Typical Predator-Prey Relationship Can Be Thrown Off Balance By Evolution, Study Finds

By Ben Wolford on May 5, 2014 3:05 PM EDT

Here, a mink devours a frog, but in some cases — excluding frogs — the hunted can become the hunter. (Photo: Shutterstock)
Here, a mink devours a frog, but in some cases the prey can gain the upper hand. (Photo: Shutterstock)

Studying the predator-prey relationship between minks and muskrats, scientists have noticed a surprising ecological mechanism at work. Normally, when muskrat populations are low, their mink predators starve and their population shrinks, too. On the other hand, when there are plenty of muskrats, minks feast and their numbers grow. Biologists have believed this relationship to be true for all predators and prey since the 1920s when mathematical biologists first described the process.

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But a new study from researchers at the Georgia Institute of Technology contends that small evolutionary changes can wobble the standard balance — and even reverse it, leaving predator populations crippled despite ubiquitous prey. "When you include evolution, the classic prey-predator dynamics have a much greater range of possible outcomes," says co-author Joshua Weitz, a Georgia Tech biologist, in an announcement of the findings. "We are not replacing the original theory, but proposing a more general model that opens the door to these new phenomena."

The paper is to be pulished Monday in the journal Proceedings of the National Academy of Sciences. In it, the authors explain how previous theories could not explain population trends in three predator-prey species pairs: minks and muskrats; two birds, the gyrfalcon and the rock ptarmigan; and phage, which is a virus, and Vibrio cholerae, a kind of cholera-causing bacteria. The scientists said studying the latter two could aid epidemiologists in predicting disease outbreaks.

In some cases, the typical population model was inverted in these species. "You can see oscillations in which there are lots of prey around even when there are many predators, or lots of predators around even when there are very few prey," says Michael Cortez, who led the research. He attributes this to what the scientists call "co-evolution" — slight mutations in each species passed down to offspring that can lead to longer lifespans and more reproduction.

Cortez explains it this way: "When prey is abundant and there are few predators, it may be because there are lots of defended prey — prey that the predators can't eat. When there are lots of predators around and few prey, it's because the prey are very good food sources and the predators are doing quite well."

The study is not the first to highlight the role of variables in the oscillation of predator-prey populations. One study published last year from the University of Leicester, for example, showed that even individual "personalities" can affect the balance. But it may be the first to look at how evolution can tip the scales. "We can now explain broad trends that occur in vastly different systems using a theoretical approach, and the fact that we can identify the mechanism responsible for it is unique to our study," Cortez said.

Above photo courtesy of Shutterstock

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