The Sheldon spectrum has been broken by overfishing. Credit: AI-generated, DALL-E 3.

World - How humans broke a natural law that governed ocean life for millions of years

For millions of years, one of the largest power law distributions known in nature has governed marine life -- that's until humans came along.

Over 50 years ago, researchers circumnavigated the Americas sampling ocean life along the way at regular intervals. In the process, they discovered a striking relationship: the abundance of most marine species is linked to their body size such that their collective mass winds up being the same. Krill are a billion times smaller than tuna, but their numbers are a billion times more abundant. If you were to weigh all the krill in the ocean, the mass should be close to all the tuna — but not anymore.

A new study found that human activity like industrial fishing has broken this mathematical relationship known as the Sheldon spectrum, after Ray Sheldon, a marine ecologist who first reported this relationship in 1969.

Humans are at it again

The Sheldon spectrum — the total mass of a marine population stays the same even as the individual size changes — applies to virtually all ocean life, from the tiniest bacteria to the largest whales. Even though a whale is trillions of trillions of times larger than a bacterium, its population size is smaller by the same order of magnitude, so their mass evens out.

It kind of suggests that no size is better than any other size,” Eric Galbraith, a professor of Earth and Planetary Sciences at McGill University in Montreal, told Wired. “Everybody has the same size cells. And basically, for a cell, it doesn’t really matter what body size you’re in, you just kind of tend to do the same thing.”

But Galbraith was shocked to find that the Sheldon spectrum has been broken, specifically for larger marine creatures. Generally, the larger the fish or crustacean, the easier it is to catch. Global fisheries are notoriously unsustainable, with the United Nations Food and Agriculture Organization (FAO) pointing out that one-third of fish stock worldwide is experiencing depletion due to “overfishing and habitat destruction.”

Galbraith and colleagues, led by Max Planck Institute ecologist Ian Hatton, used modern satellite imagery and recent in situ ocean measurements to estimate the abundance of plankton and fish. They also used a reliable estimate of marine mammal populations from the International Union for the Conservation of Nature, the organization that designates threatened or endangered species.

“One of the biggest challenges to comparing organisms spanning bacteria to whales is the enormous differences in scale,” says Hatton.

“The ratio of their masses is equivalent to that between a human being and the entire Earth. We estimated organisms at the small end of the scale from more than 200,000 water samples collected globally, but larger marine life required completely different methods.”

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