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Predators at the top of the food chain - you know, lions and tigers and bears, for example - are relatively scarce in nature, which is normal, because if you have too many, they will eat themselves out of prey. But top predators are now so rare, that many are in danger of disappearing. NPR's Christopher Joyce reports that is creating ripple effects throughout the natural world.

CHRISTOPHER JOYCE, BYLINE: People who study ecology - the interplay of animals and plants in nature - say it's not rocket science. It's harder.

ROLF PETERSON: We're dealing with the most complicated systems in the universe, and we hardly even know what the moving parts are.

JOYCE: Rolf Peterson studies large carnivores. He and scientists like him are finding that as the number of big predators dwindles, everything around them changes. It's like a cascade down the food chain. Take cougars and wolves, for example. Fewer of them means their prey - deer and elk, mostly - multiply. More plant eaters means more plants get eaten, and everything that depends on those plants, from birds to butterflies, is affected. Carnivore biologist William Ripple from Oregon State University says even streams are affected, because armies of deer and elk can eat all the vegetation along the banks.

WILLIAM RIPPLE: The stream actually changes course. So, we're finding that the predator can actually affect the shape of the stream.

JOYCE: These cascade effects take all sorts of paths. Bears, for example, grab salmon out of rivers and eat them on the banks. The leftovers decay and add nutrients to the soil that help plants grow.

RIPPLE: It's just a type of connecting-the-dots in nature. And it shows the inter-connectedness.

JOYCE: Ripple and other carnivorists have published a study in the journal Science today that lists the benefits that predators provide. They note that where predators are reintroduced - such as in Yellowstone National Park - deer and elk and vegetation return to a more natural state. That may seem obvious. But Peterson, a biologist at Michigan Tech, says the extent of so-called carnivore benefits is not well-known, even as big carnivores disappear.

PETERSON: You know, we have trashed the large carnivores for sure, and they're becoming more and more scarce. And we don't even have the science to tell us what we're losing.

JOYCE: Scientific questions, such as: How many wolves or cougars or grizzlies do you need in, say, a national park to keep the other animals and plants under control? Peterson says the wolf has been an especially difficult case. It's made a comeback in the U.S. and Canada, but wolves sometimes prey on livestock. They compete with hunters for deer and elk. Many people have a deep-seated fear of them.

PETERSON: How do we live with these creatures, and how will we accommodate them? And what will stop their increase once we put them back? So, we are in the driver's seat.

JOYCE: Scientists are now calling for a global Large Carnivore Initiative to organize research on carnivore ecology, and, as Peterson points out, to illustrate just how predators has shaped our world.

PETERSON: It was the large carnivores, to a great extent, that maintained that fabric of life that formed us.

JOYCE: Christopher Joyce, NPR News.

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