News about ocean ecosystems is almost without exception grim. Collapsing fisheries, expanding dead zones, and the prospect of ever warmer and more acidic seas makes optimism a tough sell. When sounding the alarm bells, marine conservation scientists have focused primarily on the loss of "ecosystem services" -- things like food, shoreline protection, and tourist-attracting seascapes -- that translate into direct economic benefits for humankind. The fate of the basic building blocks of marine ecosystems, namely the many species that together make up the assemblages that provide those services, is far less often discussed.
This stands in contrast to the land, where worries about a sixth mass extinction associated with habitat destruction often underpin conservation concerns. To the public, mass extinction conjures up the demise of the dinosaurs due to a killer asteroid, but past cataclysms can be seen just as clearly in the records of marine species of clams, corals, crinoids, coccolithophores, and the like. These organisms may not grip the imagination like Tyrannosaurus rex, but their fossils tell us that things can go dreadfully wrong in the sea, with many once-dominant creatures vanishing in a geological instant. Scientists who study mass extinctions may disagree about causes in some cases, but they agree that such an event is not something we want to initiate. In the most extreme case, the mass extinction that occurred at the end of the Permian Age about 250 million years ago, over 90 percent of all marine species went extinct. A repetition of an extinction event even remotely approaching this level would have consequences for humans that are so grave they defy imagination.
Why have marine conservation scientists focused so little of their attention on extinction? In part, this stems from earlier assumptions that marine species are more resilient than their terrestrial counterparts, thanks to their geographically broad distributions. We now know, however, that many marine species are much less widespread than we once imagined, as genetic studies have revealed sharp breaks between otherwise similar organisms living in slightly different habitats or on opposite sides of ocean basins. Thus, the potential of marine species to escape extinction by being in many places has probably been exaggerated. Nevertheless, it has been hard to imagine that humans could cause massive marine extinctions because of the vastness of the ocean, its inherent ability to buffer change, and the fact that people cannot populate its surface.
Indeed, the number of marine species known to have gone extinct globally is relatively small, in one analysis just 21. Some of these were spectacular large organisms, such as Steller's sea cow, the Caribbean monk seal, and the great auk, whereas others, such as most of the fishes, invertebrates, and seaweeds on the list, are known primarily to specialists. Interestingly, many of them went extinct not recently but before the first half of the last century. Not surprisingly, hunting was often the culprit.
Of course, the numbers of extinct species could soon grow, and grow exponentially. Catastrophic declines associated with fisheries have brought some species to the brink of extinction, either directly as targets or indirectly as bycatch. Leatherback turtles, the vaquita porpoise, and the white abalone are a few examples where only heroic efforts will suffice. It is worth noting that we do not have to directly kill every single member of a species to cause the species' extinction; rather, once a critical minimum population size is reached, the ability of the species to successfully reproduce can be lost, and its numbers inexorably decline. Marine species are also often difficult to breed. The vaquita, for example, are so shy that the only good photographs we have of them were taken after the porpoises were killed by drowning in fishers' nets.
Even more worrying is the prospect of mass extinctions through wholesale degradation of marine environments, something once unimaginable but now all too real a prospect. So many people live on or near the coast that almost all shallow waters have suffered, and even if some healthy habitat remains, extinctions can occur because the amount of suitable habitat available determines the number of species that can survive. Thus, some of the species that are still with us may in fact be doomed, a concept known as the extinction debt, which looms over the biosphere much as financial debt looms over the global economy.
If humans affected only the shallow waters fringing our coastlines, that would be bad enough. But thanks to greenhouse gas emissions, even the open oceans have seen and will continue to see increases in temperature and acidity. Both are bad because species often have rather narrow physiological limits, and the two changes together represent a kind of double jeopardy (triple jeopardy when you add in the fact that industrial fishing fleets go nearly everywhere and have stripped the oceans of most of its top predators). Often lost in the debate on climate change is the fact that carbon dioxide stays in the atmosphere for at least a century; even if we reform our ways instantly, extinctions are still likely to occur. Failure to take substantive action soon will bring large changes to the fundamental physical and chemical state of our oceans, something once thought impossible, and make the lessons of past mass extinctions increasingly and scarily relevant.
For these reasons, marine scientists no longer just talk about lost goods and services but also discuss permanent losses in the biological diversity of the planet, which we humans call Earth but whose surface is mostly ocean. Coral reef scientists have been among the first to raise the warnings, because they study the most diverse and arguably the most threatened of all marine ecosystems. Reefs today take up an area smaller than the size of Texas, and yet they house about one-quarter of all the species of the sea. Overfishing, pollution, invasive species, warming-caused bleaching, disease, and acidification (think coral osteoporosis) have combined in perfect storm -- like fashion to decimate reefs around the world. Living coral has declined by 80 percent in places like the Caribbean, and a recent study suggests that one-third of all corals are at risk of extinction. This is a level even higher than that estimated for frogs, once considered the most vulnerable of all life forms. Unlike frogs, however, corals create the three-dimensional structures, visible even from space, that shelter somewhere between 1 million and 9 million species. Reefs are indeed the canary in the oceanic coal mine, a canary that has passed out on the floor of its cage. Miners of the past paid close attention to their canaries, and so should we.