Environmental and Demographic Drivers of Hawaiian Reef Corals

Abstract

Coral reefs are one of the most diverse and productive ecosystems on earth. While coral abundance is decreasing on a global scale, some monitoring sites show increasing coral cover on a regional scale. Spatially and temporally variable patterns in coral abundance are dependent on local topography as well as the type, scale, and frequency of disturbance. The objectives of this dissertation were to 1) identify colony scale rates and causes of mortality, 2) investigate the relative importance of top-down and bottom-up effects on the distribution of coral cover at Kahekili Beach Park (KBP), a site at risk for a phase shift from coral to macroalgal dominated community, and 3) to describe spatial variability in demographic rates of Montipora capitata at sites with varying trajectories of coral cover. Observations of individual colonies at six sites around Maui showed a baseline level of chronic partial mortality associated with inter-specific competition and predation that affected a small proportion of colony surface area. Baseline mortality was punctuated by losses of larger proportions of colony surface area, through partial mortality associated acute disturbances, such as thermal bleaching and mechanical damage to colonies. To investigate the relationship between coral cover and top-down (herbivore abundance) vs. bottom-up (submarine ground water discharge) drivers, a spatially explicit Geographically Weighted Regression (GWR) was compared to a non-spatial Ordinary Least Squares (OLS) model. Both models showed that the all main effects and 2-way interactions between top-down and bottom-up drivers were related to the distribution of coral cover. The GWR improved the fit of the model and allowed for visualization of spatial patterns in relationships between the different variables, which will aide researchers and managers in developing and testing hypotheses to improve our understanding of these systems. An Integral Projection Model approach, used to model the complex demographics of M. capitata, showed site specific sensitivity to rates of sexual vs. clonal recruitment and regrowth of damaged colonies. Findings concerning the population biology of M. capitata around Maui can be used to understand processes driving variability in reef structure and facilitate development of effective management practices.