Changing hiding patterns in sago pondweed tubers in response to predation by tundra swans

Plants may evolve defence against herbivory if the benefits to fitness exceed the costs. An avoidance strategy reducing the accessibility of costly tissues has been found for the freshwater macrophyte sago pondweed Potamogeton pectinatus in the shallow lake Lauwersmeer in the Netherlands. The plant, facing high levels of herbivory on tubers by tundra swans Cygnus columbianus in autumn, hides its tubers by burying them deep in the sediment. We hypothesized that changes in the predation pressure would result in alterations in the size of tubers and their distribution across sediment depth. Two trade-offs underlie this idea. Firstly, deep tubers are less accessible to foraging swans but they must be larger to meet the higher energy demands when sprouting. Secondly, at a given depth larger tubers are competitively superior but face a higher risk of being detected by swans. In 2004 we compared tuber size and burial depth in a heavily predated pondweed population (Lauwersmeer, the Netherlands), a moderately predated one (Lake Peipsi, Estonia) and a virtually predation-free pondweed population (Camargue, France). We also included historical data collected in 1980 in the Lauwersmeer, when exploitation by swans had started a few years before. The tuber size-depth distribution from the Lauwersmeer in 1980 was similar to the current situation in the virtually swan-free Camargue. In accordance with our hypothesis, tubers appeared to be buried deeper, and at a given depth to be smaller, in the moderate and high predation environment than in the predation poor environments. This strongly suggests an adaptive avoidance response by sago pondweed to foraging by tundra swans.