Intellectual Merit: Large-bodied marine predators have both direct and indirect effects on other members of their communities through predation-related removals and intimidation-related (that is, non-consumptive) changes in behavior of other organisms, respectively. Thus, large-bodied predators can produce cascading effects on other aspects of the ecosystem. Understanding the nuances of top predator effects on ecosysytem structure and function is a topic of keen ecological interest with strong implications for conservation and management. The PIs have been working for 15 years in the pristine seagrass ecosystem of Shark Bay, western Australia. Their work has shown that tiger sharks induce a reduction in seagrass grazing by dugongs which allows a slow-growing but highly palatable seagrass (Amphibolis antarctica) to persist and even dominate. The slow-growing seagrass has recently experienced a widespread die-off, and two important questions are: (1) what are the immediate effects on the ecosystem; and (2) how does the presence or absence of sharks affect re-establishment of this species, with long-term consequences for the system? The natural die-back of A. Antarctica provides a unique opportunity to study these questions of diversity, structure, and persistence, and to examine the effects of a natural disturbance in a pristine ecosystem that has been studied for many years. A RAPID award is needed to take advantage of this situation shortly after disturbance.Broader Impacts: The project will provide important insights into the impacts of climate change and overfishing on ecosystem dynamics, and the PIs will integrate results into their on-going outreach programs, including a website (www.sberp.org), public lectures, and video-based projects for elementary, middle, and high school students. Data will be provided to the Department of Environment and Conservation of Western Australia to assist in their management of Shark Bay, as well as US data depositories. The project will provide support for research by a PhD student, a postdoc, and multiple undergraduate assistants.