State-dependent risk-taking by green sea turtles mediates top-down effects of tiger shark intimidation in a marine ecosystem

1. A predictive framework of community and ecosystem dynamics that applies across systems has remained elusive, in part because non-consumptive predator effects are often ignored. Further, it is unclear how much individual-level detail community models must include. 2. Previous studies of short-lived species suggest that state-dependent decisions add little to our understanding of community dynamics. Body condition-dependent decisions made by long-lived herbivores under risk of predation, however, might have greater community-level effects. This possibility remains largely unexplored, especially in marine environments. 3. In the relatively pristine seagrass community of Shark Bay, Australia, we found that herbivorous green sea turtles (Chelonia mydas Linnaeus, 1758) threatened by tiger sharks (Galeocerdo cuvier Peron and LeSueur, 1822) select microhabitats in a condition-dependent manner. Turtles in poor body condition selected profitable, high-risk microhabitats, while turtles in good body condition, which are more abundant, selected safer, less profitable microhabitats. When predation risk was low, however, turtles in good condition moved into more profitable microhabitats. 4. Condition-dependent use of space by turtles shows that tiger sharks modify the spatio-temporal pattern of turtle grazing and their impacts on ecosystem dynamics (a trait-mediated indirect interaction). Therefore, state-dependent decisions by individuals can have important implications for community dynamics in some situations. 5. Our study suggests that declines in large-bodied sharks may affect ecosystems more substantially than assumed when non-lethal effects of these top predators on mesoconsumers are not considered explicitly

Biology of sea turtles under risk from tiger sharks at a foraging ground

It is important to understand the population structure and space use of sea turtles because of their potential effects on the dynamics of nearshore ecosystems. Much of our current understanding is skewed because the ecology of sea turtles on their foraging grounds is known best from areas where there are, or have been, major human impacts. We studied green Chelonia mydas and loggerhead Caretta caretta sea turtles on the relatively pristine feeding ground of Shark Bay, Western Australia, where tiger sharks Galeocerdo cuvier are a major mortality agent. Recapture rates were lower for green than for loggerhead turtles, possibly reflecting a larger population rather than low site fidelity for greens. The sex ratio of loggerhead turtles was not significantly different from 1:1, whereas green turtles showed a strong female bias. Size distributions of both species were skewed towards larger (and presumably older) individuals relative to a human-impacted feeding ground in eastern Australia. Body condition varied temporally for green turtles, but not for loggerhead turtles, possibly due to longer distances traveled to nesting beaches by green turtles. Rates of shark-inflicted injuries were higher for loggerhead turtles, especially males, than for greens. Sublethal effects of these injuries were not evident. There are notable differences between sea turtles threatened primarily by tiger sharks in Shark Bay and populations on feeding grounds where historical and current mortality causes are anthropogenic. We conclude that without baseline data from relatively pristine habitats our understanding of human impacts on sea turtle populations suffers from a limited scope