Mathematicians show how beetles can share a niche

New equations help solve decades-old puzzle of why one species doesn’t always outcompete another

WASHINGTON, D.C. — Placing two species of flour beetle in the same jar of flour needn’t always result in one species driving the other to extinction, as ecologists have thought. A new mathematical model presented January 5 at the annual Joint Mathematics Meetings shows how sometimes these two competing species can coexist.

The new research raises questions about a common assumption in ecology, the idea that only one species can survive in a specific ecological niche. Like similar species of flour beetles living in and eating the same flour, two species that share a niche ought to compete until one wipes out the other, according to the long-held theory.

But the theory assumes that no evolutionary changes occur in the beetles over a few dozen generations. By expanding the theory to include equations for subtle evolutionary changes even on such short time scales, mathematicians found that evolution can sometimes steer the two species toward coexistence.

“I think it opens some questions about this dogmatic view in ecology,” says Jim Cushing, coauthor of the study and an applied mathematician at the University of Arizona in Tucson. “It reopens the issue of what you consider a niche to be.”

Cushing and his colleagues were trying to explain an anomalous result from a classic flour-beetle experiment performed in the 1960s. In the series of experiments, one of the two species pushed the other to extinction every time — except once. In that one case, the two species coexisted in a jar of flour for more than 30 generations. Trying to explain why that one case would be different from all the others has challenged scientists ever since.

At the time, researchers noted a small evolutionary change in the traits of the beetles in the one anomalous case. Flour beetles sometimes eat the eggs of their own species as well as those of closely related species. The beetles in the oddball case had evolved to become more-voracious egg eaters.

Cushing and his colleagues designed a mathematical model that, unlike previous models, allowed the egg-eating trait to evolve — which affected the birth and death rates of each species. Neither species went extinct after more than 30 generations.

“What makes this work very, very exciting is the assumption was that if you start with equal conditions, you will always get one species outcompeting the other,” comments Joel Brown, an evolutionary ecologist at the University of Illinois at Chicago. “They’re showing how just a tiny bit of evolution might actually explain the discrepancy.”

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