英文摘要: | Rising sea temperatures are likely to increase the frequency of disease outbreaks affecting reef-building corals through impacts on coral hosts and pathogens. We present and compare climate model projections of temperature conditions that will increase coral susceptibility to disease, pathogen abundance and pathogen virulence. Both moderate (RCP 4.5) and fossil fuel aggressive (RCP 8.5) emissions scenarios are examined. We also compare projections for the onset of disease-conducive conditions and severe annual coral bleaching, and produce a disease risk summary that combines climate stress with stress caused by local human activities. There is great spatial variation in the projections, both among and within the major ocean basins, in conditions favouring disease development. Our results indicate that disease is as likely to cause coral mortality as bleaching in the coming decades. These projections identify priority locations to reduce stress caused by local human activities and test management interventions to reduce disease impacts.
The 2014 boreal summer was the warmest on record1, breaking air temperature records in hundreds of cities and causing unprecedented highs in sea surface temperatures in the North Pacific2. Concurrently, a catastrophic outbreak of starfish wasting disease decimated US West Coast populations of ~20 starfish species3 and outbreaks of eelgrass wasting disease resulted in declines in habitat area as high as 90% in parts of California and Washington (Wyllie-Echeverria, personal observation). Pathogens causing these wasting disease outbreaks have been in the environment for at least decades4, although the causative virus for sea-star wasting is newly described3. These recent examples serve as reminders that disease outbreaks can rapidly and extensively devastate populations of keystone species and key habitat builders. Both events also caught the scientific and management communities by surprise, underscoring the importance of developing forecasts and long-term projections of conditions that increase outbreak likelihood. Forecasts of conditions conducive to disease onset have been most extensively developed for the agricultural crop sector5, 6 because of the economic value of optimizing the timing of pesticide application. Studies presenting longer-term, climate-model-based projections of conditions that promote disease onset for other plants and animals are far more rare. So far, climate models driven by Intergovernmental Panel on Climate Change (IPCC) emissions scenarios have been used only to develop projections of conditions related to the causative agents and vectors of human diseases7, such as malaria8, 9, 10 and Chikungunya virus11. Overall, the science of developing forecasts and projections for wildlife diseases is in its infancy and warrants much greater research focus7, especially in the marine environment, where disease outbreaks have been increasing in frequency and severity over recent decades12. Climate-related diseases have already severely impacted the primary framework builders of coral reef habitats12, 13, 14, 15. Of the range of bacterial, fungal and protozoan diseases known to affect stony corals16, many have explicit links to temperature, including black band disease17, yellow band disease18, 19 and white syndromes13, 20, 21. Here, we apply the climate models used in the IPCC 5th Assessment Report (see Supplementary Table 1 for list) to project three temperature conditions that increase the susceptibility of coral hosts to disease or increase pathogen abundance or virulence. We posit that temperature conditions that increase host susceptibility, pathogen abundance and pathogen virulence will substantially increase the likelihood of disease outbreaks once the set threshold frequencies and stress levels are surpassed. The output from the climate model ensemble for each of these three conditions is a projected year by which the target frequency or stress level is reached. All projections are presented for RCP 8.5, the emissions scenario that best characterizes current conditions and emission trends, and for RCP 4.5, which represents a pathway to stabilization at 4.5 W m−2 (~650 ppm CO2 equivalent) after 2100 (ref. 22). Along with the individual projections, we present maps of the earliest and latest projected year one of these three conditions favourable to disease development is projected to occur. We also present: comparisons between the projected timing of these conditions and annual severe coral bleaching, a map of a composite metric of stress caused by local human activities that can also increase host susceptibility, and a map of disease risk under RCP 8.5 that combines global climate and local anthropogenic stress.
The year in which host susceptibility is projected to exceed the set threshold (that is, sublethal bleaching stress three times per decade) varied spatially throughout all reef regions, but with a clear latitudinal trend. Reef locations in the tropics (<23° latitude) suffered thermal stress conducive to disease before subtropical reefs (23°–32.5° latitude), a pattern that was similar under both RCPs (Figs 1a and 2a). There was little variation (<5 years) in the projected timing of this condition among locations in the tropics (Fig. 1a). In contrast, some northern hemisphere subtropical reefs, such as in the Red Sea and Persian Gulf, were projected to experience these conditions ~20 years later than subtropical reefs in the south of Australia and Madagascar. Overall, under both RCP 8.5 and RCP 4.5, the median year this threshold will be surpassed was 2011; most (~76% as of 2014) of the world’s reefs are already experiencing thermal stress potentially conducive to disease outbreaks. Under both RCP 8.5 and RCP 4.5, the metric for increased host susceptibility will be reached at >90% of reef locations by 2020 (Fig. 2a).
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