aters teeming with five-foot long sturgeon and codfish. Oyster reefs so high and numerous that ship captains fear navigating the waters. Gray whales swimming offshore. It sounds like an exotic, untouched land, and it was—the East Coast in the 1500s and 1600s.

Charles Peterson, professor of marine sciences, barely stops for breath as he tells story after story of marine animals lost to human exploitation—in North Carolina, Virginia, the Caribbean, Florida, Alaska. But the problem isn’t just that we don’t have as many pretty animals to admire. According to a study conducted by Peterson, his friend and colleague Jeremy Jackson, and other researchers, the overfishing and overharvesting of marine animals that began long ago has markedly changed the balance of life in our waters.

Peterson was drawn into these stories by Jackson, of the Scripps Institution of Oceanography. Jackson loves reading historical accounts written by sea captains, especially a pirate named William Dampier. Many of these captains had to become amateur natural historians to survive waters rife with coral reefs or other dangers. "Sometimes they had to feed a crew that had run out of supplies, and here they were on a strange shoreline, and so they had to know how to survive," Peterson says.

With Dampier’s accounts as well as British records of turtle hunting, Jackson concluded that tens of millions of turtles, some as large as 500 pounds, lived in the Caribbean in the 1700s. But people heavily harvested the animals for their meat and for their shells. Records show that between 1688 and 1730, people harvested 13,000 turtles from one island alone. As the turtle population dwindled, turtle grass went ungrazed and grew out of control. The grass eventually began to rot, depleting the water’s oxygen and killing fish and shrimp.

Jackson suspected that similar scenarios had played out in many marine habitats. A "keystone" species is overharvested, disrupting an important function—such as the turtles eating the sea grass. An alternate species may take over the keystone’s function and keep the system going. But when that second species runs into trouble, the whole system can collapse, resulting in poor water conditions or overgrowth of pests. The effect may not be felt until years after the keystone species is first disrupted.

Jackson called on Peterson and 17 other colleagues to explore patterns of decline of species worldwide. The team looked at tropical coral reefs, kelp forests, sub-tidal shores, and estuaries in different parts of the world. Contributors included ecologists, who studied trade records and other historical documents; archeologists, who studied historical records and data from excavations of kitchen middens (scrap piles left behind by humans); and paleontologists, who studied fossils. The results of the group’s work were published in the July 27, 2001, issue of the journal Science.

he team searched back hundreds of years to get a baseline of how ecosystems looked before human intervention. Peterson explains why. "We have some feeling of how natural systems have changed over one, two, or, at most, three generations of human lifetimes." And ecology as a discipline began only within the last 100 years. "That doesn’t come anywhere close to bringing us back to the period where we really started affecting these populations."

As the team, including Karen Bjorndal of the University of Florida in Gainesville, studied turtles further, they found that the exploitation in the Caribbean began even before Europeans arrived. Middens created soon after American Indians reached the area between A.D. 600 and 800 are full of turtle bones, Bjorndal found. She estimates that today green turtle numbers are just 5 to 10 percent of their original level.

The story of another overexploited animal shows the value of diversity in the environment. "We talk about problems of declining diversity, and we have the Endangered Species Act to protect it," Peterson says. "But people may ask, what damn good is it? Why do we need all that stuff?" One answer: in the Caribbean, herbivorous fishes grazed the seaweeds and kept them under control while carnivorous fishes ate and controlled, among other things, sea urchins. But people seriously overharvested both kinds of fishes. With its primary predators all but gone, the sea urchin population exploded. A stroke of luck was that the urchins grazed the seaweeds too. "So the immediate impact of the overfishing was not evident," Peterson says. But when a virus struck the sea urchins, they were so numerous that it spread quickly. Now there are no fish, no urchins, and little coral on the reefs—the overgrown seaweeds block out the light that coral needs to survive.

"With the redundancy in function between the grazing fish and the grazing urchin, we were okay," Peterson says. "Having removed those grazing fishes, the urchin was the last bastion of prevention of the overgrowth by seaweeds. So when that went, the whole system fell apart."