MODELING REEF-CORAL RESPONSE TO CLIMATE CHANGE
CACCIAPAGLIA, CHRISTOPHER WILLIAM
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Coral reefs are one of the most diverse ecosystems on the planet. They provide goods and services to millions of people living in the coastal tropics. Recently, however, rising sea-surface temperatures have been threatening reef corals, causing episodes of thermal stress that lead to coral bleaching, mortality, and changes in reef composition. This increase in ocean temperature, along with the melting of glaciers and polar ice caps, is also causing an increase in sea level, which is threatening to ‘drown’ reefs that cannot keep up with sea-level rise. Some early models predicted that nearly all coral species would be unable to survive the +2–3°C increase in seasurface temperature predicted by the year 2100. However, ocean warming was most often modeled without considering geographical variation. Indeed, within that spatial variation are locations that may act as climate-change refugia, where temperatures are not rapidly increasing, and in which corals can persist into the future. This study aims to identify potential climate-change refuges for coral reefs in the Indian and Pacific Oceans. Species-distribution models are increasingly being used as a tool to identify suitable habitats for terrestrial and marine species under future climate change. These models consider: (i) the contemporary distribution of species, (ii) the environmental conditions in which the species is found, and (iii) the forecasted environmental conditions from climate models. These models fit the distribution of a species to its current habitat and use projected climate to predict where the species will most likely persist into the future. The first model in this study identified twelve potential climate refuges in the Indo-Pacific. In the Indian Ocean, climate-change refugia were identified in south western Madagascar, the Maldives, the Chagos Archipelago, Western Australia, and the Seychelles. In the Pacific Ocean, climate-change refugia were identified in northern Indonesia, Micronesia, the northern Marshall Islands, the southern Great Barrier Reef, the Solomon Islands, Vanuatu, and French Polynesia. All twelve of these reef-coral refugia deserve high-conservation status. Nine of the twelve species examined were predicted to lose 24–50% of their current habitat, with most reduction predicted to occur between the latitudes 5°–15°, in both hemispheres. Yet when these species were modeled with a 1°C capacity to adapt, they were predicted to retain much of their current distribution. By contrast, the thermally tolerant Porites lobata is expected to expand its current distribution by 8%, particularly southward along Australia’s western and eastern coasts. In the second model, light-blocking turbidity reduced the harmful effect of increased sea-surface temperature in 9% of the Indo-Pacific’s shallow water reefs. Turbidity-driven mitigation was identified in the northwestern Hawaiian Islands, northern Philippines, the Ryukyu Islands (Japan), eastern Vietnam, western and eastern Australia, New Caledonia, the northern Red Sea, and the Arabian Gulf. Turbidity also prevented coral growth by reducing light for photosynthesis in 16% of shallow-water reefs in the Indo-Pacific. In the third model, genetic isolation reduced Porites lobata’s predicted suitable habitat by over 60%, irrespective of climate-change scenario. These results indicate that genetic isolation will likely play a major role in the persistence of coral species under climate change. Most loss in suitable habitat occurred in the Pacific Ocean and small, isolated populations were most vulnerable to climate change. In the fourth model, sea-level rise threatens to outpace vertical accretion in habitats that are unable to support reef-building corals in a warming ocean. In locations where primary reef-accreting species were able to withstand the regional increase in thermal stress, reefs were predicted to be able to vertically accrete and keep pace with sea-level rise. These locations were aligned with previously identified climate and turbidity-driven refugia. The largest effect of erosion was biological erosion, considered as function of human-population density. Reefs were predicted to drown primarily at the distributional edge of the representative reef-accreting species. Although the models in this study show significant habitat loss as the oceans are predicted to warm, the work also highlights regions where corals might survive over the next century. This study provides critical information on where we should invest conservation effort, and identifies refugia that deserve protection and consideration as global sanctuaries.