Predation-sensitive foraging behaviour of dugongs (dugong dugon)

Foraging decisions by herbivores are of great interest to ecologists, as spatial and temporal patterns of herbivory may structure plant communities. Yet, in marine systems, the factors governing foraging choices of large herbivores (e.g., sea turtles, sirenians) remain poorly understood. Between 2002 and 2004, I assessed the use of habitat and alternative feeding tactics by dugongs (Dugong dugon) foraging under risk of predation by tiger sharks (Galeocerdo cuvier) in Shark Bay, Western Australia. Use of two primary foraging habitats (shallow banks, deep channels) by dugongs was monitored systematically using transect passes through 14 survey zones allocated evenly to the habitat types; use of two microhabitats within shallow areas (edge, interior) by foragers was also quantified using transects. The time allocated to each of two alternative feeding tactics (cropping, excavation) by individual dugongs was quantified using focal animal follows. The threat of tiger shark predation was indexed using catch rates on drumlines. Overall, foraging dugongs showed a consistent preference for shallow habitats, which are rich in food (seagrass). However, the degree to which foragers used deep habitats, where seagrass is sparse but safety from sharks is relatively high, changed in a threat-sensitive manner: forager densities in deep habitat were equivalent to those predicted by food supply when sharks were scarce, and greatly exceeded levels predicted by food alone when sharks were common. Within shallow habitats, foragers preferred interior feeding microhabitats when sharks were scarce, and shifted to edge microhabitats, which offer swift escape into deep water, when sharks were common. Finally, foraging dugongs spent the majority of their time cropping seagrass. However, during months when excavation occurred (Feb - May of all years), use of this tactic was inversely related to shark abundance, likely because it constrains vigilance. These results suggest that dugongs adaptively manage their risk of mortality by reducing their use of dangerous habitats, microhabitats, and feeding tactics when the likelihood of encountering predators is elevated. It follows that, by mediating patterns of dugong herbivory, tiger sharks may exert a powerful indirect effect on Shark Bay's seagrass meadows.