Predicting Effects of Water Regime Changes on Waterbirds: Insights from Staging Swans

Predicting the environmental impact of a proposed development is notoriously difficult,especially when future conditions fall outside the current range of conditions. Individualbasedapproaches have been developed and applied to predict the impact of environmentalchanges on wintering and staging coastal bird populations. How many birds make use ofstaging sites is mostly determined by food availability and accessibility, which in the case ofmany waterbirds in turn is affected by water level. Many water systems are regulated andwater levels are maintained at target levels, set by management authorities. We used anindividual-based modelling framework (MORPH) to analyse how different target water levelsaffect the number of migratory Bewick’s swans Cygnus columbianus bewickii staging ata shallow freshwater lake (Lauwersmeer, the Netherlands) in autumn. As an emerging propertyof the model, we found strong non-linear responses of swan usage to changes in waterlevel, with a sudden drop in peak numbers as well as bird-days with a 0.20 m rise above thecurrent target water level. Such strong non-linear responses are probably common andshould be taken into account in environmental impact assessment