While recent reports suggest Stone Age hunters drove dozens of species of huge land creatures to extinction, the cover story of the July 27 edition of Science describes the ecological extinctions of marine megafauna from overfishing at a global scale never before realized. The study, led by Jeremy Jackson of Scripps Institution of Oceanography at the University of California, San Diego, includes vast populations of whales, manatees, dugongs, monk seals, sea turtles, swordfish, sharks, giant codfish, and rays.
Recognition of what has been lost, however, also shows what could be gained. The scientists claim this revolutionary historical perspective is essential to management because historic data provide a framework for remediation and restoration that is otherwise invisible.
"Every marine ecosystem I have ever studied during my entire 30-year career looks unrecognizably different from the way it used to be, and I wanted to know why," says Jackson, a renowned marine ecologist who initiated the two-year study of human impacts on oceans over time.
"Comparing the magnitude of the mass ecological extinctions in the ocean to those on land may not be enough," states study co-author Roger Bradbury of the Australian National University in Canberra, Australia. "On the land, as we killed off the giant mammals and destroyed the ancient forests, we replaced them with a new suite of farmed species. In the coastal seas, we took out animals and replaced them with nothing."
Jackson convened an international team of 19 leading marine researchers at the National Center for Ecological Analysis and Synthesis (NCEAS) in Santa Barbara, California. Drawing on paleoecological, archeological, and historical data, the scientists uncovered past evidence of seas teeming with large animals as well as abundances of oysters and shellfish so vast they posed hazards to navigation. The new data also show that historical overkill of this marine life triggered current ecological collapses–many of which have been mistakenly attributed to pollution.
"We started out to study everything that people had ever done to oceans historically and were astounded to discover that in each case we examined, overfishing was the primary driver of ecosystem collapse," states Jackson.
The data demonstrate that overfishing triggered changes in ecosystem structure and function as early as the late aboriginal and early colonial stages. Even more chilling, the scientists show that grinding down marine food webs is responsible for many of the problems we face today. Removal of key predators and entire layers of the food chain set off sequences of events that are now culminating in toxic algal blooms, dead zones, outbreaks of diseases, and other symptoms of ecological instability.
Responding only to current events on a case-by-case basis cannot solve the ocean’s problems because impacts of human disturbance are synergistic and have deep historical roots.
Instead, the scientists say, problems need to be addressed by a series of bold experiments to test the success of integrated management on the scale of entire ecosystems. With few exceptions, such as the Steller’s sea cow, and Caribbean monk seal, most species that are ecologically extinct probably still survive in sufficient numbers for successful restoration with proper management. This optimism is in stark contrast with many terrestrial ecosystems where many or most large animals are already extinct.
Historical data not only help clarify underlying cause and rates of ecological change, but they also demonstrate achievable goals for restoration, management, and exploitation of coastal ecosystems that far exceed what we contemplate today. Scientific expectations for the recovery of marine ecosystems are too low because they are based on fisheries data that long postdate the decimation of fisheries.
"The many tens of millions of sea turtles in the Caribbean before Columbus easily exceeded the abundance and biomass of large animals in East Africa," states Jackson. "All we do today is micromanage remnants of once vast populations."
The scientists state that fisheries regulators and marine managers need to move beyond their fixation on quotas and boundaries and devise ways to restore the productivity and function of coastal seas. "We need to change the way we think about our coastal seas: not pristine, but damaged, and equally not hopeless, but salvageable," states Bradbury. "Our research points the way."
Other co-authors of the study include: Wolfgang Berger, Michael Kirby, Carina Lange, and the late Mia Tegner of Scripps Institution of Oceanography (the authors have dedicated the paper to the memory of Tegner, who died after the paper was submitted); Karen Bjorndal, University of Florida; Louis Botsford, University of California, Davis; Bruce Bourque, Bates College, Maine; Richard Cooke, Smithsonian Tropical Research Institute, Washington; Jon Erlandson, University of Oregon; James Estes, University of California, Santa Cruz; Terrence Hughes, James Cook University, Townsville, Australia; Susan Kidwell, University of Chicago; Hunter Lenihan, Hatfield Marine Centre, Newport, Oregon; John Pandolfi, Smithsonian Institution, Washington, D.C.; Charles Peterson, University of North Carolina; Robert Steneck, Darling Marine Centre, University of Maine; and Robert Warner at University of California, Santa Barbara.
The research was conducted as part of the Long-Term Ecological Records of Marine Environments, Populations and Communities Working Group supported by the National Center for Ecological Analysis and Synthesis (National Science Foundation), the University of California, and the University of California, Santa Barbara.