In July 2014 Super Typhoon Rammasun swept over Hainan, an island province of southern China, destroying at least 23,000 houses and causing $2 billion in damage. It was not just people who experienced the impact, though. In Hainan Bawangling National Nature Reserve, home to the world’s 30 remaining Hainan gibbons—the rarest primate on earth—landslides tore through sections of the forest. With critical parts of their habitat now cleaved by deep gullies, the gibbons, a tree-dwelling species, were forced to leap across a distance of up to 50 feet to get from one area to another.
The gibbons came to depend on just two places to cross the tree chasm, and sometimes they would barely make it—breaking their fall only by managing to grab hold of the tip of a particular palm frond. “It was very scary!” says Bosco Chan, head of the Chinese conservation department at the Kadoorie Farm and Botanic Garden in Hong Kong, who manages its Hainan gibbon conservation project. As months passed, Chan noticed that constant use was causing the frond to fray and droop, threatening to bring the gibbons crashing down with it—and breaking the critically endangered species’ habitat into disparate islands. “I didn’t know how long the frond would be able to hold the gibbons,” he says. “I thought, ‘It’s just time for us to build something.’”
As recently described in Scientific Reports, Chan and his colleagues turned to aerial bridges—the arboreal answer to terrestrial wildlife underpasses and overpasses. They hired professional tree climbers to brave the steep terrain to install a simple bridge, 33 feet at its highest point, consisting of two parallel mountaineering-grade ropes. Including labor and materials, the bridge’s installation cost $5,000. It took the gibbons more than five months to catch on, likely because they have never encountered an artificial structure before. But once they learned the ropes, they started regularly using the bridge to make safe, quick crossings. (Check out a video.)
The take-home message is that solutions to similar conservation problems “needn’t be very complicated and needn’t be very expensive,” Chan says. “Sometimes simple is beautiful.”
Chan and his colleagues’ work adds to a growing body of evidence pointing to the usefulness of artificial bridges for helping arboreal animals cross fragmented landscapes to access habitat and unite populations, says Kylie Soanes, a conservation biologist at the University of Melbourne in Australia, who was not involved in the Hainan project. “The more information we have coming in from studies like this on different species in different situations, the better.”
Aerial bridges have been used in Peru, the U.K., India, Kenya, Brazil, Australia and other countries to help a diversity of animals ranging from marsupial gliders and squirrels to sloths and capuchin monkeys. But many aerial-bridge installations are small, one-off projects carried out by local organizations or even individuals, with little or no scientific study of what does or does not work. In addition, solutions for one species or habitat may not be applicable for another.
“In the recent paper on gibbons, they created a specific design—two parallel ropes—that’s perfect for gibbons but maybe is not going to work for spider monkeys in the Amazon,” says Fernanda Abra, a postdoctoral researcher at the Smithsonian Conservation Biology Institute, who was not involved in the study. “What we want is to build knowledge in tropical countries about what structures are most suitable for different species with different behaviors.”
At the beginning of 2021 Abra will begin testing the effectiveness of artificial bridges for helping various species cross hundreds of miles of highway running through Brazil’s Amazon rain forest from Manaus to Boa Vista. She will start by comparing two different designs—a single large rope and a two-dimensional rope lattice—for helping two threatened species: pied tamarins and Guiana spider monkeys, both of which are regularly killed by cars. If successful, her findings will provide a proof of concept that she hopes will be scaled up.
Fortunately, there is already promise of that happening. Members of the Waimiri Atroari tribe, whose land the road cuts through, are enthusiastic about helping Abra locate the best places to install bridges and maintain the structures after her study comes to an end. The Waimiri Atroari have also already expressed interest in learning to build the bridges themselves so they can install more of them. “They want to avoid animals being killed because they see it as a waste—because animals are their food, their source of meat,” Abra says. “Now that they’re involved in this project, this is gold for me.”
Unlike the gibbon study, which took place in a strictly protected natural area, most artificial bridges are installed in places where humans dominate or at least are present in the landscape. In those cases, buy-in from local people is often critical for long-term success, says Anna Nekaris, a primate conservationist at Oxford Brookes University in England, who was not involved in the gibbon research. Nekaris, who studies threatened slow lorises in agricultural areas of the Indonesian province of West Java, learned this lesson firsthand. Originally, she and her colleagues sought to connect highly fragmented trees that the animals depend on by installing strips of industrial rubber. But while the slow lorises loved the rubber bridges, the farmers on whose land the animals lived “didn’t care,” Nekaris says. “If there was a windstorm, and the bridges blew down, they wouldn’t even tell us.”
To win the farmers over, Nekaris realized that she could use PVC pipes that would simultaneously act as bridges for slow lorises and valuable irrigation sources for agriculture. The pipes—which are tied to trees on either side and supported by a strong wire—run from natural streams down to fields, allowing them to use the mountain gradient to deliver water. Farmers would normally have to shell out about six months’ worth of salary for such a system. At the same time, Nekaris and her colleagues built up local pride and interest in slow lorises by teaching children about the animals and assigning each family an individual slow loris mascot. As the team reported last month in Diversity, slow lorises are now just one of at least 20 species—including Javan palm civets, collared scops owls, Javan kingfishers and Horsfield’s tree shrews—that use the well-maintained bridges for crossing or perching.
While aerial bridges are no replacement for protecting intact habitat, they can play an important role in helping species survive in an increasingly human-dominated landscape, Soanes says. “In many places, so much has already been heavily modified, and so much already lost,” she says. “We should be looking for ways to help species persist in these modified environments, too.”