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Probabilistic analysis of ecological effects - Cause-effect chain modeling

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 Tool - 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

General Approach

To be able to predict the ecological impact of a specific human intervention, the pathways by which this activity can interact with flora, fauna or ecosystems have to be known. For a deterministic as well as a probabilistic approach, the assessment of this interaction starts with insight into the cause-effect chains or networks. Figure 1 shows an example of a cause-effect chain. Figure 3 illustrates the different steps in the approach for modelling ecological effects.
For a quantitative prediction of the effects of the above interaction, it is necessary to find out which quantitative relations exist between the actions and responses. Figure 2 shows an example of a model set-up that can be used in a deterministic as well as a probabilistic approach. The deterministic approach will yield a single value for each effect, the probabilistic approach an approximation of the probability distribution function.

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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 Topic - 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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