Resetting predator baselines in coral reef ecosystems: population dynamics, behavior, and ecological characteristics
- Author(s): Bradley, Darcy
- Advisor(s): Gaines, Steven D
- Kendall, Bruce E
- et al.
What did coral reef ecosystems look before human impacts became pervasive? Although we cannot effectively manage the marine environment without knowledge of what it looked like in a pristine state, human impacts have wreaked such havoc on the ocean that it is often impossible to accurately characterize marine populations pre-exploitation. Lacking a baseline is particularly critical for species of conservation concern such as sharks, which have experienced severe population declines at local and global scales. I use the unique setting of Palmyra Atoll, a remote, U.S. National Wildlife Refuge in the central Pacific Ocean, to describe the population dynamics, ecological characteristics, and behavior of reef sharks absent significant human impacts. With a combination of extensive field surveys and experiments and spatially and temporally explicit analytical tools, I attempt to resolve a set of ecological debates regarding the trophic structure of coral reef communities, the drivers of life history variability, and the ecological effects of non-consumptive human impacts on marine predators. Ultimately, I find that the trophic structure of an unexploited reef fish community is not inverted and that even healthy top predator populations may be considerably smaller, and more precarious, than previously thought (Chapter 1). At the same time, I show that there is substantial but not predictable variability in life history traits observed for the grey reef sharks across its range, suggesting that regional management may be necessary to set sustainable harvest targets and to recover this shark species globally (Chapter 2). Finally, I demonstrate that humans can interact with reef sharks without persistent behavioral impacts, and that well-regulated shark diving can capture the economic benefits of tourism without undermining conservation goals (Chapter 3). Throughout this dissertation, I focus on coral reef associated shark species, but the challenges I consider and the methods I employ are important for many systems and many predators, which tend to move more than their prey. We often find ourselves managing a moving target, and so counting things correctly, assessing ecological variability, and understanding behavioral responses due to human interactions with wildlife can profoundly change the way we understand and manage both terrestrial and marine systems.