Overfishing Long Ago Tied to Modern Ecosystem Collapse

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In every case they looked at, overfishing by humans preceded ecosystem collapse.

Removing important marine species has a profound effect on the food chain, which ultimately leads to ecosystem breakdown, the authors say. The impacts of historic depopulation of sea turtles is one example.

"The accepted wisdom among sea turtle researchers has always been that sea turtle stocks were in pretty good shape when Columbus arrived, and that it wasn't until the Europeans started to arrive that the populations began to crash," Bjorndal explained.

When the scientists examined archaeological evidence of coastal Amerindian settlements, they found that sea turtles were an important food source for the people who lived there. Over the 100 to 200 years that followed, the amount of turtle remains found in ancient trash dumps diminished until there were no more traces of sea turtles as a food source.

The findings challenge a common assumption held by marine biologists that the consumption or use of a species by indigenous groups generally has a negligible or strictly localized impact.

"We had always thought that the impact of subsistence-level fishing would be limited to a local area," said Bjorndal. "But sea turtles travel long distances to forage for food and then return to their nesting site. By overharvesting the species at a local level, the Amerindians had a region-wide impact on the ecosystem."

Now, several hundred years later, the depletion of sea turtle populations is having a profound effect on the health of coral reefs in the Caribbean, Bjorndal said.

Sea turtles were one of many species that controlled the growth of algae. Other algae-eating species were also slowly eliminated over time, until only the sea urchin remained. In the 1980s, sea urchin populations plummeted following a well-documented outbreak of disease.

With no plant-grazing species left, the reefs were swamped by an overgrowth of algae, which killed many corals and prevented new ones from growing.

Common Pattern

The researchers found the same pattern in all the other cases they studied.

Co-author Jim Estes, a research ecologist with the Western Ecological Research Center in Santa Cruz, California, described the chain of events that occurred in the North Pacific after aboriginal Aleuts greatly reduced sea otter populations starting about 2,500 years ago.

Sea otters are the major predators of sea urchins. As their major predators were removed from the ecosystem, sea urchin populations soared.

Overgrazing by the sea urchins eventually killed off the kelp beds, resulting in changes in wave action, water quality, and siltation rates. These changes, in turn, had a major impact on other near-shore flora and fauna.

In another instance, the Chesapeake Bay now has vast areas in which algae is so abundant that the level of oxygen in the water is inadequate to support other organisms. The authors tie this process, known as eutrophication, in part to the collapse of oyster populations caused by overfishing in the 19th century.

Management Implications

"Our study shows that marine ecosystem collapse is not entirely due to recent factors, and that to really understand what's happening we need to view the problem in its proper historical perspective," said Bjorndal.

Estes said the emphasis on recent human activities as the cause of ecosystem collapse may have arisen in part because ecological data on coastal marine systems has only been collected and studied since the 1950s. As a result, researchers' insight is limited mainly to the recent structure and function of ecosystems.

The new report points to the need to manage marine ecosystems for long-term effects and not just immediate problems.

"For ecosystem restoration and management to be effective, we need to go back far enough in time to truly identify the problems and set our goals appropriately," said Bjorndal.

The study also highlights the importance of maintaining the biodiversity of an ecosystem, she added.

"It's not enough to bring back one species," said Bjorndal.

"In a healthy marine ecosystem, there are a suite of animals that fill the same ecological niche," she said. "The loss of redundancy—where numerous species fill the same environmental niche—leaves an ecosystem extremely vulnerable."

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