It was long thought that the ocean harbours far fewer species than the land. Of the 1.8 million species identified and named by scientists, only around 275,000, or 15 per cent, are marine. Yet, as only a fraction of living species are known to science, these numbers fall far short of the world’s real diversity. Estimates vary wildly, but a middle-of-the-road guess is that, in reality, there are actually 10 to 30 million species on the land alone, suggesting that only a tenth or less of terrestrial species has been identified.

Many marine scientists think this problem is much more acute at sea. They think that the real reason that so few marine species are known is not because the biodiversity really is low, but because the ocean has been so poorly studied. It is vast, taking up some 95 per cent of the living space on Earth, and is mostly remote and extraordinarily hard to access. Scientists have sampled only about 7 per cent of it.

Speculation and controversy
There may be at least as many species in the seas as on land. A 1992 study estimated that 10 million or more invertebrate species live on the deep-sea floor alone. By another estimate, there are at least a million coral-reef species.

These estimates are controversial, but there is no question that the oceans harbour most of the world’s biodiversity at a higher biological level. Life originated in the sea, and most life forms never left. Of the 34 major animal groups, or phyla, all but one occur in the ocean and 15 are found there alone. By contrast only 15 animal phyla live on land; 17 in freshwater. While most terrestrial species belong to a single group – insects – the richness of marine species is distributed among a number of major groups, with fundamentally different body architectures and lifestyles. Thus, the sea nurtures most of our planet’s biological heritage.

This store of biodiversity is of immense value. It provides food, fertilizer, pharmaceuticals, jewellery, clothing and additives to food, cosmetics and household products. With so many species undiscovered, the potential for new products is vast.

The global community must take urgent steps to protect marine diversity

The services provided by marine ecosystems – such as regulating the atmosphere and climate, protecting coastlines, purifying water and providing opportunities for tourism and recreation – are even more important. By one recent calculation, marine goods and services are worth $20 trillion annually, comparable to the entire global gross domestic product. Regardless of its value in economic terms, marine biodiversity is essential to the very existence of life on Earth and therefore truly invaluable.

Unfortunately there is growing evidence that, as on land, it is under threat. Rich coastal habitats around the world have been destroyed by landfills, dredging, damming or channelling rivers, and other coastal developments. Half the world’s mangrove forests, for example, are already gone.

Most of the world’s commercial fisheries are under intense pressure, threatening not just the relatively few species targeted but the many more that are taken as by-catch or whose habitats are disrupted. For example, trawling for cod and other species has driven once abundant skate on the east coast of North America to the brink of extinction. Parts of the North Sea floor are trawled eight times a year, presumably with dramatic impacts on seabed communities.

Pollution gets everywhere. Persistent toxic substances like DDT and PCBs have poisoned seabirds and marine mammals and may have broader, more subtle effects on ecosystems. The massive alteration of ecological cycles is even more alarming. More than half the nitrogen entering the ocean now comes from human sources: the effects are not well understood, but have been associated with blooms of toxic phytoplankton and great ‘dead zones’, nearly devoid of oxygen, in the seas. The ecology of phytoplankton, vitally important in removing carbon dioxide (CO₂) from the atmosphere and producing oxygen, could be altered on a global scale.

Exotic issues
The introduction of species to places far from their home waters, where unchecked by natural controls they sometimes crowd out native biota, is an ever increasing problem. The possible effects of rising temperature, increased CO₂ and ultraviolet radiation levels – as a result of global warming and the thinning of the ozone layer – are still uncertain, but there is increasing cause for concern. Coral reefs, for example, may be directly affected by increased CO₂ in the atmosphere as well as by the rising temperatures it brings.

Biologists once thought that marine species were practically immune to extinction because currents spread their drifting larvae over huge areas, making it hard to wipe them out. Recent evidence, however, suggests that many marine species range much less widely over the oceans than previously realized, and so are more vulnerable. And even widely distributed marine species have been driven to extinction.

Losing certain ‘keystone’ species can have cascading effects. Sea otters like to eat sea urchins, which in turn eat kelp and other seaweeds. As otter populations have crashed in the Aleutian Islands, urchins have proliferated and are eating their way through species-rich kelp forests. Killer whales, unable to find enough of the seals and sea lions they prefer, seem to have switched to eating otters. And scientists speculate that the seals and sea lions have declined because their food has been decimated by commercial fishing.

We know too little about such interlinkages to predict how life in the oceans – and therefore the entire planet – will respond to our continuing insults. There is much to learn at even a basic level. We now realize that the deep sea is highly diverse, though thought lifeless only a century and a half ago. New discoveries have recently revolutionized our view of marine ecosystems. Microbes so tiny that they went undetected for centuries have been found to dominate much of the sea’s primary production, for example.

Superficial understanding
We do not really understand the ‘biological pump’ of CO₂ from the atmosphere to the deep sea, which critically modulates the greenhouse effect. We have only recently learned that the availability of iron controls primary production over huge areas of the sea. Entire ecosystems, depending on chemical energy rather than the sun, have been discovered at deep-sea hot springs. With our superficial understanding of the oceans, we cannot gauge the prospects for global marine biodiversity, or the consequences of losing it – but we do know the stakes are high.