Elsevier

Animal Behaviour

Volume 74, Issue 1, July 2007, Pages 93-101
Animal Behaviour

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

https://doi.org/10.1016/j.anbehav.2006.11.016Get rights and content

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.

Section snippets

Study Site

We undertook this study in the Eastern Gulf of Shark Bay (∼25°45′S, 113°44′E; Fig. 1). Shark Bay is home to a large population of dugongs (10 000–14 000 individuals; Marsh et al., 1994, Preen et al., 1997, Gales et al., 2004), but their use of our study area fluctuates seasonally (Wirsing et al., in press a). In 1991, Shark Bay was listed as a World Heritage Area, and anthropogenic activity in the region has always been minimal. Thus, we were able to evaluate the microhabitat use of dugongs

Results

We sighted 105 foraging dugongs while conducting transects through the seven survey zones; 64 individuals were found in edge microhabitat and 41 were found in interior microhabitat. Most encounters involved solitary animals (53/75), and mean ± SD group size was 1.40 ± 0.81, facilitating robust estimation of daily densities for survey zones and across microhabitats. The best model of foraging dugong density incorporated large tiger shark abundance (S) and the interaction between shark abundance and

Discussion

Our results are consistent with the hypothesis that microhabitat use by dugongs foraging over seagrass meadows is influenced by the danger of predation. Foraging dugong densities were not equivalent in edge and interior microhabitats throughout the year (rejecting the food quantity hypothesis), nor did foragers consistently overuse meadow interiors (rejecting the food quality hypothesis). Instead, the degree of dissimilarity between forager densities in edge and interior microhabitats was a

Acknowledgments

We thank R. Abernethy, V. Alla, T. Bujas, C. Chow, M. Davis, A. Greenley, K. Harper, L. Heithaus, M. Kerford, S. Kowalewsky, A. Krickan, J. Laski, L. Marshall, K. Martin, R. McPhie, B. Stalvey, J. Wilder and K. Wirsing for their assistance in the field. This material is based upon work supported by the National Science Foundation under Grant Number 0526065. Other grants and support were provided by Humminbird, Mercury Marine Australia, the Monkey Mia Dolphin Resort, Monkey Mia Wildsights

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