Healthy Ecosystems Need Fear
How a "landscape of fear" transformed the ecology of an African park.
Posted Oct 13, 2020
At Mozambique’s Gorongosa National Park, a bold experiment is playing out to rescue a wilderness on the brink—and demonstrate the powerful role that fear plays in healthy ecosystems.
A new film from NOVA and HHMI Tangled Bank Studios documents one of the most ambitious wildlife recovery projects ever: the reintroduction of African wild dogs into Gorongosa. The project could be a victory for conservation but it’s also an opportunity to test a hypothesis about the role of fear in nature.
“Nature’s Fear Factor” premieres Wednesday, Oct. 14 at 9 p.m. ET on PBS and will be available for streaming online and on the PBS video app.
A Wilderness Without Predators
Gorongosa was once a celebrated and species-rich national park. But a decades-long civil war ending in the 1990s decimated the park’s large wildlife. The landscape, however, which includes many diverse habitats, remained intact. Since the 2000s, herbivores have returned to Gorongosa, with some of them approaching pre-war numbers.
At first glance, the park looks like a paradise. There are large grazers everywhere. But ecologists like Princeton’s Rob Pringle see an ecosystem out of balance.
While GPS-collaring and tracking the antelopes of Gorongosa, Pringle noticed that a species called bushbuck was no longer hiding in the bush.
“Everything we know about bushbuck says they are forest-dependent antelope confined to woodland and thicket habitat,” he says. “These bushbuck were venturing out onto the wide-open floodplain, sometimes to areas miles from the nearest tree.”
Pringle thought this aberrant behavior could be linked to two critical elements missing from Gorongosa: large predators and the fear they instill.
The traditional view of how predators affect ecosystems measures their influence by the animals they kill and consume. According to more recent theories, this view underestimates the total impact of predators.
Another factor gives predators the power to shape the behavior of many, while killing a few. The idea is known as the ‘landscape of fear’—the mere presence of predators inspires wariness in prey and changes their behavior, with repercussions that spread through the ecosystem.
The landscape of fear has been tested in controlled experiments with small animals (for instance, spiders and grasshoppers). There is some debate over whether and how the idea applies in the case of larger animals, such as large grazers and apex predators. Gorongosa, a wilderness devoid of predators, provided an ideal opportunity to scale up these experiments from the lab to the real world.
Last year, Pringle and his colleagues reported how bushbuck brazenness has altered the park’s vegetation. The researchers confirmed that, in the absence of predators, bushbuck were leaving their comfort zone in search of more nourishing food. DNA analysis of bushbuck scat revealed that the animals were heavily feeding on the legumes and forbs on the open plain. By fencing off a section of the plain to exclude bushbuck, Pringle and his colleagues verified the toll the grazers were taking on the vegetation.
And it wasn’t just bushbuck changing their diets. In a healthy ecosystem, the herbivores have different, specialized diets. In Gorongosa, the herbivores’ diets shifted and overlapped, resulting in competition for the same food.
Could an injection of fear back into the environment restore balance to the ecosystem? An experiment by Pringle and his colleagues suggests it is possible. When the researchers played leopard calls or laid down the scent of other carnivores, nearby bushbuck immediately knew what to do—they headed for the safety of the forest.
Still, temporary predator cues are a lot different than living your life in a landscape of fear. There were many unknowns about how Gorongosa’s herbivores would react to sharing the park with predators again.
The plan to reintroduce a pack of African wild dogs to the park was both wide-reaching and risky. In April 2018, head of Gorongosa’s predator program Paola Bouley and senior wildlife veterinarian Antonio “Tonecas” Paulo oversaw the reintroduction of fourteen wild dogs into the park.
Since then, the wild dogs have been thriving—hunting, splitting into new packs, and having lots of puppies.
Behavioral observations and analysis of wild dog scat show that the new carnivores’ diet is made up primarily of bushbuck. The bushbuck appear to have noticed the change, retreating from the open plain and sheltering in the forest once again. Initial analyses of bushbuck behavior suggests they are less restful and more vigilant now that wild dogs are back.
Although Pringle is encouraged by these early analyses, he says it is difficult at this point to interpret their results as definitively demonstrating that the landscape of fear has been restored. It’s only been two years and in March 2019 a massive cyclone caused an atypically large flood in Gorongosa, confounding the results. Still, he finds the behavioral data convincing.
“For the 2019 paper with fake predator cues, we saw this significant response of increased vigilance within 48 hours,” he says. “We would expect the presence of actual predators and the increasing predation risk in the park to fairly rapidly change the behavior of animals.”
Hope for Damaged Ecosystems
One year after the first 14 wild dogs were released in Gorongosa, an additional fifteen dogs were introduced. The park is now home to more than forty wild dogs.
But the park will require more than just these predators to return to its former state. Gorongosa will eventually need several predators, targeting different habitats and prey. There are plans to replace more large carnivores in the park in the coming years, with the priorities being leopards and hyenas. Some predators may be returning on their own. Shortly after the 2018 reintroduction of the wild dogs, a leopard was spotted in the park, the first sighting in a decade.
Pringle says Gorongosa’s example of reintroducing large animals to help restore ecological health has the potential to be applied to many other ecosystems. After all, there are a lot of places in the world where humans have pushed aside big carnivores.
“Gorongosa is showing it might not be easy but it can be done—and it can be done on a timescale that matters to people,” Pringle says. “We can see the change.
The difference in how the park feels, its vibrancy, since I first visited it nine years ago is really dramatic.”
Pringle calls Gorongosa a testament to the resilience of nature, given a chance.
“If you give these systems a little bit of space and breathing room, we are discovering they are actually very good at repairing themselves,” he says.
Murdock, David (Director and Producer). (2020). Fear Factor [NOVA episode]. A NOVA Production by HHMI Tangled Bank Studios for GBH Boston.
Atkins JL, Long RA, Pnsu J, Daskin JH, Potter AB, Stalmans ME, Tarnita CE, and Pringle RM. Cascading impacts of large-carnivore extirpation in an African ecosystem. Science 12 Apr 2019; 173-177. Doi: 10.1126/science.aau3561.