Imagine you are a grazing animal, an antelope or an elk. The lush grasses of a streambank or an open plain tempt you, but predators lurk there. You avoid this "landscape of fear," keeping to the safety of the forest and leaving the grass to flourish. It's a plausible but still controversial scenario for how predators can shape an ecosystem. Now, ecologists have taken advantage of the effects of war on Mozambique's Gorongosa National Park to give the idea new support.
After a surge in poaching during the country's civil war from 1977 to 1992 extirpated leopards and wild dogs from the park, a secretive antelope that used to stick to the forest started to forage out in the open. In a paper published online in Science this week, ecologists show the untrammeled consumption has altered the park's vegetation—and that the sound or smell of the predators is enough to reverse the effect, by driving the antelopes back toward the forest.
"It is a rare and important study because it uses experimental methods to study the ecology of fear," says William Ripple, an ecologist at Oregon State University in Corvallis. And it demonstrates that the loss of fear "can have far-reaching ecological effects."
Something similar appeared to happen after wolves were reintroduced to Yellowstone National Park in Wyoming in 1995. Elk that had previously browsed in open areas began to avoid them, and willows on streambanks and floodplains thrived. Studies of smaller predators and prey, such as spiders and the grasshoppers they eat, also supported the existence of a landscape of fear, which "blocks out some resources that would otherwise be very desirable" to those prey, explains Robert Pringle, an ecologist at Princeton University.
But last year, ecologists Michel Kohl and Daniel MacNulty from Utah State University in Logan published a reanalysis of the elk data from Yellowstone and concluded the animals were not avoiding the open habitats. The willows were doing better, they argued, simply because wolves were preying on the elk, reducing their numbers. More data still in press confirm that conclusion, MacNulty says.
The Mozambique park offered a chance to test the landscape of fear more rigorously. It once teemed with elephants as well as lions, leopards, and herds of antelopes, attracting visitors including movie stars John Wayne, Joan Crawford, and Gregory Peck. But almost all of its large wildlife perished during the civil war. Since the mid-2000s, herbivores have returned, but large predators have not.
After noting that some bushbucks, small antelopes once known only to live in the park's forests, now graze in the open, Pringle and Princeton graduate student Justine Atkins tested whether fear could push these animals back into the forest. They played recorded leopard calls and laid down the scent of other carnivores near about a dozen bushbucks wearing radio tracking collars. The tracking data showed the bushbucks headed back toward the trees when they sensed a predator nearby.
Atkins, Pringle, and their colleagues also analyzed plant DNA in bushbuck feces to confirm that, having lost their fear of predation, the animals were heavily feeding on the legumes and forbs on the open plain. By fencing off part of the plain to exclude bushbucks, the researchers verified that the animals were taking a toll on the vegetation. "Just changing fear on a landscape changes everything," Ripple says. "It's a really cool study."
MacNulty is not convinced the sounds and smells the researchers used are a good stand-in for real predators. He also questions older data indicating the bushbucks kept to the forest before the predators vanished. And even if fear guides bushbuck behavior, he says, it may not affect larger herbivores such as elk, which can fend off wolves. "The jury is still out in terms of [the] general importance" of the landscape of fear idea, MacNulty concludes.
Yet even he agrees that Gorongosa National Park is the perfect place to investigate this idea further. An ambitious public-private recovery plan helped bring back herbivores. And last year, park officials began to reintroduce wild dogs. More predators are on their way. Soon, instead of having to mimic these hunters and scavengers, Atkins says she and her colleagues will "get to observe what happens to the bushbuck community as the predators come back."