Tens of thousands of years ago, the arrival of people in the Americas, and in Australia and New Zealand, was followed by a wave of extinctions, particularly of the largest species, which made the most attractive game. More recently, rats, cats and goats have eaten their way through the native plants and animals of small and not so small islands; and California is home to four hundred introduced plant species, which have almost entirely displaced the native prairie. But in the next hundred years or so, we are likely to see something new, as human activities cause the disappearance of ecosystems on a global scale. Species living on mountain-tops are going to find their habitat disappearing, as warmer climates rise up to engulf them. And Steve Jones and J.E.N. Veron warn that climate change may well bring about the end of coral reefs – if overfishing, disease, invading species and pollution don’t get them first.
The surface waters of the open tropical ocean are poor in nutrients and almost lifeless because the warm upper layer does not mix with the cool water below; as a result, everything edible sinks, never to return. In shallow coastal waters, corals get round this by symbiosis. Reefs are built by polyps (tentacled creatures related to the sea anemone) that secrete limestone and live on the growing pile of rock. Polyps take living algae into their cells, which photosynthesise, and provide most of their hosts’ energy, as well as giving them their colour. In return, the algae get protection, and feed off the polyps’ waste nitrogen and carbon dioxide. Polyps in symbiosis with algae can grow up to a hundred times more quickly than a lone polyp. In waters where an efficient nutrient cycle prevents materials from washing away, reefs can be the most productive environments on earth, laying down carbon at twice the rate of an equivalent area of rainforest, and producing several centimetres of rock per year.
A hectare of tropical reef can support two tonnes of fish, and a quarter of all known fish species live on reefs. Reef fisheries account for less than 5 per cent of global catches (reefs cover only 0.1 per cent of the sea’s area), but they provide employment, and protein, to people in developing countries who would otherwise struggle to obtain them. More than half a billion people live within 100 kilometres of a coral reef, and conservation groups estimate that hundreds of millions, mostly in Asia, have some dependence on reefs for their welfare.
Like nearly all other fisheries, reefs are severely overexploited. Last year, British and Canadian researchers compared the actual catch in reef fisheries with an estimate of how much fishing they can support, based on the rate at which fish populations grow. They found that the one million tonnes of fish caught from the world’s reefs is three times more than is sustainable. Put another way, we would need four more Great Barrier Reefs to support current levels of fishing without depleting stocks.
The damage fishing causes is not restricted to tropical reefs. Two-thirds of coral species live in the deep sea, making do without algae by filtering food particles out of the water. Like most things in the deep, we don’t know much about them, but we do know of the existence of several hundred reefs, sometimes tens of kilometres long. There are nearly a hundred in the north-east Atlantic alone; the largest are the Darwin Mounds, discovered in 1998 a thousand metres down and two hundred kilometres north-west of Cape Wrath. Deep-water reefs support large fish populations and are often discovered by fishing boats. But because reefs damage fishing gear, trawlers have been known to fix chains or rollers in front of the nets, which smooth the seabed by scrubbing the reef away. And when tropical fisheries become so depleted that hooks, nets and traps are no longer effective, fishermen turn to explosives, or sometimes cyanide, which leaves fish alive but unhealthy (the method was pioneered by fishermen working to supply the aquarium trade). Both methods are dangerous for the fish, the reef, and the people using them.
By altering the food web, overfishing can trigger what ecologists call a trophic cascade, where the removal of one consumer has knock-on effects across the entire reef. Corals rely on herbivores, such as fish and sea urchins, to prevent them from being overgrown with seaweed. When grazers become scarce, the plants can smother the coral. And the removal of large predatory fish can lead to an increase in the population of coral-eating invertebrates, such as starfish.
Since the proportion of the global population living on the coast is rising, it’s difficult to see how fishing is going to be reduced unless the fish run out. But locally there have been some successes. The best way to regulate the industry is to limit where and when people are allowed to fish. Quotas are not as good, because they ignore ‘bycatch’, the undesirable species or illegally small fish whose corpses are dumped overboard.
Where areas of reef are closed to fishing, the effect on fish populations can be dramatic. Few ‘marine protected areas’, as they are called, have been in place long enough for their effects to have become clear; but in one of the oldest, Exuma Cays in the Bahamas, where fishing has been banned since 1986, there are now seven times more large predatory fish – usually the first to be caught – inside the reserve than in nearby unprotected areas. There are also more large herbivorous fish, which keep the seaweed down. And off St Lucia, which established a network of reserves in 1995, catches have gone up even though the area open to fishing has gone down, because the booming fish populations in reserves spill over into surrounding areas.
Some countries have already protected large areas of reef. In the past five years, about a third of the Great Barrier Reef has been made off-limits to fishing. Norway has banned fishing on all its deep-water reefs; the UK government and the European Commission have moved to stop trawling on the Darwin Mounds; and last September, deep-water trawling was banned across a large stretch of the South Pacific. But in poorer countries – the most species-rich reefs lie off the Philippines and Indonesia – a lack of resources hinders the establishment, management and enforcement of marine-protected areas. In the developing world, the best way to stop reserves from becoming ‘paper parks’ – areas that are marked on maps and protected in legislation, but not in practice – is to involve the people that live and work on reefs in their management. Sometimes this has been overlooked in the drive to protect wildlife.
Overfishing is now the biggest threat to reefs, but the growth and urbanisation of coastal populations has other effects. Sediment and chemicals washed into rivers and out into the sea clog coral, and run-off also carries nutrients and microbes that have increased the level of coral disease. About thirty diseases have been identified, with a palette of names such as black band, brown band, pink spot and yellow blotch; not all their causes are known. Over the past decade or so, 90 per cent of the Caribbean’s Elkhorn coral – one of the main reef-building species – has died, mainly as a result of white pox, which causes the soft coral bodies to melt off their bony skeleton. White pox is caused by a human gut bacterium washed into the ocean in sewage. Recently, the human herpes virus turned up in corals off Panama. Diseases also move around in the ballast water of ships, as do alien species: Pacific corals are showing up in Florida. The lion fish is another Indo-Pacific import that’s increasing in the Caribbean, probably after being emptied out of aquariums. (Following the release of Finding Nemo in 2003, many tropical fish were given their freedom, usually via the toilet. In response, several conservation groups, and plumbers in Los Angeles, mounted a ‘Don’t flush the fish’ campaign.)
A fifth of the world’s reefs have already been destroyed; another quarter are critically endangered, and another quarter are under threat in the long term. Reefs in the Caribbean and the Indian Ocean are in the worst state; the Great Barrier Reef is healthiest, though still in decline. A Reef in Time scarcely mentions the local threats: an indication of how completely climate change dominates discussion of conservation. We could block every sewage pipe and make every reef a reserve, and still the reefs will vanish over the next hundred years or so because of global warming, and two of its effects in particular: coral bleaching and ocean acidification.
Bleaching results from a combination of strong sunlight and warm water. In such conditions, algae photosynthesise faster and produce more oxygen. Oxygen is toxic in high concentrations, so the polyp has to spit out its overactive algae. A 1°C rise above the hottest summer temperature on record, sustained for a few weeks, would be enough to cause coral and algae to part company. So far the worst episodes of bleaching have occurred in El Niño years, when the South Pacific warms up. In 1998, one-sixth of the world’s corals were affected. Bleaching isn’t necessarily lethal. The associations between algal species and corals are loose and promiscuous: each can make do with any of a range of different partner species, and the algae can survive in open water. Bleached corals might pick up an algal species that prefers higher temperatures, or get their old partner species back when the water cools. But it would be over-optimistic to think that polyps can expel and ingest algae at will: thousand-year-old reefs died in 1998, and about half of those bleached have yet to recover. Since then, there have been several more local bleaching events: more than half the Great Barrier Reef bleached in 2002, which wasn’t an El Niño year, showing that the ocean is warming to the point where bleaching doesn’t need a helping hand.
But even bleaching is insignificant compared with ocean acidification. The ocean is a massive, though not limitless, sink for carbon dioxide: close to half of the carbon dioxide emitted by people has dissolved into its waters. This slows down the rate of global warming, but makes the sea more acidic, which in turn reduces the levels of carbonate that corals, molluscs, crustaceans, starfish, sea urchins and the vast quantities of shell-building creatures in the plankton use to build their shells and skeletons. Shells will get thinner, more brittle and more vulnerable to erosion.
Acidification is already affecting plankton growth in the cold southern ocean: carbon dioxide dissolves more easily in cold water, so acidification will hit the parts that bleaching misses. The danger is not so much climate change itself as its speed, which is historically unprecedented and faster than many species can adapt to. If emissions continue unabated, the rate of shell-building could halve by the end of the century, which means that many reefs will probably be unable to resist erosion. They will wither away, by a process Veron compares to osteoporosis.
Such changes, Veron writes, ‘will be permanent as far as humans are concerned’. He has spent more than thirty years diving on and studying the Great Barrier Reef, and he embeds his concerns about current environmental change in a story of climate and corals spanning the past half-billion years. On that timescale, few things look exceptional or irreversible. Reefs have always sprouted where they can, and died when the conditions changed: when they sank too far beneath the waves for light to reach their algae, when they were lifted up during earthquakes or exposed by falling sea-levels during ice ages, or when continental drift carried them into colder waters. Coral building has evolved, and ended, time and time again as various groups of sea anemones have found the habit useful or harmful. On five previous occasions, reefs disappeared entirely for several million years. The largest mass extinction in Earth’s history, at the end of the Permian period 250 million years ago, seems to have been linked to the global warming caused by a massive rise in atmospheric carbon dioxide. Only the resulting acidification of the oceans would have been widespread and powerful enough to cause the reefs’ disappearance during this and, probably, the other mass extinctions. Like many biologists, Veron believes a sixth mass extinction is in progress.
As Hugh Pennington has pointed out, flux is the rule for living communities, and one species’ extinction is another’s opportunity.Perhaps a shift from a world of corals and sea snails to one of soft-bodied worms and jellyfish is ethically neutral, and perhaps the best arguments that conservationists can muster, beyond a sense that it’s polite to leave things as you found them, are subjective, aesthetic and conservative (I might never see the Taj Mahal, but I’d be unhappy if it was demolished). Over the past decade, conservation biologists have sought to make nature politically easier to grasp by putting cash values not just on wild products, but also on the ecosystem services provided by nature. Besides fish, reefs provide the large but complicated economic benefits of tourism, and, on a much smaller scale, bioprospecting; there is already one drug from a reef-dwelling animal on the market, a powerful painkiller derived from the venom of a Pacific cone snail. Reefs also provide natural sea defences, shielding coasts from wave and storm damage. The estimate is that coral reefs worldwide are worth $30 billion per year, or $100,000 per square kilometre.
This moves the focus away from species and towards habitats, which in conservation terms is the right way to go. But it’s difficult to know what to make of numbers like these. They always seem huge (at least in comparison to the amounts spent on conservation), and if it’s money that matters, there might be more lucrative ways of using a shoreline – by building a hotel, say. Economic rationalism got us where we are now: overfishing and carbon emissions are tragedies of the commons, in that there’s no point stopping if everyone else is going to carry on, and if everyone else stopped then one’s own actions wouldn’t be a problem. That said, economics and ecological thinking aren’t necessarily at odds. British conservation biologists calculated in 2004 that a worldwide network of protected areas covering a quarter of the seas would create a million jobs and cost less than the subsidies currently given to the fishing industry.
The deeper significance of the disappearance of coral reefs is, in the end, what it tells us about things to come. Corals are peculiarly sensitive to environmental change. The biosphere can get by without them, and we probably can too. But if, over the next hundred years, every aspect of nature as we have known it for the past few million years were to undergo a similar shift – if the Amazon were to dry up and burn, or the tundra melt and give up its methane – we’d have more than the aquarium trade to worry about. ‘A change in a property that has been constant for millions of years could cause great loss of life among the organisms that have come to rely on that constancy,’ Veron writes. Human beings are organisms, too.