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Coral reefs worldwide face unprecedented cumulative anthropogenic effects of interacting local human pressures, global climate change and distal social processes. Reefs are also bound by the natural biophysical environment within which they exist. In this context, a key challenge for effective management is understanding how anthropogenic and biophysical conditions interact to drive distinct coral reef configurations. Here, we use machine learning to conduct explanatory predictions on reef ecosystems defined by both fish and benthic communities. Drawing on the most spatially extensive dataset available across the Hawaiian archipelago-20 anthropogenic and biophysical predictors over 620 survey sites-we model the occurrence of four distinct reef regimes and provide a novel approach to quantify the relative influence of human and environmental variables in shaping reef ecosystems. Our findings highlight the nuances of what underpins different coral reef regimes, the overwhelming importance of biophysical predictors and how a reef's natural setting may either expand or narrow the opportunity space for management interventions. The methods developed through this study can help inform reef practitioners and hold promises for replication across a broad range of ecosystems.

Coral reefs worldwide face an uncertain future with many reefs reported to transition from being dominated by corals to macroalgae. However, given the complexity and diversity of the ecosystem, research on how regimes vary spatially and temporally is needed. Reef regimes are most often characterised by their benthic components; however, complex dynamics are associated with losses and gains in both fish and benthic assemblages. To capture this complexity, we synthesised 3,345 surveys from Hawai'i to define reef regimes in terms of both fish and benthic assemblages. Model-based clustering revealed five distinct regimes that varied ecologically, and were spatially heterogeneous by island, depth and exposure. We identified a regime characteristic of a degraded state with low coral cover and fish biomass, one that had low coral but high fish biomass, as well as three other regimes that varied significantly in their ecology but were previously considered a single coral dominated regime. Analyses of time series data reflected complex system dynamics, with multiple transitions among regimes that were a function of both local and global stressors. Coupling fish and benthic communities into reef regimes to capture complex dynamics holds promise for monitoring reef change and guiding ecosystem-based management of coral reefs.

Aim: Species ranges provide a valuable foundation for resolving biogeographical regions, evolutionary processes and extinction risks. To inform conservation priorities, here we develop the first bioregionalization based on reef fish abundance of the Hawaiian Archipelago, which spans nearly 10° of latitude across 2,400 km, including 8 high volcanic islands in the populated main Hawaiian Islands (MHI), and 10 low islands (atolls, shoals and islets) in the remote northwestern Hawaiian Islands (NWHI). Location: The Hawaiian Archipelago. Taxon: Fishes (276 taxa). Methods: We compiled 5,316 visual fish surveys at depths of 1–30 m from throughout the Hawaiian Archipelago. Geographical range (km2) for each species was measured as extent of occurrence (EOO) and area of occurrence (AOO). PERMANOVA and PCO were used to investigate drivers of fish assemblage structure. Distance‐based multivariate analyses were used to evaluate the relationship between fish assemblage structure and predictor variables including latitude, reef area, temperature, chlorophyll‐a, wave energy and human population density. Results: Distinct fish assemblages exist in the MHI and NWHI, with two additional faunal breaks driven primarily by endemic species abundance. Latitude explained 37% of the variability in fish assemblages, with reef area accounting for an additional 9%. EOO showed a significant correlation with latitude. Endemics comprised 52%–55% of the numerical abundance at the northern end of the archipelago but only 17% on Hawai'i Island in the extreme south. Maximum size and activity regime (day vs. night) explained the most variation in the abundance of endemics. Main conclusions: The Hawaiian fish assemblages are strongly influenced by endemic species, affirming the archipelago as a biodiversity hotspot of high conservation value. The higher abundance of endemics in the NWHI may represent preadaptation to oceanic (oligotrophic) conditions. Resolution of distinct bioregions across the archipelago provides a better understanding of reef fish macroecology, with implications for management at the archipelago scale. [ABSTRACT FROM AUTHOR]

A major challenge for coral reef conservation and management is understanding how a wide range of interacting human and natural drivers cumulatively impact and shape these ecosystems. Despite the importance of understanding these interactions, a methodological framework to synthesize spatially explicit data of such drivers is lacking. To fill this gap, we established a transferable data synthesis methodology to integrate spatial data on environmental and anthropogenic drivers of coral reefs, and applied this methodology to a case study location–the Main Hawaiian Islands (MHI). Environmental drivers were derived from time series (2002–2013) of climatological ranges and anomalies of remotely sensed sea surface temperature, chlorophyll-a, irradiance, and wave power. Anthropogenic drivers were characterized using empirically derived and modeled datasets of spatial fisheries catch, sedimentation, nutrient input, new development, habitat modification, and invasive species. Within our case study system, resulting driver maps showed high spatial heterogeneity across the MHI, with anthropogenic drivers generally greatest and most widespread on O‘ahu, where 70% of the state’s population resides, while sedimentation and nutrients were dominant in less populated islands. Together, the spatial integration of environmental and anthropogenic driver data described here provides a first-ever synthetic approach to visualize how the drivers of coral reef state vary in space and demonstrates a methodological framework for implementation of this approach in other regions of the world. By quantifying and synthesizing spatial drivers of change on coral reefs, we provide an avenue for further research to understand how drivers determine reef diversity and resilience, which can ultimately inform policies to protect coral reefs. [ABSTRACT FROM AUTHOR]

Abstract: Large declines in reef fish populations in Hawai‘i have raised concerns about the sustainability of these resources, and the ecosystem as a whole. To help elucidate the reasons behind these declines, a comprehensive examination of reef fish assemblages was conducted across the entire 2500 km Hawaiian Archipelago. Twenty‐five datasets were compiled, representing >25 000 individual surveys conducted throughout Hawai‘i since 2000. To account for overall differences in survey methods, conversion factors were created to standardize among methods. Comparisons of major targeted resource species (N = 35) between the densely populated main (MHI) and remote north‐western Hawaiian Islands (NWHI) revealed that 40% of these species had biomass in the MHI below 25% of NWHI levels. In total, 54% of the species examined had biomass <50% of NWHI biomass. The moku or district was a basic unit of resource management in pre‐contact Hawai‘i and was used as a unit of spatial stratification for comparisons within the MHI. Biomass of resource species was negatively correlated with human population density within moku boundaries, with extremely low biomass in areas with highest human population densities. No such relationship was found for species not targeted by fishing. A number of remote areas with small human populations in the MHI still support high standing stock of fished species, and these areas are likely important refugia for maintaining fisheries production and biodiversity functioning. These results highlight the large gradient of human impacts on fish assemblages across the Hawaiian Archipelago and the potential in using landscape and seascape units, such as those that are watershed and bio‐physically‐based, when managing in part based on a framework of traditional ecological knowledge. [ABSTRACT FROM AUTHOR]