Jeremy Woodhouse / Getty

There was something odd about the bushbuck, the scientists finally decided.

They’d been watching the small antelope in Mozambique’s Gorongosa National Park in the Great Rift Valley for years, and they’d noticed more and more of the normally cautious, wood-dwelling creatures brazenly grazing on the plains, where they would be easy prey. They were seeking out the lush food that grows in sunny, open places; they were fearless. And they had reason to be, says Justine Atkins, a graduate student at Princeton University who studies antelope at Gorongosa, because almost every predator in the national park had been dead for more than a decade.

In a new paper in Science, Atkins and her collaborators show that there is now a population of bushbuck in Gorongosa that prefer to graze in the open, that their numbers have been increasing, and that they are, by and large, better fed than their woodland brethren. But on some level, they still possess that age-old fear. When scientists released the scent of a lion on the wind or played the growl of a leopard, the bushbuck headed for the trees.

Mozambique’s years of civil war, which ended in 1992, wiped out every predator from the national park except for a small handful of lions. Hungry soldiers and desperate civilians killed most of the herbivores as well, though with more than a decade of restoration work, some have slowly begun to recover. Still, with a radically changed ecosystem may come radically changed behavior.

Ecologists have theorized that when predators are eliminated, prey species will start using resources they would normally have forgone as too risky. This idea had been tested in computer models and in ecosystems such as tide pools, where researchers could remove the predator just by picking it up. The civil war and the decimation of Gorongosa’s wildlife provided a rare, if tragic, opportunity to watch these dynamics on a grand scale, with mammals and large predators.

To confirm their impressions, the researchers put radio collars on bushbuck to track their grazing locations over time. This soon revealed that only some antelope were behaving fearlessly, while others consistently played it safe. To see whether the fearless ones were getting a nutritional boost—an enticement into risky behavior—they sequenced plant DNA from the bushbuck’s feces and developed a detailed picture of their diet. The hunch was correct: Plains-grazing bushbuck were consuming more calories and drastically more protein than woodland-grazers were. They also seemed to be overgrazing plants that would have had refuge from bushbuck in the past, signaling another potential shift in the ecosystem.

As the numbers of large carnivores around the world shrink, the study suggests that the world they leave behind will change, perhaps fairly swiftly, in their absence. There will, sadly, be many more chances to observe such shifts, in ecosystems all over the planet. However, whether those changes persist depends on what happens next.

Curious whether the bushbuck would respond to the signs of a predator, Atkins and her colleagues devised an elaborate spoof: For 48 hours, they deployed speakers playing a leopard’s cries and planted mock lion feces and a repellent developed to mimic the smell of carnivores around the antelopes’ habitat. They found that the animals reverted to grazing in the woods, suggesting that they aren’t immune to the implications of such cues.

That will be an interesting behavior to watch over the coming years, says Atkins. Several follow-up experiments are under way, but “the main thing is seeing what happens to this population as the apex predators are reintroduced to Gorongosa,” she says.

African wild dogs were one of the predator species destroyed during the war, and in the summer of 2018, 14 were released into the park. Interestingly, they appear to be eating mainly bushbuck, which suggests that the species’ days in the sun may be numbered.

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