should pay attention to the issue
of weighting. Weighting applies to different tools, i.e. weighting within a line of
evidence, and applies to weighting across different lines of evidence in the Triad
approach. Some general principles apply to this. As a default, the three lines of evi-
dence in the Triad should be equally weighted. Each line of evidence has its own
weaknesses and strengths. However, together they form the strongest basis for ERA
according to the principles of a balanced WoE approach. In specific cases, specific
considerations demand for a differential weighting between the different lines of
evidence in the Triad approach. The absence of adequate reference sites is typically
the most problematic with ecological field surveys at strongly disturbed sites. In
these cases, ecosystem changes might dominantly be caused by other factors than
soil contamination. Another example of differential weighting is a difficult chem-
ical assessment, because of complicated exposure routes and limited toxicity data.
In that case it is defendable to give a lesser weight to the chemical based assess-
ment ( chemical characterization ) than to the two effect values from the other lines
of evidence ( determination of toxicity and ecological observations ).
Within one line of evidence attention should be given to a suite of aspects within
the ecosystem. Typically, the starting point is an equal weight for all organisms
and processes, applying the following popularized statement: 'All organisms are
unequal, but equally important'. Another possibility is to collate data in different
trophic groups like primary producers, decomposers of organic matter (fragmen-
tation and mineralization) and consumers, and give these different trophic groups
equal weights. Within any individual line of evidence of the Triad approach,
differential weighting of results may be applied for three possible reasons:
1. Ecological considerations, e.g. from different land use classes, may trigger
a differential weighting, which should be defined in the conceptual model.
This allows extra attention to specific (functional) groups, key species, endan-
gered species, 'charismatic' species or even specific ecological processes in the
terrestrial ecosystem.
2. Differential weights can be applied in order to account for the uncertainty or
variation within the end-points. Tests with a high level of uncertainty, or with
a high variation in results, may be given a smaller weight in the ERA (Menzie
et al. 1996 ).
3. Differential weights might be applied in order to correct for biases in the
expected number of false positive or false negative results. For instance, the
geometric mean of the inverted effect value gives extra weight to those observa-
tions with a positive response. This acknowledges the fact that many bioassays
or ecological field surveys are sometimes not able to demonstrate ecological
effects on the screening level, although in reality these effects are present (false
negatives). This is especially a problem with tight budgets or highly dynamic
systems, because the number of replicates is often too limited for demonstrating
significant effects.