An individual-based model of swan-macrophyte conflicts on a chalk river

Foraging mute swans Cygnus olor can cause a substantial reduction of aquatic macrophyte biomass in chalk rivers in southern England. To reduce the adverse effects of this on ecology, hydrology and fisheries interests a river management strategy needs to be able to predict where and when grazing pressure will be greatest. To assess the suitability of mathematical modelling for this purpose, an individual-based model (IBM) was constructed and tested for swans grazing a 1.1km length of the River Frome and adjacent pasture land in Dorset, England. Comparison of predicted and observed data indicates that the model was good at estimating the number of swan days, macrophyte biomass depletion, swan feeding time and feeding patch use. The results indicate that IBMs used at the appropriate scale could help to reconcile river management conflicts involving swans.