Review: Hughes TP, Baird AH, Bellwood DR, Card M, Connolly SR, Folke C, Grosberg R, Hoegh-Guldberg O, Jackson JBC, Kleypas J, Lough JM, Marshall P, Nyström M, Palumbi SR, Pandolfi JM, Rosen B, Roughgarden J (2003). Climate change, human impacts, and the resilience of coral reefs. Science, 301: 929-933.

Feature Paper: Hughes TP, Baird AH, Bellwood DR, Card M, Connolly SR, Folke C, Grosberg R, Hoegh-Guldberg O, Jackson JBC, Kleypas J, Lough JM, Marshall P, Nyström M, Palumbi SR, Pandolfi JM, Rosen B, Roughgarden J (2003). Climate change, human impacts, and the resilience of coral reefs. Science, 301: 929-933.

Author Abstract: The diversity, frequency, and scale of human impacts on coral reefs are increasing to the extent that reefs are threatened globally. Projected increases in carbon dioxide and temperature over the next 50 years exceed the conditions under which coral reefs have flourished over the past half-million years. However, reefs will change rather than disappear entirely, with some species already showing far greater tolerance to climate change and coral bleaching than others. International integration of management strategies that support reef resilience need to be vigorously implemented, and complemented by strong policy decisions to reduce the rate of global warming.


Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.


Review: Now that we've looked at reasons for coral reef decline and looked at what happens to coral reefs once they decline, as well as the role of Marine Protected Areas (MPAs) in protecting coral and fish stocks, we will turn our attention to evaluating how coral reefs can fare given climate change effects that are outside of the control of most coral reef managers, and therefore, can still impact MPAs.


Today's paper looks at growing global threats to coral reefs, while our past papers dealt more with specific impacts that could more easily be addressed and policed.

As the authors state in their abstract, "the diversity, frequency, and scale of human impacts on coral reefs are increasing" globally. The authors also point out sadly that "there are no pristine reefs left" and from my own experience in the field, even remote uninhabited and unfished reefs suffer from floating rubbish, nets, and regional fisheries declines. Furthermore, "local successes at protecting coral reefs over the past 30 years [for example, through MPAs and grassroots efforts] have failed to reverse regional-scale declines."

And with global carbon dioxide increasing steadily (levels are already higher than anything in recorded recent history since about 600,000 years ago), the oceans will absorb the carbon dioxide and lead to decalcification of oganisms (as discussed in two past Science Corner reviews, or in the author's words, "future changes in ocean chemistry due to higher atmospheric carbon dioxide may cause weakening of coral skeletons and reduce the accretion of reefs, especially at higher latitudes."

The authors include some good figures in their paper to show the global extent of impacts on coral reefs as organisms and fisheries resources are traded globally. The authors also point out that as atmospheric carbon dioxide has increased steadily, coral bleaching has also increased in severity as atmospheric changes have lead to elevated temperatures and increasingly frequent El Niño-Southern Oscillation (ENSO) events.

The authors point out that corals start to bleach at about 1ºC above their mean summer maximum temperature exposure, and that "this threshold will be chronically exceeded as temperatures rise over the next 50 years, leading to predictions of massive losses of all corals."

While some corals may adapt, when I was working in the Maldives I was exposed to reefs that still had not recovered beyond 5-25% live coral cover 10 years after a particularly intense 1998 ENSO killed over 90% of corals on reefs throughout the central and northern Maldives (and 30-70% of corals in the southern Maldives died). Therefore, as bleaching events increase in frequency, the adaptation of a few coral species may be outweighed by the loss of most corals. 

If reefs continue to be impacted before they can fully recover, they may never reach a pristine state again and rather end up slowly (or rapidly in the case of the Maldives and Caribbean) dying off.

So, to get back to the title of this paper, we've discussed climate change and impacts, but what about the natural resilience or coral reefs? As shown in past papers, healthy coral reefs are more likely to ward off disease and to recover faster from impacts. But can corals adapt to climate change?

Yes, corals can adapt. They have symbiotic algae within their tissues that they rely on for photosynthesis, and some researchers (such as Dr. Rob Rowan of the University of Guam Marine Laboratory) have shown that there are multiple clades (or genotypes) of zooxanthellae (the algae living within coral tissues and providing food for corals through photosynthesis) and that some clades prefer warmer water and others colder water. So when corals bleach, the corals that survive have been shown to often switch their zooxanthellae clades to warmer-adapted symbionts so that they have fewer bleaching events in the future. 

However, as the authors point out, "a major concern is that the accelerating rate of environmental change could exceed the evolutionary capacity of coral and zooxanthellae species to adapt." And during the 1998 ENSO event, scientists showed through core samples drilled in large corals that many specimens of 500 - 1000 year old corals died in 1998 from bleaching stress. ENSOs aren't new... they've been around at least 13,000 years from the shift out of the last glacial ice age. But the main point is that in the last 1000 years, corals did not face the kind of stress that they did in 1998, and so many corals were stressed that year from high sea-surface water temperatures that 13% of corals died worldwide and nearly 70% of corals on many reefs in the Indian Ocean.

You can think of corals as very flexible to environmental changes when young but very specific to those conditions once they are a couple years old. So, corals in the Persian Gulf, for instance, often get exposed to temperatures in excess of 36ºC, while those same species might have only 25ºC maximums near the equator. Yet, even though two corals might be the same species, once a coral adapts to such conditions, if you were to take a specimen from more stable temperatures and move it to conditions like the Persian Gulf, it would almost certainly perish it's first year.

The authors show that MPAs do help corals after bleaching events because healthy herbivorous fish communities keep algae from overgrowing newly bleached (and dead) coral skeletons, thereby giving corals time to grow back quicker. However, there are still many research and management challenges, since MPAs do not stop the cause of global climate change, which effects corals worldwide.

The authors point out that more large-scale experiments and sampling of coral reefs needs to occur to model the scale of impacts and key biological processes to "go beyond the current emphasis on routine monitoring and mapping." Critically, the authors state that "most coral reef research is parochial and short-term, and provides little insight into global or longer-term changes." More emphasis on "temporal, regional, and global patterns of relative abundance or functional attricutes of species that could be exploited for management of resilience" needs to occur. The authors also call out for "studies of intergenerational (genetic) responses to climate change" for coral reef organisms, "particularly corals and zooxanthellae."

The take-home message is that conservation efforts and MPAs need "to be complemented by heightened protection of adjacent areas and by strong international policy decisions to reduce the rate of global warming." In the meantime as governments work for such changes in greenhouse gas emissions (the main cause of global climate change), the authors suggest that "30% of the world's coral reefs should be no-take areas to ensure long-term protection and maximum sustainable yield of exploited stocks." 

Since coral reefs are mostly tropical, and many tropical countries are poor, incentives need to be given to developing nations to protect their coral reef resources. Unfortunately, industrialized nations don't bear the onus of responsibility though they have the majority of coral reef researchers. As the authors point out, "even in affluent countries, such as the United States and Australia, less than 5% of reefs today are no-take areas. Wealthy countries have an obligation to take the lead in increasing the proportion of reefs that are no-take areas, while simultaneously controlling greenhouse-gas emissions."

Next we'll shift gears away from coral reefs, but stick with an important component of the marine environment: fisheries resources. One way to protect coral reefs and fisheries resources is to aquaculture sea life. Yet, it is not without its own challenges, often centered around on reproduction in confined spaces, nutrition, and diseases.