Abstract
Using expert elicitation results in the displacement matrix tool for assessments
In this section we provide recommendations on the usage of the expert elicitation results for the mortality rate of displaced birds (hereafter ‘displacement mortality rate’) within the main tool that is currently used in assessing risk from displacement – the “Displacement Matrix”, which requires this parameter as an input to the tool.
A very broad operational definition of the displacement mortality rate is that it is equal to the ratio of the absolute level of displacement mortality from a windfarm to the number of individuals experiencing displacement by a windfarm, so that the absolute level of displacement mortality from a windfarm is equal to the number of individuals experiencing displacement by a windfarm multiplied by the displacement mortality rate. The number of individuals experiencing displacement by a windfarm can itself, in general, be calculated to be equal to the baseline number of individuals using the windfarm footprint area prior to construction multiplied by the proportion of these individuals that would experience displacement if there were a windfarm (the “displacement rate”). The absolute level of mortality is therefore equal to:
Absolute level of displacement mortality from a windfarm = Baseline number of individuals using windfarm * Displacement rate * Displacement mortality rate
[Equation 1]
The current Displacement Matrix approach, which is extensively used in assessments, performs this calculation for a pre-specified set of values for the displacement rate and displacement mortality rate. A range of values for both of these parameters are used to provide a visual means of examining the consequences arising from the uncertainty associated with both rates. Within the current Displacement Matrix approach the baseline number of individuals using the windfarm is defined in a specific way -- as the maximum monthly estimated baseline abundance within the windfarm footprint within the season of interest, as estimated using at-sea survey data. The current approach focuses only on individuals that use the windfarm and so does not account for indirect effects on individuals that do not use the windfarm but may be impacted by it – e.g., in particular, it does not account for indirect effects on chicks driven by displacement impacts on breeding pairs.
There is currently no direct empirical evidence regarding the displacement mortality rate (see WP1 report), leading to considerable uncertainty in the appropriate parameter values to use within the Displacement Matrix tool. The expert elicitation (EE) within this project used expert judgement to assign values to the displacement mortality rate, for six species in breeding and non-breeding seasons. The displacement mortality rate is a key input to the Displacement Matrix approach, so the results of the EE have clear relevance to the way that the Displacement Matrix is used and should be considered in relation to updating SNCB guidance for this approach. However, there are a number of important issues that need to be taken into account when using the results of the EE within the Displacement Matrix approach:
(1) the way in which the EE accounts for uncertainty and variability
(2) the way in which baseline abundance is estimated within assessments, and how this relates to the biological interpretation of seabird space use of windfarm footprints developed by the experts and used to frame displacement mortality rate estimation within the EE
(3) the fact that the EE produced separate estimates for indirect displacement effects on chicks as well as for direct effects on adults