Top-predators as structuring agents in dynamic marine environments

Global declines in top-predators are occurring due established and ongoing fisheries throughout the world’s oceans. In particular, dramatic declines have been observed for Atlantic cod (Gadus morhua) in the boreal sub-arctic and for reef sharks (mostly Carcharhinidae spp.) in coral reefs. The impact of these declines on marine communities still remains largely unclear due to food web complexity, interacting factors, confounding variables, and fluctuating ecosystem states. Furthermore, as the impact of disturbances on communities can be press (e.g. fisheries), pulse (e.g. environmental variability) or combine, fisheries contribute to disturbance regimes that can generate heterogeneity in communities, meaning that their effects are likely not uniform across space and time. Determining the ecological role of top-predators, as top-down structuring agents, alongside ecosystem disturbances is fundamental to understanding baseline conditions and ultimately may help to inform conservation efforts. This thesis investigates the relative roles of top-predators and disturbances to build an understanding of how marine communities and food webs may be structured. This thesis aims to address: (i) how environmental variability may impact the role of top-predators, (ii) determine the ecological role of top-predators in coral reef environments, (iii) how top-down and bottom-up structuring agents impact variability in food webs, and (iv) how humans are modifying the role of top-down and bottom-up structuring agents. In this thesis I present three main findings: (i) top-predators have a strong top-down influence on marine communities and food webs alongside other disturbances, (ii) combined effects (between top-down and bottom-up structuring agents) can impact communities at broad and fine spatial scales, and (iii) spatial heterogeneity in structuring agents caused by human activities, impacts food web dynamics across multiple spatial scales. The findings in this thesis provide a foundation from which management decisions can be made to ultimately address restoration and conservation goals.Ph