It's simple: Tiny animals build the great limestone sea walls, the banks and islands and atolls and archipelagos we call reefs, some of them hundreds of miles long, thousands of feet thick. These tiny builders we call corals. Corals have seen it all—they were here long before the dinosaurs came and went—but like any animal, when they're poisoned or dynamited, buried or crushed, smothered or cooked, they die.

So what? So these are precisely the things that people do to coral reefs, and so the reefs are in trouble. Marine scientists lately have taken a hard look at the world's coral reefs, and they say 10 percent are already destroyed, and 70 percent could be gone just 40 years from now unless we change our ways. Here before the dinosaurs, 70 percent wiped out in just a couple human generations.

Who cares if the corals go the way of the dodo? You should. Science has progressed to the point where we can understand this fact: We can't live without those tiny animals and their coral reefs—at least not all of us. People need the coral reefs. Let's count the ways:

Food. Coral reef zones are home to one quarter of all marine plants and animals: Nearly a million species of fish, crabs, eels, mollusks, sponges, worms, grasses, algae, and other marine animals live on reefs or use them as nurseries to protect their young. Corals also provide natural filtration of seawater for their neighbors. These reef ecosystems support vast fisheries that people, especially in coastal nations, depend upon for much of their protein. Collapse could mean famine.

Shelter. Natural harbors that take millennia to build, coral reefs provide people with living sea walls against tides, storm surges, and hurricanes. They also act as giant sand factories, creating limestone from dissolved minerals in seawater and leaving it behind as sand to keep shorelines from eroding.

Medicine and other resources. Like the tropical rainforests, coral reefs are a center of extreme biodiversity, a great reservoir of intriguing DNA we've hardly begun to explore and natural compounds we don't yet understand. Australian scientists in Queensland have developed a sunscreen from substances that corals use to protect themselves from ultraviolet light. It has an SPF of 50+. In Menlo Park, Calif., Neurex Corp. has developed an extraordinarily potent pain-killing drug from the poison of reef-dwelling sea snails. The paralytic agent, used by snails of the species Conus magus to render their prey helpless, is hundreds of times more potent than morphine. It is injected directly into a patient's spinal fluid, providing relief for those suffering from cancer and other agonizing conditions.

Fun and profit. Coral reefs are one big underwater amusement park for snorkelers and divers, a searingly colorful undersea world of Cousteauian delights—which drives a tourist industry worth tens of billions of dollars, in many cases propping up the economies of entire nations.

And what do people do for coral reefs? Some of us kill them to make a buck, but most of us just kill them. Let's count the ways:

Overfishing: In areas blessed with an abundant human population, the collapse of the world's fisheries is a familiar story, and tropical regions are just another chapter. Coral reef fisheries are hitting bottom in many regions, notably South and East Asia, where many overexploited reefs have been scoured of nearly all edible life.

Blast fishing: In depleted fisheries, people resort to desperate tactics to catch the fish that remain—one of those is dynamite. The explosions send dead fish to the surface and destroy living reefs; they can be heard from the Philippines to Kenya to the Caribbean.

Cyanide fishing: Restaurants and markets, especially those in East Asia, like to buy live fish; fishermen oblige them by stunning big fish with cyanide sprayed into the water. The fish are caught live, the market momentarily sated, the coral reefs killed.

Sewage: Organic wastes from human cities flood to the sea, bringing an overload of nutrients; algae take over the reefs, blotting out the sunlight corals need to live. It's called eutrophication and it's a major problem, especially in the Caribbean and Central America, where just 10 percent of sewage is properly treated before it's dumped in the sea.

Farm runoff: More eutrophication. Carried to the sea by rivers and streams, chemical fertilizers act much like sewage, overloading reef areas with nutrients for algae, choking the corals. Herbicides and pesticides are a toxic bonus. Florida is a prime example.

Oil and industrial pollution: Petrochemicals and heavy metals are a persistent threat to all marine life in coral reef zones, especially near urban areas and in the seas of the Middle East.

Sedimentation: When people clearcut forests or bulldoze new housing tracts and parking lots, tons of loose dirt is washed downstream and into near-shore reef areas, where it buries corals under a layer of silt and smothers them.

Tourism: Clumsy or just uninformed, tourists crush, scrape, gouge, and break off fragile corals with their hands, their scuba fins, and their ship anchors. Resort development destroys coastal mangroves, creates new sewage sources, and stirs up more silt that smothers reefs.

Disease: Abuse adds up, and reefs that aren't killed directly by people may be getting sick anyway from the accumulated stresses. Recent years have seen epidemics of many coral diseases and the discovery of several new ones previously unknown to science. Coral bleaching, a deadly ailment on the rise, is associated with higher water temperatures—but even that can be attributed to humankind if global warming models are correct. And the diseases seem to be getting meaner: In April, scientists reported in the journal Nature that a new species of coral-bleaching and -killing bacteria was wiping out reefs in two or three days, rather than the weeks or months it took previously.

Climate disruption: Coral bleaching aside, global warming will cause some obvious problems for corals, like decreased ocean salinity and rising mean ocean depth. Then there are the less obvious problems: Australian scientists warned in March 1998 that increasing CO2 in the earth's atmosphere was raising the acidity of surface water in the world's oceans, making it harder for corals to form the (basic) limestone skeletons that make up the reefs.

Coral mining: People excavate coral reefs for their limestone and sand, for use in building materials, resort hotel beaches, tourist souvenirs, even snake-oil medicines: A Swedish company, Ericssons Preventive Medical Group, claims its Alka-Mine Coral Calcium will "naturally detox the body by neutralizing the acidity with which we are all...too apt to pollute our systems." Sweden, of course, has no coral reefs; the product's mineral-rich coral sand is mined off Okinawa.

Mangrove destruction: The familiar tree of swamps from Mississippi to Mozambique, the mangrove provides a crucial service to coral reefs: It filters silt and even pollution out of terrestrial runoff before it can taint the clear water of the reef zones. People chop down mangroves for firewood and clear them for coastal construction.

At the MoJo Wire we like to end on a hopeful note, so here: Marine scientists, like their counterparts elsewhere in the conservation biology community, are making small strides toward the design of large-scale bioreserves. Coral experts at the University of York in England have discovered that concentrations of protected coral areas "upstream" in ocean currents can help regenerate damaged areas "downstream."

Scientists may even have a new form of first aid for injured reefs: Researchers at the University of Guam have managed to cultivate 10 species of coral, which they hope will lead to full-blown aquaculturing. This would allow aquarium suppliers to "farm" coral, hopefully reducing pressure on threatened reefs. Another possible application would be to grow live "patches" to be applied to damaged reefs, helping them regenerate. Stateside aquaculturists in Pittsburgh and the Treasure Valley area of Idaho also are breeding captive corals, both as a hobby and to establish a bank of corals that could be transplanted to damaged reefs.

Photograph by Wolcott Henry