Biodiversity
Laws and regulations
Acceptable Levels of Impact (ALI) of offshore wind on birds
At Waardenburg Ecology, we undertake a range of effect studies including innovative methods based on the latest findings. We use population models to test the effects of additional mortality at the population level.
Within the Wozep programme (Wind Op Zee Ecological Programme, Rijkswaterstaat) we have developed population models for species that are sensitive to the effects of offshore wind farms. With these models, population trends can be estimated based on species-specific survival and reproduction. In this way, different wind farm scenarios can be compared.
Acceptable threshold value
In order to subsequently determine whether a modelled impact is acceptable, a threshold value is required. Together with Wageningen Marine Research, Waardenburg Ecology have drawn up a framework with which policymakers can determine these threshold values; so-called Acceptable Levels of Impact (ALI).
This ALI is defined as:
“The probability that after 30 years, the predicted population size is X% lower with impact compared to without impact does not exceed Y%.”
Within this methodology, a clear distinction is made between science and policy. The population models are based on scientific knowledge, as are the estimates of casualty numbers. From a policy perspective, two species-specific choices must be made within this method.
This ALI is defined as:
“The probability that after 30 years, the predicted population size is X% lower with impact compared to without impact does not exceed Y%.”
Within this methodology, a clear distinction is made between science and policy. The population models are based on scientific knowledge, as are the estimates of casualty numbers. From a policy perspective, two species-specific choices must be made within this method.
Acceptable decrease (X)
The first species-specific policy choice is for X, which indicates a certain level of decline. The value for Y gives a maximum acceptable probability for such a decrease. When this probability is exceeded for the chosen value for X, the conclusion is that the impact is not acceptable.
Acceptable probability of such decline (Y)
Although Y is given in the ALI definition as the maximum acceptable probability of such a decline, the policy choice is not the choice of Y, but the choice of a 'causality probability' (Pc). This is because the effect of the impact is measured against the median population size after 30 years of the no-impact scenario. As variation is included in the population models, each simulation gives a slightly different outcome. The threshold value for X can therefore also be exceeded in the scenario without impact.
The hypothetical frequency distribution below shows simulations of both the no-impact scenario (dashed line) and the impact scenario (solid line). Simulations where the decay from the reference point is greater than X are shown in grey and orange. The policy choice for the causality probability (Pc) indicates how certain we want to be that the exceedance of X is the result of the impact (for example, offshore wind farms). In fact, Pc indicates how much greater the probability of exceedance may become as a result of the offshore wind farms, compared to the scenario without impact (in the figure this is the size of the orange area in relation to the grey area).
The hypothetical frequency distribution below shows simulations of both the no-impact scenario (dashed line) and the impact scenario (solid line). Simulations where the decay from the reference point is greater than X are shown in grey and orange. The policy choice for the causality probability (Pc) indicates how certain we want to be that the exceedance of X is the result of the impact (for example, offshore wind farms). In fact, Pc indicates how much greater the probability of exceedance may become as a result of the offshore wind farms, compared to the scenario without impact (in the figure this is the size of the orange area in relation to the grey area).
Application
This method is used to test the effects of offshore wind farms at a population level. It has been used in the cumulative assessments for planned offshore wind farms in the southern and central North Sea within the Ecology and Cumulation Framework (KEC), as well as in Environmental Impact Assessments (EIA) for offshore wind farms. This method can also be used for other forms of impact.