
The quantification of ecological effects in Environmental Impact Assessments is mostly done by deterministic modelling of causeeffect chains. However, these causeeffect chains are subject to a large number of uncertainties. Part of them are inherent to natural dynamics, others are caused by a lack of knowledge on the relevant processes. In a deterministic approach these uncertainties cannot be taken into account and worstcase assumptions have to be made. The accumulation of worstcase assumptions will yield highly conservative estimates of the ultimate effect with an unknown uncertainty margin. A probabilistic approach treats uncertainties in a different way, which enables incorporating the most relevant ones in the modelling of the ecological effects. A probabilistic approach leads to insight into the probability of occurrence of the possible effects, which can be of use in discussions about the design of the project or the necessity of mitigating and compensating measures.
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Next to insight into the probability of occurrence of possible effects, a probabilistic analysis also leads to insight into the relevance of different uncertainty factors on the expected ecological effect. This indicates which factors further research should focus on, in order to reduce uncertainty in the predictions (if the nature of the uncertainty allows for such a reduction). Moreover, it is valuable information for the development of a monitoring plan in the Construction or Operation and Maintenance phase. Building with Nature interestA probabilistic approach gives more insight in the functioning of the ecological system, its internal dynamics and its response to human interventions. It shows, for instance, which factors have a relevant influence on the possibly affected species, with what intensity and what probability of occurrence. This information can be used in the design process, during construction works and for the development of an effective monitoring plan.
Sometimes significant effects (in the sense of the Birds and Habitats Directives) cannot be excluded on the basis of a deterministic approach. In that case, probabilistic analysis may be useful in order to decide whether or not mitigating or compensating measures should be taken. Deciding on the basis of a deterministic approach, without information on the probability of occurrence of the adverse effects, may lead to the implementation of unnecessary measures. Probabilistic analyses can provide a better foundation to the environmental regulations applicable to hydraulic engineering works.

This tool focuses on the application of probabilistic analysis in causeeffect 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 causeeffect chains is stateoftheart knowledge on these causeeffect chains, together with knowledge on probabilistic computation methods. This section provides more information on the following:
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General ApproachTo 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 causeeffect chains or networks. 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. For an example of a model setup that can be used in a deterministic as well as a probabilistic approach, refer to the figure in the middle. The deterministic approach will yield a single value for each effect, the probabilistic approach an approximation of the probability distribution function.
Especially in the case of long impacteffect chains, the number of uncertainties will be very large, and they may grow throughout the process. Feedback loops may even lead to their unbounded amplification. A proper elaboration of the probabilistic analysis requires an overview of the uncertainties and their influence on the probability distribution function of the effect to be considered. Depending on its influence on the final result (compared to the effect of other uncertainties), it may be necessary to incorporate a specific uncertainty in the model. The relative influence of an uncertainty can sometimes be estimated analytically, otherwise model testruns must give insight into the sensitivity of the result to a specific uncertainty variable.
Probabilistic modellingWithin causeeffect chains several types of relations and uncertainties can be distinguished. Each type of uncertainty will require a specific method of incorporation in the probabilistic model. Although different data will be necessary for each hydraulic engineering project and each impacteffect chain, the same types of uncertainties will be encountered. Therefore, the method is formulated in such a way that it can be applied to different projects and different impacteffect chains.
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Probabilistic modelling and the precautionary principleThe result of probabilistic modelling is a probability distribution function, in this case of an ecological effect. The inputs and the model used should be as realistic as possible. In view of the precautionary principle, however, the resulting probability distribution function should not give a too optimistic picture of reality. So, the probabilistic analysis should aim for 'as realistic as possible, and certainly not too optimistic'. In the following, examples are elaborated on how one can deal with the precautionary principle in a probabilistic approach.
Precautionary principle: ‘if a reasonable suspicion exists that activities can have negative consequences for the environment, measures should be taken in order to prevent these consequences or, if the prevention of these consequences is not possible, to offer protection against these consequences'. This also means that a conservative assumption will be necessary in the quantitative effect analysis, if no scientific evidence indicates that the real situation might be worse.
Example 1: Estimating probability density functions
Example 2: Lack of knowledge

In the Netherlands, the probabilistic analysis of causeeffect chains was applied 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 it is possible to give insight into the probability of occurrence of ecological effects by using a probabilistic analysis.
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The study focused on the possible impact of the sand extraction activities for Maasvlakte 2, the Netherlands, on protected seaducks 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 of sand mining using Sandwich terns', the probabilistic analysis was applied to assess the causeeffect chain from dredging activities to Sandwich Terns. The methodology is applied on a fictitious case, which shows how the probabilistic analysis can be used if effects on Sandwich Tern populations are expected.
Lessons learnedApplying a probabilistic analysis in causeeffect chain modelling is particularly useful, if the deterministic modelling requires very conservative or even worstcase assumptions. In such cases, the probabilistic approach can make the difference between a very conservative and a realistic estimate of effects. If the deterministic approach is based on realistic assumptions, a probabilistic analysis will still provide extra information, viz. the probability density function of the effect. Using a probabilistic approach for the simulation of the impact of dredging on mussels by a Dynamic Energy Budget model (see Knowledge  Cause  effect chain modelling of sand mining using mussels) turned out to have little benefits, because the deterministic estimate provided enough information. Even if no conservative assumptions in the deterministic model are needed, a probabilistic approach can still be useful to place the estimated ecological impact in the context of natural variability.

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Van Kruchten, Y.J.G. (2008) A probabilistic analysis of the ecological effects of sand mining for Maasvlakte 2, MSc Thesis Faculty of Civil Engineering and Geosciences, Delft University of Technology, Port Research Centre Rotterdam  Delft, ISBN/EAN: 9789056381974 
Ecological landscaping of seabed
Adaptive monitoring of sand extraction areas  Maasvlakte 2 extension, NL
Ecological landscaping of sand extraction sites, NL
Knowledge  Cause  effect chain modelling of sand mining using mussels
Knowledge  Cause  effect chain modelling of sand mining using Sandwich terns
Knowledge  Sediment and ecology in delta lakes
EcosystemBased Design Rules for Sand Extraction Sites
Monte Carlo simulation and Social Cost Benefit Analysis
Probabilistic effect analysis  The Species Sensitivity Distribution