Hiding from swans: optimal burial depth of sago pondweed tubers foraged by Bewick’s swans

1 We used a combination of laboratory and field experiments to test the hypothesis that the burial depth of Potamogeton pectinatus tubers will vary with local sediment type and swan predation pressure. 2 In the field, mortality due to predation by swans decreased linearly with burial depth (from 100% at the surface to 55% at 225 mm depth) and with sediment clay content. Average tuber size showed an eightfold increase when burial depth increased from 25 to 275 mm. 3 A laboratory experiment showed that plant emergence in spring decreased with increasing planting depth and with decreasing tuber size. 4 An optimization model combining these empirical results showed that optimal tuber survival is achieved if tuber size and burial depth increases as swan predation pressure rises. The model predicted different optimal combinations of tuber size and burial depth for sandy and clay-rich sediments. 5 A common-garden experiment showed that plants grown from large tubers produce larger tubers. Significant clonal variation in tuber size was also detected: after standardizing for tuber production, plants originating from sandy sites produced larger tubers than those originating from clay-rich sites. 6 Our results suggest the existence of spatial refuges for pondweed tubers against swan predation.