Human and natural drivers of multiple coral reef regimes across the Hawaiian archipelago

Sammanfattning: Loss of coral reef resilience can lead to dramatic changes in benthic structure, so called regime shifts, which significantly alter ecosystem processes and functioning. Loss of ecosystem services associated with these regime shifts can be substantial and have significant impacts on human well-being and societal development. Reversing regime shifts once they have occurred can be very difficult and costly. In the face of global change and increasing human impact, there is an urgent need for anticipating and preventing unwanted regime shifts, or conversely, forcing (reversing) shifts of already degraded reef states. Such challenges require a better understanding of the dynamics and processes (i.e. resilience) that support or undermine different reef regimes. The Hawaiian archipelago extends across a wide gradient of natural and anthropogenic conditions and provides a unique opportunity to investigate the relationships between multiple reef regimes, their dynamics and potential drivers. Here I apply a combination of exploratory ordination methods and inferential statistics to one of the most comprehensive coral reef datasets available, in order to detect, visualize and define potential multiple ecosystem regimes. The present study demonstrates the existence of three distinct reef regimes occurring across the whole archipelago, dominated by hard corals, turf algae, or macroalgae. Boosted Regression Trees (BRT) against a broad set of human and natural predictor variables were used in order to determine what might drive the different reef regimes. Results show that herbivore biomass, effluent, latitude and depth play out as particularly important variables. Finally, I highlight some trends in benthic habitat that depend on fish biomass and reciprocally, some critical thresholds in measures of fish diversity that occur across gradients of regimes, providing novel insights into the multi-scale dynamics of reef resilience and regime shifts.