Earth’s habitats continue to be exposed to anthropogenic stressors including eutrophication and climate change. There is thus a critical need to understand what makes ecosystems resilient to disturbances. Apex predators, such as large sharks, may be drivers of resilience in part because of their ability to generate widespread risk effects; such predation risk can alter prey behavior and generate cascading effects to primary producers. It is therefore important to test whether changes in prey behavior generated by predation risk can increase coastal ecosystem resilience, especially as shark overfishing and as climactic disturbances continue. Here we present a 16 month field experiment to determine whether a dominant apex predator, the tiger shark (Galeocerdo cuvier), contributes to stability of the recently disturbed seagrass community of Shark Bay, Australia. The experiment was established following a climate-driven heat wave (2011) and subsequent catastrophic loss (70-90%) of seagrass cover. We mimicked shark loss by simulating shifts in the risk-sensitive foraging patterns of dugong mega-grazers that are expected to occur if sharks were removed. We used mixed effects modeling to determine the importance of grazing treatments and initial seagrass cover to the change in percent cover of a climax (Amphibolis antarctica) and pioneer seagrass (Halodule uninervis).
Results/Conclusions
A. antarctica cover declined as grazing intensity increased. Interestingly, change in A. antarctica cover was also influenced by initial cover of this species, resulting in a significant interaction between initial cover and grazing treatment. Surprisingly, H. uninervis cover did not increase in response to grazing as pioneer seagrasses have been recorded to do elsewhere, and there was likewise no interaction between initial cover and grazing treatment. Results indicate the fragile current state of Shark Bay and the likelihood for a phase shift from an ecosystem dominated by A. antarctica to one denuded of complex seagrass if tiger sharks were removed from the ecosystem. Furthermore, the significant interaction of initial Amphibolis cover and grazing treatments on changes in A. antarctica cover suggest that predator overfishing and climate disturbance may interact. Because overfishing and climate change co-occur widely, the potential for interactions between overfishing and climactic disturbances may be widespread. This work also highlights the contribution apex predators can make to the resilience of marine ecosystems, which are likely to be exposed to more frequent and intense disturbances as climate change continues.