A Research Review of Interventions to Increase the Persistence and Resilience of Coral Reefs (2019) / Chapter Skim
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1 Introduction
1 Introduction
Pages 17-40
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From page 17... ...
Recent losses of coral cover have increased dramatically following the multiple global bleaching events from 2014 to 2017 (Hughes et al., 2017a, 2017b, 2018; see Figure 1.1b) , driven by an extensive and prolonged ocean warming event (NOAA, 2018)
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From page 18... ...
as these factors exert long-term steady pressure on coral growth, survival, and reproduction. Pulse stressors include storms, bleaching events, or disease outbreaks that are more acute, periodic, and destructive over short time periods.
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From page 19... ...
Temperature Stress By far the most attention has been paid to the global effects of ocean warming on coral health. The link between temperature and corals derives largely from the bleaching response of coral cells to higher-than-normal temperatures (see Box 1.1)
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From page 20... ...
Thus, bleaching susceptibility typically is linked to the sum of excess degrees of heat over weeks of exposure -- measured by the National O ceanic and Atmospheric Administration's (NOAA's) Coral Reef Watch program as Degree Heating Weeks (DHWs)
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. Cellular mechanisms of coral bleaching are much less understood than ecological patterns of bleaching.
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Although detailed study of the genetic mechanisms of coral bleaching or heat tolerance is still in progress, little evidence currently exists to suggest that there are a few master genes of large effect controlling heat tolerance in many species. Ocean Acidification A second global stressor related to climate change is the gradual acidification of the oceans because of increasing carbon dioxide (CO2)
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(b) Community calcification rates in the Biosphere 2 coral reef mesocosm as aragonite saturation changes.
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, mean Ωa in coral reef waters would drop another full unit. Recent work on the southern Great Barrier Reef demonstrates that OA has already caused an approximately 6% drop in reef calcification (Albright et al., 2016)
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. Importantly, future climate scenarios have been projected to result in increased disease outbreaks, impacting coral populations globally (Fabricius, 2005; Harvell et al., 1999, 2002; Maynard et al., 2015)
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Nevertheless, the clear links between anthropogenic stresses on reefs and higher disease prevalence highlights that as we move to climate regimes outside previously experienced thresholds, the potential for diseases to impact coral populations at broad ecosystem scales increases as well. LOCAL STRESSORS AND STRESS PREVENTION Because of the high severity and widespread occurrence of local human pressures on coral reefs, there have been many studies, and decades of effort, to measure the impact and mitigate the effect of human activities on reef ecosystems.
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From page 27... ...
In each case, interventions that address the source of local stressors have been identified, and in some cases, stressors have been mitigated. Despite decades of understanding the nature and extent of these local stressors in causing coral reef health declines (e.g., Russ et al., 2015)
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From page 28... ...
. Therefore, while local- and regional-scale management of local stressors will continue to be critical under climate change because it reduces chronic mortality and facilitates recovery from pulse stressors, these efforts on their own are inadequate in the face of ongoing climate change (Anthony, 2016)
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From page 29... ...
In addition, indirect reef destruction occurs through increases in sedimentation associated with coastal development, pollution, and other kinds of deleterious human impacts. Habitat destruction can also occur from overharvesting colonies for the aquarium trade or nursery construction, and from damage from tourism (Davenport and Davenport, 2006)
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It is worth noting here that healthy coral reefs provide natural coastal protection against storm waves ( errario et al., 2014) , an ecosystem service worth trillions of dollars on a F global scale (Beck et al., 2018; Costanza et al., 2014)
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. Recently, even terrestrial invaders have been shown to harm coral reefs; rat-plagued islands in the Indian Ocean have fewer seabirds, which in turn results in a variety of impacts on the surrounding reefs, including fewer nutrients and fewer algae-eating fish (Graham et al., 2018)
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. To avoid this outcome, the majority of the world's coral reefs would require an adaptation rate of approximately 0.2°C-0.3°C per decade, with rates of up to 0.5°C-1.0°C per decade in certain regions (Donner et al., 2005)
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From page 33... ...
(2018) modeled expected reef accretion rates under projected sea level rise for tropical western Atlantic and Indian Ocean reefs and found that most reefs are expected to experience a mean water depth increase of more than 0.5 meters by 2100 under RCP8.5.
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The committee's final report will build on this review to develop a sciencebased environmental risk assessment and decision framework to help inform actions to be taken by coral reef managers. As described in the committee's task, this report addresses item 1, with the remaining items to be covered in a subsequent report (see Box 1.2)
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From page 35... ...
However, they are important to consider as part of a toolkit of options for increasing coral survival. While a focus on enhancing stress-tolerant genotypes or species is inherent to many of the interventions described in this report as a way to maintain overall coral cover, maintenance of diversity of genotypes or species is another consideration for supporting the goal of increasing resilience of coral reefs and their associated ecosystem services.
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From page 36... ...
and long-term environmental scenarios as part of an overall risk assessment in an ecosystem context. The coral intervention strategies will be assessed with regard to the goal of increasing the long-term persistence and resilience of tropical coral reefs and their ecological functions.
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Interventions should be characterized, using designations such as "not appropriate," "needs further investigation," "feasible for field testing," "feasible for implementation." Atlantic/ Caribbean coral reef systems are specified for this assessment due to their advanced state of coral reef degradation, less complex ecological conditions (e.g., smaller basin, lower diversity) , and imperiled status of foundational reef building coral species, compared to the Indo-Pacific. Two reports will be produced.
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From page 38... ...
Whether coral reefs exhibit alternative stable states is controversial (Mumby et al., 2013; Zychaluk et al., 2012) , where the possibility of both coral-dominated and algal-dominated states being locally stable is likely most relevant under low herbivore density and diversity that would otherwise control algae dominance.
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From page 39... ...
. In addition to adaptive capacity or resilience to a single stressor such as thermal stress, having a diversity of genotypes or species might increase the likelihood of reef persistence when exposed to multiple stressors (e.g., disease, pollution, sedimentation, acidification, sea-level rise)
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From page 40... ...
levels are considered. As directed by the statement of task, the committee does not emphasize management of local stressors.
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