War wrecked an African ecosystem. Ecologists are trying to restore it
Predators and prey roam Gorongosa in Mozambique once again, but there’s still a long way to go
The national park at the southern end of Africa’s Great Rift Valley was once considered a wildlife paradise. Hippopotamuses lolled in the lush waters of Mozambique’s Lake Urema, and thousands of antelope bounded across the park’s savannas and floodplains. Elephant herds and prides of lions drew international tourists.
Then civil war erupted in the southeast African nation in 1977, leaving Gorongosa National Park in shambles. Closed in 1983, the sanctuary became a battleground, with animals slaughtered for food or — in the case of elephants — ivory to fund the fighting. Populations of African buffalo, blue wildebeest and zebra, thousands strong, plummeted until 15 or less of each remained. Hundreds of lions, leopards and wild dogs fled, starved or died in snares and steel-jaw traps. By the war’s end in 1992, only lions remained, their numbers in the single digits.
Science News headlines, in your inbox
Headlines and summaries of the latest Science News articles, delivered to your email inbox every Thursday.
Thank you for signing up!
There was a problem signing you up.
The park’s ruined condition has inspired a complex scientific effort by Mozambique officials and an international team of scientists to restore not just the park’s wildlife but an entire ecosystem — an enormous challenge that has rarely been attempted. Probably the most well-known example of such an effort to date is the reintroduction of gray wolves into Yellowstone National Park about 25 years ago, so far with uneven results.
Bringing back Gorongosa will require far more than the reintroduction of one species. It will take the reestablishment of at least 10 species and curtailing rampant poaching. About a decade into the project, the scientists have had mixed success.
“It is a big challenge,” says Princeton University ecologist Robert Pringle. He’s one of at least a dozen scientists from countries including Mozambique, Kenya and the United States who have been tracking the growth in animal populations and other changes in Gorongosa’s ecosystem.
Whether restoring Gorongosa is even possible is debatable. A goal of ecological science is to be able to heal natural habitats ravaged by war, development or resource extraction, Pringle says. But scientists are still working out the right recipe, looking at such factors as how the mix of species shapes an ecosystem and how predators affect the behavior of prey.
Subscribe to Science News
Get great science journalism, from the most trusted source, delivered to your doorstep.
“What Gorongosa provides, because it’s so dynamic, is the ability to test some of these ideas in unusually powerful ways,” Pringle says.
Still, he says, “I don’t think it will ever be exactly how it was.”
The Gorongosa Restoration Project, a U.S. nonprofit launched in 2004 just over a decade after the war ended, began with $40 million and a 20-year agreement with Mozambique’s government to rebuild and conserve the park. The motivation wasn’t just scientific. Located on Africa’s east coast and known for its porcelain-white beaches and rich Indian Ocean coral reefs, Mozambique was eager to build up its tourism. Visitors’ spending, which accounted for 4.2 percent of Mozambique’s economy in 1998, shortly after the end of the civil war, is roughly 10 percent today, according to the World Travel and Tourism Council.
Park rangers were hired and trained to combat poaching in park, which at roughly 4,000 square kilometers is a little bit bigger than Rhode Island. From 2015 to 2016, rangers removed 12,363 snares and 317 steel-jaw traps set by poachers and hunters throughout the park, according to a November 2018 Biological Conservation study.
Removing the traps appears to have helped Gorongosa’s sole remaining large mammal predators. The lion population grew rapidly from 30 to 50 individuals in 2012 to 150 individuals in 2018, says conservation biologist Paola Bouley, the associate director of Gorongosa’s carnivore program. Along with 100 lions living outside the park, “we’re sitting at about half the lions needed to sustain a viable population,” she says.
On the prey side of the equation, from 2007 to 2014, park managers reintroduced six herbivore species, including 210 African buffalo and 180 blue wildebeest, researchers reported March 13 in PLOS ONE.
So far, animal population trends are moving in the right direction. The number of buffalo went from 15 in 2001, the first count of those animals postwar, to almost 1,000 by 2018, while hippopotamuses, in the dozens in 2000, boomed to 546.
Still, the park’s mammal population hardly resembles the mix of animals at Gorongosa in its heyday. The park’s large herbivore biomass — a measure used by ecologists to estimate how much energy is available to the next level of the food chain — has recovered to roughly 95 percent of prewar levels. But the biggest plant eaters — elephants, hippos and buffalo — which once made up 89 percent of the park’s herbivore biomass, accounted for only 23 percent in 2018, researchers reported in the PLOS ONE paper.
In their place, smaller antelope, including reedbuck, bushbuck and kudu, now comprise 98 percent of the park’s largest herbivores in numbers and 80 percent of its large herbivore biomass. The most abundant is the waterbuck (Kobus ellipsiprymnus), a spiral-horned antelope species, with more 55,000 individuals, more than 10 times as many animals than before the war. Scientists suspect the waterbuck’s success has to do with its keeping to the open floodplain grasslands around Lake Urema and the streams that flow into it, making the animal harder to hunt for both people and lions.
“They’re extremely difficult to get to without trudging through a bunch of swamp,” Pringle says.
The antelope boon stems partly from the fact that they reproduce twice as often as buffalo, and 10 times as often as elephants. But it can also be explained by what the park is still missing: the bulk of its carnivores. Predators help balance an ecosystem by keeping plant-eating animal numbers in check. Too few predators can lead to herbivores overbrowsing, which in turn can jeopardize plants’ ability to endure.
The lack of carnivores may also have emboldened animals further down the food chain, who are now displaying unusual behaviors.
The power of predation
Before the war, Gorongosa’s bushbuck antelope (Tragelaphus sylvaticus) hugged the shadows of the forests, where their white-flecked, cinnamon-brown fur blended in with the background. Now they brazenly amble through open plains where their natural predators, leopards and wild dogs, would easily spot them. And the antelope have started eating the waterwort Bergia mossambicensis, a leafy flowering plant in the park’s floodplains that almost no other animals eat.
The bushbuck’s daring offered scientists a chance to test how carnivores affect the behavior of other animals — just through fear.
“We are just at the beginning of understanding the total impact that predators can have on prey populations, and also how they can affect community structure,” says Liana Zanette, a wildlife ecologist at Western University in London, Ontario who has not been involved with Gorongosa’s restoration.Just the threat of being killed can affect where an herbivore goes, what it eats and how quickly it reproduces. “Predators have effects far beyond what we ever imagined they could have,” Zanette says. But the idea has typically been documented in simpler or artificial scenarios, such as spiders and grasshopper nymphs in a terrarium.
Scientists thought bushbuck might revert to their woodland ways if the antelopes thought predators were back in the park, even if they actually weren’t.
So Princeton ecologist Justine Atkins, Pringle and their colleagues exposed bushbuck to a slew of deceptions: playing leopard growls over speakers and spreading fake carnivore urine and lion-scented scat purchased online. Within two days, bushbuck, born generations after the war so likely never having encountered a predator, were retreating into tree cover more frequently than before the experiment, the researchers reported April 12 in Science. The antelope stayed 150 meters on average away from the growling speakers.
Atkins and her team also set cages around 42 waterworts to protect them from the antelope and simulate the antelope’s return to the woods. About two weeks later, the plants had grown two centimeters taller and covered roughly 100 square centimeters more ground than unprotected waterworts.
Being able to demonstrate these cascading effects in the wild and in such a complex ecosystem compared with earlier, simpler experiments was surprising, Atkins says. So far, the scientists have tried this experiment only with bushbuck, so it’s unclear how other herbivores might respond. Still, “if we want Gorongosa to return to what it was, or at least be more similar to what it was before the war, large carnivores are really a key part,” Atkins says.
Bringing back the dogs
In the spring of 2018, Gorongosa’s conservation department took the first steps to expand the ecosystem’s pool of predators. It released 14 endangered African wild dogs (Lycaon pictus) flown from South Africa. Another pack, 24 strong, will be released in July.
“It’s a wave of teeth that’s coming,” Bouley, of Gorongosa’s carnivore program, says. So far, the dogs have hunted primarily antelope, even trekking into the swamps to tackle waterbuck. The dogs kill at least two animals a day, and sometimes up to six, she says.
More dogs will be needed to establish a larger population to avoid inbreeding and more effectively withstand threats such as disease and natural disaster. “Ultimately, we’re restoring a balance that was here just a few decades ago,” says Bouley, who is based in Sofala, Mozambique. That balance included more competing carnivores, not just dogs and lions. Building up those populations, and establishing balance between them and their prey, will likely take several more decades — if it’s even possible.
In complex food webs with several predators and connections, “you’d be crazy to think that it would quickly reorganize to its preremoval state,” says Tom Hobbs, a population ecologist at Colorado State University in Fort Collins who is not involved in the work in Gorongosa. Putting predators back into a broken ecosystem doesn’t necessarily reverse the results of having taken them out.
Hobbs has seen this while studying the effects of the 1995 reintroduction of 14 gray wolves (Canis lupus) into Yellowstone National Park, which stretches across Montana, Wyoming and Idaho. After hunters and rangers wiped out the park’s wolves in the 1920s, its elk population rapidly grew. The elk chomped down young willow trees that beavers in the park’s northern range depended on for food and building their dams. So beavers stopped making dams, which allowed streams to flow faster and tear deeper into the ground.
Now, almost 25 years after the wolves returned, Yellowstone’s elk population has dropped. But both the willows and the beaver population have yet to fully recover, researchers reported January 2019 in Ecohydrology. And streams still race through the park’s northern range. The future of Yellowstone’s willows remains hard to predict, says Hobbs, who was not involved in the Ecohydrology study. “But we can say without any ambiguity that they are not rapidly returning to pre–wolf removal state.”
Gorongosa is expanding, in part to make room for predators. About 2,800 square kilometers of an old private hunting reserve obtained in 2017, and on which another small pack of wild dogs was found, is set to become part of Gorongosa National Park in August. Park officials also oversee conservation in a large area outside park boundaries, including a buffer zone at the park’s edge where about 200,000 people live.
“We don’t know whether our current state is going to be a permanent state,” says conservation scientist Marc Stalmans, the Sofala-based director of scientific affairs at Gorongosa National Park and a coauthor on the PLOS ONE and Science papers. He suspects Gorongosa is still in transition, noting that Kenya’s Lake Nakuru National Park also went through a period in the 1970s of species imbalance — between zebras, buffalos, waterbucks and warthogs — that eventually worked itself out, as researchers reported in the April 2012 issue of Biodiversity and Conservation.
Ideally, the scientists want to see all of Gorongosa’s historical species back and in self-sustaining populations, even if not in their original numbers. That goal may face a number of unknown obstacles, from shifting climate conditions to increased competition for water resources with surrounding human communities. But the scientists can focus on reestablishing the fundamentals — a balance between the park’s original herbivore and carnivore species.
“We need to maintain the essential values of what the park used to be,” Stalmans says, though scientists can’t say for sure what that looks like. Before the civil war, “was the system moving in a certain direction?” he asks. Or maybe ecologists are aiming for something that’s just a snapshot in time. “Those are questions we don’t have answers for.”