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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 occurence of the possible effects, which is valuable information for ecological assessments.

    General Tool Description

    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.

    A probabilistic approach treats uncertainties in a different way, which enables incorporating the most relevant ones in the modelling of the ecological effects. It gives insight into the probability of occurrence of these effects, which can be of use in discussions about the design of the project or the necessity of mitigating and compensating measures.

    Next to insight into the probability of occurrence of possible effects, a probabilistic analysis also leads to insight into the relevance of different uncertain factors on the expected ecological effect. This shows on which factors further research should focus, 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.

    Usage skills

    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: the Species Sensitivity Distribution (SSD). 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.

    BwN interest

    A 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, prababilistic 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 occurence 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.

    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.

    Especially in case of long impact-effect chains, the number of uncertainties will be very large and they may grow through 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 will be necessary or not to incorporate a specific uncertainty in the model. The relative influence of an uncertainty can sometimes be estimated analytically, otherwise model tests runs must give insight into the sensitivity of the result to a specific uncertain variable.
    To identify all relevant uncertainties, it is useful not only to look at the cause-effect chain 'bottom-up' but also 'top-down'. For example, in case of the cause-effect chain of Figure 2 this means that not only the question 'which factors may be affected by dredging' (bottom-up) needs to be answered, but also 'which factors influence the number of sea ducks?' (top-down). This may reveal more relevant factors than the bottom-up approach only, factors that have to be taken into account in the probabilistic analysis. The number of sea ducks, for instance, may not only depend on the possibly affected bivalve population, but also on a second food source. In case of an abundance of this food source, the eventual decrease of the bivalve population does not necessarily mean a decline of the number of sea ducks.

    Figure 3 shows the phasing of the modelling process (which may be iterative).

    Probabilistic modelling

    Within cause-effect 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 impact-effect 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 impact-effect chains.
    For the overall modelling of ecological effects, in most cases a Monte Carlo analysis is likely to be the most suitable probabilistic computation method. The Monte Carlo method relies on repeated random sampling of input variables from probability density functions. The ecological impact is computed for a large number of input variable sets, using the (deterministic) quantitative relations of the cause-effect chain. Subsequently, all results can be analysed statistically, to yield probability density functions of the impact variables.

    For the random sampling, probability density functions have to be defined for each relevant stochastic input variable. How this can be done for different types of uncertainties is illustrated in the following examples:
    1. Uncertainties on quantitative relations;
    2. Uncertainties caused by natural variations;
    3. Uncertainties caused by a lack of knowledge;
    4. Uncertainties if impacts occur only under specific conditions.

    Probabilistic modelling and the precautionary principle

    The 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 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 if on cannot exclude on a scientific basis that things might be worse in reality, a conservative assumption will be necessary in the quantitative effect analysis.

    Example 1: estimating probability density functions
    In case of a probabilistic analysis, probability density functions (pdf's) have to be estimated for several stochastic variables. If sufficient, good quality data are available, an 'indisputable pdf' can be derived. However, if discussion is possible on the interpretation of for example measurement data or if pdf's have to be based on limited expert judgment, a conservative assumption on the pdf's will be necessary, in accordance with the precautionary principle. Please note that good insight in the interactions between input variables and their effect on the modelling results is necessary for being able to do a conservative assumption.

    Example 2: lack of knowledge
    By using probabilistic modelling techniques, conservative assumptions that have to be made in case of a deterministic approach can be prevented. Uncertainties due to a lack of knowlegde, however, are an exception to this rule. Here the same assumptions may have to be used as in the deterministic approach. Also see 'Uncertainties caused by a lack of knowledge' and Tool - Visualising and managing uncertainties. In case of a complete lack of knowledge, even a worst-case assumption can be difficult or impossible. 

    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. 

    Lessons learned

    Applying a probabilistic analysis in cause-effect chain modelling is particularly useful if in the deterministic modeling very conservative or even worst-case assumptions are necessary. 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 Topic - Probabilistic effect analysis - Cause-effect chain modelling Sand Mining- Mussel) turned out to have little benefits, because the deterministic estimate provided enough information. In other cases without conservative assumptions in the deterministic modeling, however, a probabilistic approach can still be useful to place the estimated ecological impact in the context of natural variability.


    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: 978-90-5638-197-4

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