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How overfishing can alter an ocean’s entire ecosystem

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Some say this is worrisome evidence of a greatly changed and simplified marine ecosystem. Like investment portfolios with few holdings, simple ecosystems are prone to collapse; and collapsed or rearranged ecosystems don’t necessarily provide what humans expect. Increasingly mindful of marine ecosystems’ complexity – and wary of their collapse – some people are calling for a holistic approach to managing ecosystems, one that aims to manage for the health of the entire system rather than that of a single stock.

Just 4 percent of the world’s oceans remains free from human impact, according to a 2008 study in the journal Science. Forty percent of this is heavily impacted.

Where intact ecosystems remain, scientists are often astounded by what they find. On the remote Palmyra Atoll in the equatorial Pacific, for example, large sharks and predatory fish dominate the reefscape – an “abundance of toothy things,” says Callum Roberts, a professor of marine conservation at the University of York, England. Unlike terrestrial ecosystems, which are dominated by a few apex predators, pristine marine ecosystems support a large biomass at the top.

“Today’s oceans have got far less in the way of biomass than they used to,” Professor Roberts says. “We’re altering ecosystems in a way that reduces the level of productivity they can support.”

By one estimate, only one-tenth of the sharks, tunas, cods, and other large predatory fish that once swam the oceans remains. And their absence has ripple effects throughout marine food webs.

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