In 2015, researchers studying antelope behavior in Mozambique's Gorongosa National Park noticed that some bushbucks were acting strangely. Typically a forest-dwelling antelope species, the herbivores rely on tree cover to hide from predators that stalk grassy, open landscapes during the dry season. But some bold bushbucks were now wandering out into the treeless floodplain of Lake Urema to forage.
The researchers wondered if a lack of top predators in the park had emboldened the bushbuck to venture out of the thickets surrounding the Urema floodplain. The park's wildlife populations had plummeted during Mozambique's civil war, a 16-year conflict that ended in 1992. While herbivore species have returned to the park since then, large carnivores—like leopards, wild dogs, hyenas, and lions—have yet to bounce back to their pre-conflict populations.
To find out whether the lack of predators was influencing the herbivore's behavior, the international team of researchers tracked the movements of nearly a dozen bushbuck and then simulated the presence of their natural predators on the floodplain. In a new study, published in Science, the researchers found that reintroducing top predators to the park might lead the herbivores to fear the floodplain once again.
"Most large carnivore populations are declining worldwide," says Justine Atkins, a Ph.D. candidate at Princeton University and lead author on the new study. "They're definitely in trouble, and we're going to see in the next few years or decades more losses of large carnivores."
The new study demonstrates how those losses may well ripple through entire ecosystems, extending beyond the predator population itself.
The team began by fitting 11 bushbuck with GPS collars and followed their movements for eight months, confirming that at least some of the animals were routinely venturing out into the open floodplain. The researchers found that the diet of the bushbuck that fed in the floodplain was higher in both digestible energy and protein than the diet of the animals that stayed in the wooded areas, giving the floodplain foraging animals a competitive advantage over their warier woodland counterparts. With the reduced threat of predation, it seems, the animals were more likely to gamble on the higher-quality foliage that grows in the open plain.
Next came the challenging part, according to Atkins: designing an experiment in the park that allowed the team to monitor bushbuck behavior in the presence and absence of top carnivores. "To test the hypothesis we wanted to test required controlled experimental conditions, but those kind of conditions are not very easy to come by in the wild," she says.
With few natural predators in the park, the team settled on mimicking their presence with recordings of leopards and strategically deploying fake lion scat and generic carnivore urine in some locations. "Short of having an actual predator we could introduce, we thought this isolated exposure to predator cues would be the next best thing," Atkins says. As a control, the team also deployed white noise recordings, herbivore dung, and saline solution in other areas.
The team found that bushbuck in both the wooded areas and the floodplain avoided the predator cues, while their behavior was unaffected by the control cues. Bushbuck that encountered the predator cues on the floodplain retreated to the tree cover. Though the top predators had been gone for decades, the bushbuck retained an innate fear of the region's natural carnivores.
The researchers also found that the expansion of bushbuck territory into the floodplain could be shaping the ecosystem's plant communities. The team identified a species of waterwort that few other plains-dwelling herbivores consumed—Bergia mossambicensis—and set up wire-mesh cages around some of the plants on the floodplain. The caged plants thrived, while the uncaged plants nearby were devoured by the bushbuck. According to Atkins, their findings underscore the cascading effects that the loss of large carnivores can have on ecosystems, beyond the animals they kill for food.
Atkins notes that the Gorongosa National Park launched a restoration project to bring back at least some of the park's former biodiversity by restoring its carnivore populations. Last summer, park staff re-introduced a pack of 14 African wild dogs. The park's lion population, once decimated by the civil war and the snares of poachers, is slowly growing as well.
"So there is reason for hope," Atkins says, who, with her colleagues, plans to monitor what happens in the park as predation levels begin to rise again. "For a behavioral ecologist like me that's just the perfect opportunity," she says. "I'm really excited to see how that plays out."