“Successful management and restoration of coastal marine ecosystems has failed in part because of a lack of understanding the deeper historical causes of collapses in these ecosystems,” said Dr. Jim Estes, a USGS research ecologist with the Western Ecological Research Center in Santa Cruz, Calif., one of the authors of the article.
The scientists examined paleoecological records from marine sediments dating from about 125,000 years ago; archaeological records from human coastal settlements occupied after about 10,000 years ago; historical records from documents of the first European trade-based colonial expansion in the Americas and South Pacific in the 15th century to the present; and ecological studies from the past century to help calibrate the other records.
They found that the three cultural stages they examined --aboriginal, colonial and global -- occurred at different and distinct times in the Americas, New Zealand and Australia. This enabled the scientists to distinguish fishing in these locations by cultural stages. The scientists also were able to determine whether changes occurred due to human impacts or changing climate. They compared the function and structure of kelp forests, coral reefs and estuaries before and after fishing occurred.
The scientists found that as human disturbance occurred over time, ecosystem structures and functions changed in response to overfishing. While few species like the Steller’s sea cow of the North Pacific and the sea mink of the Gulf of Maine were fished to extinction, many became ecologically extinct like the sea otter, which did not make a comeback from intense exploitation until afforded protection in the 20th century. The scientists also found when multiple species occupied similar niches in an ecosystem’s structure and could fill a similar function of an overfished species, signs of overfishing could be masked for long periods of time.
“Since most ecological studies of coastal marine ecosystems have been conducted after the 1950’s, scientists have had first-hand knowledge of only the recent structure and function of these ecosystems,” said Estes. “We have found that the beginnings of human disturbance of these systems began well before these studies, and include changes that occurred during each cultural period -- aboriginal, European colonial, and more recent global exploitation of sea life. We fear that even more marine ecosystems may soon be at risk of collapse.”
The scientists found that unrelenting exploitation accelerated and intensified as population growth, technological advances and eventual expansion to a global market occurred. The first major human disturbance to all of the ecosystems they examined was always overfishing of the large vertebrates and shellfish. Much later other events followed: pollution, overenrichment of waters from nutrients, disease outbreaks, habitat destruction, invasion of introduced species and human-caused climate change.
This historical perspective has helped Estes better understand more recent changes in the Northern Pacific and in southern California. In the North Pacific, where Estes has studied nearshore ecosystems since the 1970’s, historical evidence showed that aboriginal Aleuts greatly reduced the number of sea otters starting about 2,500 years ago. By preying on sea urchins, sea otters prevented urchins from overgrazing the kelp forest. Hence, with fewer sea otters to control them, the sea urchins grew larger, reducing the kelp. When fur traders hunted the remaining sea otters to the brink of extinction in the 1800’s, the kelp forests disappeared from overgrazing by sea urchins and didn’t appear again until legal protection partially restored sea otters. Estes found that recent depletions of the kelp forest have occurred in areas of Alaska where killer whales are preying on sea otters. The whales recently shifted their diet to sea otters from seals and sea lions. Estes said the seals and sea lions apparently have declined because of food web changes associated with whaling and fishing, and because ocean warming is decreasing the productivity of the North Pacific.
Historical perspective also made possible the comparison of kelp forests of Alaska to those of the southern hemisphere, in research in the 1990’s. While sea otters had been hunted to near extinction in California by the 1800’s, Estes said the kelp forest response to the absence of sea otters was different than it had been in Alaska. Estes said exploring the evolutionary role of sea otter predation of sea urchins in California revealed that southern kelp had developed strong defenses against plant eaters that made them less susceptible to sea urchin overgrazing than Alaska kelp forests, which lacked such defenses.
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