To assess the impact of an activity on the environment, investigators often deal with limited knowledge about possible effects. Consequently assumptions have to be made, where the accumulation of worst case assumptions leads to unrealistically pessimistic estimates of the impact. A probabilistic approach will give more realistic estimates and also quantifies the uncertainty of this impact.
The case on Sandwich terns, as described in van Kruchten & van der Hammen (2011), shows that it can be difficult to make a reliable assessment of possible impacts of dredging activities on a tern population, because:
- most of the research reported in the literature is descriptive and does not quantify the links in the impact effect chain; as a result, many of those links remain quantitatively uncertain;
- some of the relations in the chain that could not be derived from measured data, they were made probabilistic with a given probability distribution as they could not be based on measured data;
- some pragmatic (and conservative) assumptions had to be made, by lack of information; consequently, the uncertainty margin of the expected impact on the tern population might differ from what the model results indicate;
- the connection between “prey availability” and “time to catch enough food” had to be based on very limited data and limited background knowledge. As a consequence, the estimated relationship is highly uncertain. Therefore, it is recommended to collect more data on these connections in order to narrow down the uncertainty margins in the model results.
Improving the modelling of impacts on tern populations
Further research on the following topics is recommended to improve the model of the effects of dredging on Sandwich Tern populations.
- How does an increase of turbidity affect the ‘findability’ of food by terns? A point of attention is the possible influence of wind waves (Taylor, 1983), as high turbidity levels often occur simultaneously with high wave events.
- More measurement data on the relation between breeding success and the amount of food provided are desirable. This will enable a better founded estimate of the relation between these factors.
- Do temporal variations in the amount of food that is provided to tern chicks have a significant influence on the breeding success?
- Is the relation between water transparency and prey capture probability, found by Baptist & Leopold (2009), generally applicable?
- Black headed gulls are known rob fish caught by terns (kleptoparasitism). If more quantitative information is available about the impact of kleptoparasitism on the amount of food provided to tern chicks, this effect can be incorporated in the model. A point of attention is the correlation between kleptoparasitism and turbidity, because both factors depend on weather conditions.
- Do terns migrate to other colonies if food conditions are limiting at their regular breeding colony? If so, do they return to their former breeding grounds when the disturbance has disappeared?
- The values of several parameters are estimated or based upon research on other tern species. More research on the values of these parameters for Sandwich terns is desirable. This would result in better estimates of the values and their probability density functions (pdf’s). When insight into these pdf’s is gained, it is recommended to consider whether or not these uncertainty margins can have a significant influence on the final results of the probabilistic modeling. In that case these uncertainty margins have to be taken into account in the probabilistic model.
- Within the foraging area, possibly relevant spatial differences exist in turbidity and the presence of prey fish. A relation might exist between spatial differences in turbidity and the distribution of patches of fish over the foraging area. Research into this subject is recommended, as it may be relevant to incorporate the effects of the local differences in the modeling of the impact of dredging activities on tern populations.
- Other visual hunting birds could be affected by sand mining in a similar ways as Sandwich terns. The vulnerability of a species will depend on several factors such as their diet restrictions and the location and size of their foraging grounds.