Science for sustainable reefs
This topic is sponsored by the Australian Research Council Centre of Excellence for Coral Reef Studies.
Pollution, overfishing, coastal development and climate change are putting the world’s coral reefs under increasing pressure. With millions of people relying on them, how can science help make our reefs sustainable?
A healthy coral reef is a thing of beauty and a wonder to behold. Coral reefs make a great holiday destination but for many of the world’s people they are so much more. Approximately 500 million people depend on coral reefs for food, coastal protection, building materials and income from tourism. This includes 30 million who are almost totally dependent on coral reefs for their livelihoods or for the land they live on (atolls). But this precious resource is under growing pressure and in serious decline.
According to the report Status of coral reefs of the world: 2008, 19 per cent of the world’s coral reefs have effectively been lost and a further 15 per cent are seriously threatened with loss within the next 10 to 20 years. In some regions the losses are significantly higher. The coral reefs of the Caribbean, for example, have declined in cover by 80 per cent in just the last three decades.
And what is threatening our coral reefs? Basically it’s us and our activities. Overfishing, pollution, disease and habitat destruction are some of the direct threats, but overarching and interacting with these are a suite of more serious problems connected with climate change – warming oceans, ocean acidification and rising sea levels.
However, it’s not all bad news, especially in the short term. As scientists investigate how coral reefs bounce back after major disturbances (such as storms) they’re discovering that healthy coral reefs can have enormous resilience. Possibly our best preparation in the face of climate change is to focus on keeping our reef systems healthy and help them sustain themselves, but it may be that we cannot protect them all.
How are we affecting reefs?
Overfishing has led to the collapse of many of the planet’s biggest fisheries. While coral reef fisheries only account for around 2 to 5 per cent of global fisheries, they are vital to some of the poorest people in the developing world. Losing reefs deprives many of these people of their livelihood. Fishing pressure on many coral reefs has increased dramatically with the emergence of export markets for restaurant and aquarium trades based on rapid air transport. In places like the Coral Sea fishing pressure has grown rapidly in the past 20 years, and catches are declining.
Even within the Great Barrier Reef, regarded by many as one of the best managed reef systems in the world, overfishing is being experienced among some key fish groups. Populations of coral reef sharks, for example, are in the midst of a catastrophic collapse. Grey reef shark numbers have already dropped to around 3 per cent of unfished levels.
And when fish stocks take a battering, the coral reef is deprived of the ecosystem services provided by the fish. For example, removal of herbivorous fish can cause an increase in seaweed (algae) as it is no longer kept in check by grazing fish. The algae can then outcompete corals and overfished reefs become less resilient to disturbances such as large storms and bleaching events.
Human activity at sea impacts on coral reefs but so too does human activity on land, as nutrients, sediment and pollution find their way to reefs in runoff.
Crown-of-thorns starfish Outbreaks of this coral-eating pest are not necessarily correlated with human activities, but the ability of reefs to recover is.
The overarching menace of climate change
It now seems certain that coral reefs will be the first marine ecosystem to suffer extreme damage and possible collapse from climate change. The major consequences of increasing greenhouse gases on coral reefs include:
- coral bleaching from warming oceans
- rising ocean acidification from dissolved carbon dioxide (CO2)
- more severe storms
- rising sea levels
Coral bleaching occurs when symbiotic algae are ejected from their coral homes. Repeated ocean-scale bleaching events have hammered home the message that warming oceans pose a critical threat to coral reef systems. The extreme El Niño event experienced in 1998 resulted in the most extensive coral bleaching and mortality ever recorded, with approximately 16 per cent of the world’s coral reefs being effectively destroyed (since then approximately three quarters of these have recovered). And in 2005 many coral reefs of the wider Caribbean were devastated when a series of major ‘hot-spots’ developed during the northern summer. The year 2005 was also a record hurricane year, which resulted in considerable loss of coral reefs.
But even if bleaching doesn’t occur, warmer waters have a range of other impacts as the stressed corals become more vulnerable to disease.
Increasing concentrations of carbon dioxide in the atmosphere are raising the acidity of our oceans. If seawater acidifies by only a few tenths of a pH unit more, many corals, diatoms and shellfish will be unable to grow their skeletons and shells, posing the risk of extinctions and threats to marine food chains.
Rising sea levels are already encroaching on low lying islands and coastal regions, threatening people’s land, shelter and water supplies. However, compared to warming oceans and acidification, rising sea levels are likely to have less of an impact on the reefs themselves.
Sustaining our reefs
Healthy reefs look after themselves
In the past 50 million years, coral reefs have shown extraordinary resilience, coping with massive swings in climate and ocean level. Why is it then that many seem to be failing in the face of change now? The answer lies partly in the multiple stresses human activities are placing on reef systems, but the main reason is the speed of climate change we are undergoing.
Because of its size, not all parts of the Great Barrier Reef have been equally affected by human activities. The places affected the most are inshore areas, especially those influenced by deterioration in water quality from sedimentation, which has not affected most outer reefs. Outer reefs may also be more resilient to damage eg, from bleaching and crown-of-thorns starfish because the water in which they grow is in good shape. Inshore corals are prone to losing out to seaweed due to higher levels of nutrients and sediment, as well as coral bleaching caused by hot summers. The capacity of corals to recover from damage will be critical to their future in the face of climate change. While healthy, biodiverse reefs can be resilient to disturbances such as warmer water, multiple threats to reefs decrease their ability to bounce back (Box 1: Strength in diversity for coral reefs). We can reduce the impact of run-off through effective management of coastal development and catchment areas, and we can manage the effects of overfishing. However, in the longer term, the future of the Great Barrier Reef and other reefs of the world will depend on international initiatives to curb carbon dioxide emissions.
MPAs for a coral future
An effective way to reduce the stress of overfishing and maintain reef biodiversity is to establish marine protected areas (MPAs) in which fishing is restricted or banned. The Great Barrier Reef represents the most ambitious and possibly the most successful example of what networks of MPAs can achieve. In 2004 a strict no-fishing policy was imposed on a third of the reef area to form (what was at the time) the world’s largest network of no-take reserves. In just a couple of years there was a spectacular recovery in coral trout numbers on unfished reefs. In the longer term, the reserve system should enhance the sustainability of reef fishing and the reefs' ecosystem in general (Box 2: The Great Barrier Reef MPAs: how effective are they?). This in turn will provide greater protection for the tourism industry on the reef, which is worth in excess of A$5 billion.
Room for coral (with space for humans)
So if the problem is one of human impact, is the solution globally to simply remove the threats and place our healthy reefs in reserves? The answer is yes, but unfortunately there’s nothing simple about it. Human activity is an integral part of most of our coral reef systems and it can’t be turned on and off like a switch. Indeed many studies have shown that setting up no-take marine reserves that ignore the needs of the local population usually don’t work.
Take the Coral Triangle, for example, the world’s most biodiverse marine province. Millions of people depend on its reefs for their livelihoods, yet the region as a whole is deteriorating rapidly. The Coral Triangle Initiative, the biggest reef conservation movement ever, is attempting to reverse this by getting all countries involved to agree on measures to protect reefs. One of the guiding principles of the Coral Triangle Initiative includes the support of people-centred biodiversity conservation. As Professor Terry Hughes, Director of the Centre of Excellence for Coral Reef Studies states:
'...there are around 200 million people who depend on it for their livelihoods. You cannot ignore the needs of these people in devising ways to protect their marine diversity. You have to design your conservation measures so that they also address things like ecosystem services which the ocean provides to humans, and sustainable livelihoods for people who depend on the sea, as well as protecting biodiversity.'
The success of the reserve system in the Great Barrier Reef is in part due to the support built during its development. The Great Barrier Reef Marine Park Authority sought and gained the support of the public, industry and governments at all levels for putting the management of the world’s largest coral reef system onto an ecological footing. Backing all of this was the necessary legislation and regulatory powers and also good science to constantly inform the management process. Unfortunately, the example of the Great Barrier Reef is more the exception than the rule. Marine zoning in some countries has been severely limited because of poverty, inflexible institutions, lack of public support, difficulties developing acceptable legislation, and failures to achieve desired results even after zoning is established.
The future of the world’s coral reefs is looking grim on many fronts, and climate change will certainly redraw the coral distribution maps of the world. As this century unfolds, unless we curb our emissions
and promote resilience in our reefs, corals are going to deteriorate to the point where we could lose
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Posted September 2009, edited August 2012.