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Probabilistic analysis of ecological effects - Cause-effect chain modeling

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The quantification of ecological effects in Environmental Impact Assessments is mostly done by deterministic modelling of cause-effect chains. However, within these cause-effect chains, from construction process to the impact on species, habitats or ecosystems, a large number of uncertainties play a role. Part of them are inherent to natural dynamics, others are caused by a lack of knowledge on the relevant processes. In a deterministic approach, taking into account these uncertainties within the quantification of effects is not possible, so worst-case assumptions are needed to account for them. Often, the predicted impact is based on an accumulation of worst-case assumptions, which yields a highly conservative estimate with an unknown uncertainty margin.

A probabilistic approach deals with these uncertainties in a different way. By using probabilistic computation methods it is possible to incorporate (part of) the relevant uncertainties in the modelling of the ecological effects. A probabilistic approach leads to insight into the probability of occurrence of the possible effects, which is valuable information for ecological assessments.

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Context, purpose and results

The assessment of the ecological effects of hydraulic engineering projects may require a quantitative prediction of these effects. Usually, quantification of ecological effects in Environmental Impact Assessments (Project phase Planning and Design) is done by deterministic modelling of cause-effect chains. The issue here is that within these cause-effect chains - from construction process to the impact on species, habitats or ecosystems - a large number of uncertainties play a role. Some of these are inherent to natural dynamics, others are due to a lack of knowledge on the relevant processes in the cause-effect chain (also see Visualising and managing uncertainties for a description of the different types of uncertainties). In a deterministic approach these uncertainties cannot be taken into account and worst-case assumptions have to be made. The accumulation of worst-case assumptions will yield highly conservative estimates of the ultimate effect with an unknown uncertainty margin.

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How to Use

This tool focuses on the application of probabilistic analysis in cause-effect chain modelling. A general probabilistic approach for ecological risk assessment, originating from ecotoxicology, is also available: Probabilistic effect analysis - The Species Sensitivity Distribution. Essential for the probabilistic modelling of cause-effect chains is state-of-the-art knowledge on these cause-effect chains, together with knowledge on probabilistic computation methods.

  1. General Approach
  2. Probabilistic modelling
  3. Probabilistic modelling and the precautionary principle

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Practical Applications

In the Netherlands, the probabilistic analysis of cause-effect chains was worked out for the first time in ‘A probabilistic analysis of the ecological effects of sand mining for Maasvlakte 2’ (Van Kruchten, Y.J.G. , 2008). This study showed that giving insight into the probability of occurrence of ecological effects by using a probabilistic analysis is possible. The study focused on the possible impact of the sand extraction activities for Maasvlakte 2, the Netherlands, on protected sea-ducks in the nature reserve Voordelta. The results showed that the probability of occurrence of significant effects (in the sense of the Birds Directive) was very small, which was valuable information in the discussion about the necessity of implementing mitigating or compensating measures.

In 'Knowledge - Cause-effect chain modelling - Sand mining - Sandwich terns' the probabilistic analysis is worked out for the cause-effect chain from dredging to Sandwich Terns. The methodology is applied on a fictitious case, which shows how the probabilistic analysis can be used in case effects on Sandwich Tern populations are expected. 

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References

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