Life in the ocean, they discovered, followed a simple mathematical rule: The abundance of an organism is closely linked to its body size. To put it another way, the smaller the organism, the more of them you find in the ocean. Krill are a billion times smaller than tuna, for example, but they are also a billion times more abundant.
What was more surprising was how precisely this rule seemed to play out. When Sheldon and his colleagues organized their plankton samples by orders of magnitude, they found that each size bracket contained exactly the same mass of creatures. In a bucket of seawater, one third of the mass of plankton would be between 1 and 10 micrometers, another third would be between 10 and 100 micrometers, and the final third would be between 100 micrometers and 1 millimeter. Each time they would move up a size group, the number of individuals in that group dropped by a factor of 10. The total mass stayed the same, while the size of the populations changed.Sheldon thought this rule might govern all life in the ocean, from the smallest bacterium to the largest whales. This hunch turned out to be true. The Sheldon spectrum, as it became known, has been observed in plankton, fish, and in freshwater ecosystems, too. (In fact, a Russian zoologist had observed the same pattern in soil three decades before Sheldon, but his discovery went mostly unnoticed). “It kind of suggests that no size is better than any other size,” says Eric Galbraith, a professor of earth and planetary sciences at McGill University in Montreal. “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.”