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The savannahs of Africa are among the few places where big, grazing animals still roam freely. But people are raising more cattle there and some ranchers resort to shooting wild animals. The conventional thinking is that animals, such as zebra and buffalo, eat the grass that the cattle need. But no one in Africa had really tested that notion scientifically, until now.
As NPR's Christopher Joyce reports, scientists who did got a surprise.
CHRISTOPHER JOYCE: Wilfred Odadi studies African wildlife and cattle. Cattle ranchers in his native Kenya tell him they resent the wildlife.
WILFRED ODADI: One of the reasons they do that is that they see wildlife as potentially competing with their livestock.
JOYCE: Competing for grass in places that can be bone-dry one month and lush the next. Odadi works with the African Wildlife Foundation and studies at Princeton University. He and a team of scientists decided to test this notion of dietary competition on the savannah. They fenced in some cattle to eat by themselves. They let other cattle graze outside with wildlife, mostly zebras.
It turned out that during the dry season, the zebras did eat the cattle's lunch. The cattle didn't gain as much weight as their fenced in brethren, but ecologist Truman Young says during the wet season, it was just the opposite.
TRUMAN YOUNG: The exciting result was that when the grass was growing vigorously, when it was wet, grazing by the zebras actually made the remaining grass more accessible or more palatable to the cattle. What happened was the cattle gained more weight in the presence of zebra.
JOYCE: Young is a plant specialist at the University of California at Davis. When he looks at a cow, he sees grass. That's what cows are made of. He says, when some grass gets too long, it gets rank, tough and fibrous.
YOUNG: What the zebras do is they lop that off. That encourages the regrowth of fresh shoots from the base and those fresh shoots are just more edible. It's like having the choice of having, you know, bran muffins or blueberry muffins, I guess.
JOYCE: What the zebras are doing, in ecology speak, is facilitating the cattle, actually helping the cattle eat better.
Yohan du Toit grew up in Africa and is an ecologist at Utah State University. He says the lack of competition among grazing animals, at least in the wet season, makes sense if you've seen big herds of them in the wild.
YOHAN DU TOIT: If you were to go to the Serengeti or, indeed, any other savannah, you would see multiple species of hoofed mammals, grazing mammals, standing shoulder-to-shoulder out there.
JOYCE: The research appears in the journal Science. Du Toit, who was not part of the research team, says it has real ramifications for wildlife, which is in decline in most parts of Africa. Landowners just don't see a profit in having wildlife around.
DU TOIT: They will not invest in maintaining wildlife if they can convert their land to cropland or have more intensive cattle production, et cetera, et cetera. And so those are the areas where you're getting these really major conflicts between wildlife and cattle.
JOYCE: But he says cattle ranchers could let their livestock graze with zebras in the wet season, then pen them in during dry spells.
The research did not find any advantage for cattle from sharing their meals with big plant eaters like elephants or giraffe, but du Toit says it's worth looking into. Many of these big mammals evolved together by eating different kinds of plants.
He's begun his own study to see if bison and cattle can facilitate each other's diets on the plains of Utah where the deer and the antelope still roam.
Christopher Joyce, NPR News.