Spatially-explicit predictive modeling of coral species distributions in the Hawaiian islands

Abstract

Coral reefs are an ecosystem in transition. Scleractinian corals are the foundation species of tropical and subtropical reef ecosystems, yet information about their status is woefully inadequate. In order to appreciate the changes that reefs are undergoing, we need to explore methods that better explain the current conditions of coral species populations. Using species distribution models, this dissertation examined the physical and biological factors that influence the distribution of six dominant scleractinian species, Montipora capitata, Montipora flabellata, Montipora patula, Pocillopora meandrina, Porites compressa, and Porites lobata in the main Hawaiian Islands. The primary objectives of the dissertation included: (i) compilation of a database of quantitative field observations of the six coral species from reef surveys and spatial environmental covariate data in the main Hawaiian Islands during 2000-2009, (ii) identification of models and environmental factors that were most informative for predicting the distributions of the six coral species in the main Hawaiian Islands using the field observations for model training and validation, (iii) utilization of the model outputs to map spatially-explicit presence and abundances of coral species for near-shore, shallow reefs (~30 m depth) of the main Hawaiian Islands, and (iv) comparison of the populations of coral species in a network of MPAs to unprotected reefs using data from the spatial abundance maps. The results demonstrated that species distribution modeling approaches are an effective means to characterize the distribution, presence, and abundance of corals in the Hawaiian Islands. Mean significant wave height and max significant wave height were the most influential variables explaining coral presence and abundance (as benthic cover) in the Hawaiian Islands. Models also identified relationships between coral cover with island, depth, downwelled irradiance, rugosity, slope, and aspect. The rank order of coral abundance (from highest to lowest) for the MHI was P. lobata, M. patula, P. meandrina, M. capitata, P. compressa, and M. flabellata. Abundances of the two Porites species were higher in MPAs than open areas. The three Montipora species and Pocillopora meandrina had lower abundances in most MPAs compared to open areas. Manele-Hulopoe and Molokini Marine Life Conservation Districts (MLCD) had higher abundances for four of the six coral species compared to unprotected areas while Waikiki MLCD had lower abundances than open areas for all corals. The Hawaiian Islands Humpback Whales National Marine Sanctuary (HIHWNMS) encompassed coral populations with higher abundances than areas outside the boundaries especially for the four corals (Montipora spp. and P. meandrina) underrepresented in the current MPA network. It can be concluded that species distribution modeling delivers a methodological approach to spatially-explicit marine population assessments at a macroecological scale that was not previously possible. The utility of SDMs to provide species abundances at a high map resolution across the entire geographic domain represents a significant improvement in our ability to describe the condition of these coral populations. The information on coral species is critically important as baseline data for population connectivity modeling, marine spatial planning, and especially, climate studies. Coral reefs are undergoing rapid change but species responses to environmental drivers are heterogeneous. This work will serve as the framework for future investigations to better assess the conditions of species populations and understand the changes that Hawaiian reefs are experiencing.