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. Individualbased
approaches have been developed and applied to predict the impact of environmental
changes on wintering and staging coastal bird populations. How many birds make use of
staging sites is mostly determined by food availability and accessibility, which in the case of
many waterbirds in turn is affected by water level. Many water systems are regulated and
water levels are maintained at target levels, set by management authorities. We used an
individual-based modelling framework (MORPH) to analyse how different target water levels
affect the number of migratory Bewick’s swans Cygnus columbianus bewickii staging at
a shallow freshwater lake (Lauwersmeer, the Netherlands) in autumn. As an emerging property
of the model, we found strong non-linear responses of swan usage to changes in water
level, with a sudden drop in peak numbers as well as bird-days with a 0.20 m rise above the
current target water level. Such strong non-linear responses are probably common and
should be taken into account in environmental impact assessments