Heithaus Research (tiger sharks, turtles, dugongs, and seagrass)

	Heithaus Lab: Research

Research in our lab is united by the overarching theme of understanding how predator-prey interactions structure communities with a particular focus on the role of non-lethal predator effects (i.e. predator intimidation and prey fear). I am particularly interested in the role of upper trophic level marine predators in their communities and ecosystems, and how ongoing reductions in their populations are likely to impact marine communities. While my approach to asking questions sometimes involves developing theory and laboratory tests of this theory, my lab's research is primarily field-based. Since 1997, colleagues and I have been using the Shark Bay, Western Australia seagrass ecosystem as a model system for determining the role of tiger sharks in shaping the behavior of their prey (dolphins, dugongs, sea turtles, cormorants) and how these effects might be transmitted through the community as behaviorally-mediated indirect species interactions. More information on this project is available at the Shark Bay Ecosystem Research Project Website. Some specific projects that we are currently working on include:


Behaviorally mediated indirect species interactions in a subtropical seagrass ecosystem
	In this project, we are testing whether tiger sharks indirectly influence seagrass communities through predation-sensitive habitat use and foraging behavior of megagrazers (dugongs and green sea turtles). We have found that dugongs shift their use of deep and shallow habitats, microhabitats within shallow banks, and foraging tactics in response to tiger shark predation risk. In addition, green turtles in good body condition give up foraging opportunities in order to be safe from tiger sharks. To test whether these shifts in the spatial pattern of grazer abundance are transmitted to seagrass communities, we have constructed exclosures that will allow us to quantify grazer impacts in habitats that vary in the level of risk that turtles and dugongs face from tiger sharks. In addition, we are using stable isotopes to determine the relative contributions of seagrass, algae, and jellyfish to the diets of green sea turtles. Finally, this project also investigates spatial and temporal variation in seagrass community composition and nutrient content within our study site and across Shark Bay. This project is supported by NSF grant OCE-0526065 and the PADI Foundation.
Dugongs can modify the structure of seagrass beds through their foraging

Tiger sharks cause grazers to change habitats, but does this affect seagrass communities?

Foraging ecology of rays in Shark Bay, Western Australia
Rays are large conspicuous predators in soft bottom communities and may play a significant role in structuring soft bottom communities, through predation and bioturbation. Despite their potential importance, surprisingly little research has focused on the ecology of rays. The goals of this project are to determine how rays respond to spatial and temporal variation in prey availability and the physical environment (e.g. water depth and temperature), and risk from predators like tiger and hammerhead sharks. We are assessing ray habitat use with a combination of visual surveys and acoustic tracking. Finally, we are invetigating the impacts of rays on benthic invertebrate communities using exclosure experiments. This research is supported by the National Geographic Expeditions Council.

Consumer-mediated nutrient flow in the Florida coastal Everglades
	The coastal Everglades estuary is generally oligotrophic and limitign nutrients in upsream areas tend to be derived from the Gulf of Mexico. We are using a combination of acoustic telemetry and stable isotopic analysis to determine whether large consumers might facilitate upstream transport of nutrients into the mangrove/sawgrass ecotone region. Stable isotopic analysis will allow us to estimate the relative contributions of marine and freshwater productivity to the diets of these consumers and acoustic tracking will be used to determine the factors influencing their movements, residency times, and habitat use. Our initial studies focused on juvenile bull sharks, which are found up to 30 km from the ocean and appear to feed mostly in marine-derived food webs. We are now going to expand our studies to other large consumers including Florida gar, snook, and alligators. This project is part of the Florida Coastal Everglades Long Term Ecological Research Project.

The influence of predation risk on diving behavior of air-breathing aquatic foragers
Our lab is working on theoretical, experimental, and field studies to determine the role of predation risk in shaping diving behavior. Current projects include the development of a dynamic state variable model of diving under predation risk, a laboratory study of red-eared slider turtles to test model predictions, and field studies of green and loggerhead turtles and pied cormorants that are under risk of tiger shark predation. Theoretical developments, laboratory work, and field studies of cormorants are occuring in my lab and field studies of turtles are being carried out in collaboration with Dr. Lawrence M. Dill (Simon Fraser University).
		Do pied cormorants change their diving behavior to reduce their risk of being killed by sharks?

Trophic structure of a pristine subtropical seagrass community
	We are using stable isotopes to determine the trophic structure of sand-bottom and seagrass communities in Shark Bay, Western Australia. These data will be useful in comparisons of trophic structure with similar, but more heavily impacted, ecosystems. Ultimately, I hope to construct energy-flow models of the Shark Bay community that can be used to test the relative predictive power of traditional food web models with those that incorporate non-lethal predator effects.