Living on the edge: dugongs prefer to forage in microhabitats that allow escape from rather than avoidance of predators

Abstract

When threatened with predation, foraging prey can cease feeding and seek refuge or shift to feeding sites (microhabitats) offering increased safety. Predator-induced microhabitat shifts by large herbivores are of interest to ecologists because spatial patterns of foraging by these animals shape plant communities. The influence of predation risk on microhabitat use by large herbivores in marine systems remains poorly understood. We explored the relationship between microhabitat use by dugongs, Dugong dugon, and tiger shark, Galeocerdo cuvier, predation risk in an Australian embayment over 3 years. Use by foraging dugongs of two seagrass microhabitats, edge (lower-quality seagrass, swift escape from sharks) and interior (higher-quality seagrass, fewer escape options), was monitored in seven survey zones. We indexed predation danger using catch rates of tiger sharks that were greater than 3.0 m in total length. The degree of dissimilarity between forager densities in edge and interior microhabitats was a function of tiger shark abundance: foragers underused edge (safe) microhabitat when sharks were scarce, overused it when sharks were common, and responded to daily changes in shark abundance in a threat-sensitive fashion, showing the greatest preference for edges when shark abundance was highest. We conclude that dugongs manage their probability of death by allocating more time to safe but lower-quality feeding microhabitats when the likelihood of encountering sharks is elevated. Dugong grazing can influence seagrass biomass and patch composition, so tiger sharks probably affect the microhabitat structure of seagrass meadows, and ultimately their benthic communities, indirectly by altering the way that dugongs use feeding patches.