CAREER: Does predator intimidation help structure a pristine seagrass community through multiple indirect pathways?

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A recent synthesis suggests that the role of grazing in seagrass ecosystems is generally under-appreciated and that our understanding of these critical marine communities would be greatly enhanced by long-term experimental studies that determine the impact of grazing by multiple guilds, especially megagrazers (green turtles, sirenians) that have declined dramatically in most coastal systems. Furthermore, given the steep declines in populations of marine predators, an understanding of how upper trophic levels structure spatio-temporal patterns of grazer impacts is needed to be able to predict how anthropogenic changes to marine environments will influence seagrass ecosystems. A predictive framework of community ecology in general has been hampered in part because non-lethal effects of predators, both direct and indirect, often are overlooked. Behaviorally mediated indirect species interactions (BMII) occur when a predator causes a change in the behavior of its prey that is transmitted to lower trophic levels. While BMII appear to be important in shaping community dynamics, most studies of BMII have been carried out in small-scale experiments, and have not considered that top predators might initiate multiple BMII that could either attenuate or amplify their indirect effects in natural communities. In this study, Dr. Heithaus will expand on previous investigations on one of the world's most pristine seagrass ecosystems (Shark Bay, Australia) to determine whether multiple BMII initiated by tiger sharks attenuate or amplify their indirect effects on seagrass communities. During previous NSF-funded research, the results show that tiger sharks can indirectly influence seagrass communities through two parallel non-lethal pathways rather than just one. In the first, well-established three-step BMII, tiger sharks cause shifts in megagrazer foraging locations, shielding seagrasses in dangerous microhabitats when sharks are present. In a second possible BMII, tiger sharks cause shifts in piscivore (dolphin, cormorant) microhabitat use that could result in enhanced grazing by herbivorous fishes in the microhabitats that megagrazers have abandoned. Depending on the nature of herbivorous fish interactions with seagrasses, these BMII could result in attenuation or amplification of tiger shark impacts on seagrass communities. In this new award, further monitoring visual and video surveys, experimental herbivory trials, underwater video monitoring, and turtle-borne video cameras will be used to determine whether and how spatiotemporal patterns of grazing vary. Experiments, in the field, will be employed using two types of exclosure and seagrass transplants to determine the relative importance of the BMII initiated by tiger sharks and whether they result in attenuation or amplification of shark effects on seagrass communities. This project will support some of the most comprehensive long-term studies of a relatively pristine community of highly interactive marine species (sharks, turtles, sea cows), extending the datasets to 15 years. This will enable Dr. Heithaus and collaborators to address new questions about how patterns of abundance are influenced by large-scale oceanographic and climatic factors. The project will also develop a broad education and outreach program that is integrated into the research program. This includes: 1) 5th-8th grade biology teacher research experiences in Shark Bay, 2) graduate student interactions with middle school students, 3) talks by graduate students and the PI at schools and regional and national science teacher conferences, 4) the development of lesson plans based on species in Shark Bay, 5) the development of DVDs for use in secondary school classrooms, 6) the development of video labs, 7) video content for National Geographic's online and on-demand channel - NGC Wild, 8) research experiences for undergraduate and graduate students, and 9) the enhancement of an educational website.