I heard about the study of loss of coral cover during the 12th International Coral Reef Symposium in Cairns, Queensland, this July. The peer-reviewed publication was today posted on-line at the prestigious journal, Proceedings of the National Academy of Sciences. It’s an open-access article meaning that anyone can download it for free from the site.
Beginning some 27 years ago, driven partly by an awareness that outbreaks of crown-of-thorns starfishes were causing significant coral damage, the Australian Institute for Marine Science commenced a long-term monitoring program along the full length of the Great Barrier Reef. This program used close to 300 randomly selected reefs distributed throughout the region, and made annual research cruises to monitor a random subset of reefs each year. To monitor a reef, a diver was towed on a manta board round the full reef perimeter – usually several kilometers in circumference – counting starfish and estimating coral cover. Additional studies were undertaken on a subset of reefs each year to gain additional information about coral health and other aspects of the reef ecosystem. The program continues, and is the most extensive, longest-term quantitative monitoring program on any reef system anywhere in the world. The data used in the 2012 report come from 2,258 surveys, from 214 different reefs distributed across and up the full length of the Great Barrier Reef over the years 1985 to 2012.
The main results are definitely sobering. During these 27 years average coral cover (the percentage of the substratum covered by living coral) has declined from 28.0% to 13.8%, a loss of 50.7% of coral cover initially present, and a rate of loss of 0.53% per year. There is considerable variation from place to place and year to year in the amount of coral cover lost. The losses are primarily attributable to three causes: outbreaks of Crown-of-Thorns starfishes (42% of losses), tropical cyclones (hurricanes, 48%), and coral bleaching caused by warmer than usual water (10%). All three of these stressors have become more severe because of human activities.
The Crown-of-Thorns, Acanthaster planci, is a predator of living coral that occurs naturally throughout the vast Indo-West Pacific region. It has the habit, common to many echinoderms (starfishes, sea urchins, and their allies), of erupting periodically into enormously dense populations that literally eat themselves out of house and home and then die back. It has this habit because its life cycle begins with a tiny pelagic larval stage produced in enormous numbers by breeding females. In the majority of years, nearly all these larvae succumb to starvation or predation before they complete larval life, but n occasional years when conditions are particularly favorable, far larger numbers of larvae survive to become juveniles and suddenly there are lots of starfish looking for coral to eat.
Scientists estimate that prior to European settlement in Australia, Crown-of-Thorns eruptions occurred once every 50 to 80 years. They damaged reefs severely, died off, and the reefs then recovered. With European settlement, the Queensland coast has become a major agricultural area, and run-off rich in fertilizers and other nutrients fills the rivers that flow out to the Great Barrier Reef lagoon. The nutrient enrichment that has occurred over time, has created good conditions for larval starfish survival on average about once every 15 years (patterns of flood and drought interact with farming practices to create conditions that are particularly favorable to the starfish larvae only once in a while (but far more often than in the past, prior to European settlement). The monitoring data show impacts of starfish in every year, but particularly during two periods – 1986-1992 and 1998-2003.
Figure 2 from De’ath et al. 2012. PNAS
Cyclones are another factor that has always impacted the Great Barrier Reef, but global climate models have shown that, with warming temperatures, tropical storms are going to become more intense. In Australia’s case, five severe category 5 cyclones hit the Great Barrier Reef between 2005 and 2011 including cyclone Yasi in 2011, the largest cyclone ever recorded to hit Australia, and cyclone Hamish in 2009 which travelled south-east along two thirds of the length of the Great Barrier Reef instead of crossing the reef and going inland as most cyclones do. The monitoring data show that cyclone damage has been particularly high during three periods – 1989-1992, 1997-2001, and 2009-2011.
Cyclone Yasi as it approached the Queensland coast – the largest storm ever to hit the GBR. See more here.
May of Cyclone Hamish’s route along the Great Barrier Reef. See account here.
The stress of warmer than usual water can cause corals to expel their symbiotic algae (termed bleaching), and if stressful conditions persist for about 3 weeks most of the bleached coral dies. Mass bleaching events, first noted in the early 1980s, have become frequent events around the world, including on the Great Barrier Reef. On the monitored reefs, bleaching mortality was a significant process in five years – 1998, 1999, 2001, 2002, and 2003. As is well recognized, as climate change continues, episodes of warm water will become more frequent, and bleaching events will become more common.
Pulling all these observations together, it is clear that for the past 27 years and probably longer, our impacts on the environment have been increasing the mortality caused by the starfish, the cyclones and coral bleaching. The overall rate of mortality is great enough that the abilities of the corals that survive to grow and replenish killed patches of reef are no longer sufficient to cope, and as a result, the Great Barrier Reef is progressively losing its coral, at a rate of 0.53% per year. That rate is sufficient to eliminate the entire reef well before the end of this century, but we also know that that rate is going to increase as we continue to warm the planet.
It’s important to mention that the Great Barrier Reef is the best managed coral reef system worldwide, and that Australia has even succeeded in making inroads into the problem of nutrient enrichment due to agricultural run-off. This depressing tale of loss, is a tale of loss in spite of a major effort, at considerable financial cost to look after the Great Barrier Reef, and ensure it would be here for the future. It’s also worth noting that recent reports from the Caribbean confirm it has lost about 80% of its coral cover since the mid-1970s. The proximate stressors have been different, but again they have been ones that have become more severe over time due to human activities.
Sobering news, indeed. And I am tempted to ask if we have now reached the tipping point. Are we seeing the beginning of the predicted unraveling of the natural world, the unraveling that will soon run beyond our capacity to control it? It really is time to start a major effort to shut down our fossil-fuel economy and try to bring the Earth back towards a place with which we are more familiar – a place in which we might have a much better chance of surviving. So, tonight, think about the Great Barrier Reef, that vast labyrinth of coral reefs stretching 2000 km down the Queensland coast and averaging 150 km in width. It’s awesome. And we are killing it off. And it does matter, because our changes to the Earth’s systems are impacting other parts of the biosphere as well. We are all going to be directly affected unless we fight hard to correct our behavior. We need a CO2 concentration in our atmosphere of 350ppm. That is what it was in the early 1980s. It’s now pushing up against 400ppm, and our use of fossil fuels is growing.