Oecologia (2007) 153:1031–1040 DOI 10.1007/s00442-007-0802-3 123 BEHAVIORAL ECOLOGY Fear factor: do dugongs (Dugong dugon) trade food for safety from tiger sharks (Galeocerdo cuvier)? Aaron J. Wirsing · Michael R. Heithaus · Lawrence M. Dill Received: 7 November 2006 / Accepted: 18 June 2007 / Published online: 17 July 2007  Springer-Verlag 2007 Abstract Predators can inXuence plants indirectly by altering spatial patterns of herbivory, so studies assessing the relationship between perceived predation risk and habi- tat use by herbivores may improve our understanding of community organization. In marine systems, the eVects of predation danger on space use by large herbivores have received little attention, despite the possibility that preda- tor-mediated alterations in patterns of grazing by these ani- mals inXuence benthic community structure. We evaluated the relationship between habitat use by foraging dugongs (Dugong dugon) and the threat of tiger shark predation in an Australian embayment (Shark Bay) between 1997 and 2004. Dugong densities were quantiWed in shallow (puta- tively dangerous) and deep (putatively safe) habitats (seven survey zones allocated to each habitat), and predation haz- ard was indexed using catch rates of tiger sharks (Gale- ocerdo cuvier); seagrass volume provided a measure of food biomass within each zone. Overall, dugongs selected shallow habitats, where their food is concentrated. Foragers used shallow and deep habitats in proportion to food avail- ability (input matching) when large tiger sharks were scarce and overused deep habitats when sharks were common. Furthermore, strong synchrony existed between daily mea- sures of shark abundance and the extent to which deep hab- itats were overused. Thus, dugongs appear to adaptively manage their risk of death by allocating time to safe but impoverished foraging patches in proportion to the likeli- hood of encountering predators in proWtable but more dan- gerous areas. This apparent food-safety trade-oV has important implications for seagrass community structure in Shark Bay, as it may result in marked temporal variability in grazing pressure. Keywords Community structure · Foraging · Herbivory · Predator intimidation · Shark Bay Introduction Foraging by herbivores can lead to marked changes in plant biomass, distribution, and diversity (Crawley 1983). Thus, predators may aVect plants indirectly by altering spatial patterns of herbivory (Abrams 1995; Schmitz 2003; and see review by Schmitz et al. 2004). Predation can inXuence the distribution of herbivores lethally if individuals are removed diVerentially across space, or sublethally (noncon- sumptively) if a positive correlation between resources and danger prompts individuals to trade access to proWtable foraging patches for safety (Sih 1980; McNamara and Communicated by Peter Peterson. Electronic supplementary material The online version of this article (doi:10.1007/s00442-007-0802-3) contains supplementary material, which is available to authorized users. A. J. Wirsing · L. M. Dill Behavioural Ecology Research Group, Department of Biological Sciences, Simon Fraser University, Burnaby, BC, Canada, V5A 1S6 A. J. Wirsing Honorary Research Associate, School of Animal Biology, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia A. J. Wirsing (&) · M. R. Heithaus Department of Biological Sciences, Marine Biology Program, Florida International University, Biscayne Bay Campus MSB, 3000 NE 151 St, North Miami, FL 33181, USA e-mail: wirsinga@Wu.edu