Peter Stoel/Oregon State University

Peter Stoel/Oregon State University

These days it doesn't pay to be the biggest, baddest plant eaters around. Large herbivores, especially in Africa and Asia are on the decline, reports a new study, which could empty out some of the world's most diverse landscapes.

“I expected that habitat change would be the main factor causing the endangerment of large herbivores,” said wildlife ecologist William Ripple in a statement. Ripple, of Oregon State University, led a team of international researchers studying the world's biggest plant eaters.

“Surprisingly, the results show that the two main factors in herbivore declines are hunting by humans and habitat change. They are twin threats.”

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The herbivores studied all weigh in at more than 200 pounds and are threatened in "grasslands, savannas, deserts and forests," the study reported. This group includes zebras, camels, elephants, tapirs and rhinos.

"The market for medicinal uses can be very strong for some body parts, such as rhino horn,” Ripple said. “Horn sells for more by weight than gold, diamonds or cocaine."

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The black rhino, for example, was declared extinct four years ago.

"Without radical intervention, large herbivores (and many smaller ones) will continue to disappear from numerous regions with enormous ecological, social, and economic costs," Ripple said. Losing the large herbivores could mean food scarcity for large carnivores, such as lions and tigers, less seed dispersal and changes in habitat for fish, birds and amphibians.

Of the animals studied, 25 occupy just 19 percent of their historical ranges. Livestock production has tripled since 1980, the scientists wrote, reducing access to land, food and water and increasing the risk of disease.