Canopy bridges: The answer to fragmented forests?

In November 2020, a couple of young researchers at Satchari National Park in Habiganj tried their hands at something that was a novel concept in Bangladeshi wildlife conservation.

Wildlife researchers and conservationists Hassan Al-Razi Chayan and Marjan Maria hired help at Tk 1,500 a day (for a few days) to set up aerial rope bridges and connect two fragmented sides of Satchari forest. They based their initiative on the findings from studies conducted around the world, such as the much-celebrated case of Hainan gibbon.

Anyone who has been to Satchari National Park is painfully aware of just how fragmented the forest is. Decades of economic development, a road literally slicing through the forest, and increased human footprint have not done much good to the forest. Yet, this evergreen tropical forest in northeast Bangladesh is home to six of the 10 primate species found in the country. Many of these primates are either considered "Vulnerable" or "Endangered," a status from which graduation to something better is becoming increasingly difficult, if not impossible.

To make matters worse, according to data collected by Hasan and Marjan between 2015 and 2017, 14 primates died while crossing the road in Satchari forest, and 13 died in the same manner in Lawachara forest. The data is based on information collected from news reports and other sources. This raised alarm among researchers as many of the mammals that died were either vulnerable or endangered. Casualties included the likes of Phayre's leaf monkey which is endangered, capped langur which is vulnerable, and slow loris which is endangered.

It was these casualties that gave birth to the idea of an artificial aerial rope bridge connecting the two sides of the forest. The Forest Department also supported the initiative, the researchers said. Marjan was the one who received a small grant from the US-based The Explorers Club to help move along the project. The researchers had to first decide on the material for the rope. Since it was targeted to help primates, some of whom can be quite heavy, they settled on using ropes that big ships use to set anchor. At least three canopy bridges were set up at different points in Satchari forest, connecting the two sides that have been created by man-made interventions. The team also set up camera traps to monitor whether any of the intended animals actually used their bridges.

It took some time, but in the first month of being set up, small arboreal mammals such as Irrawaddy and flying squirrels were seen using the rope bridges. It took some more time for the primates to warm up to the idea. But soon enough, camera trap footage revealed alpha males of primate groups leading the way, thus encouraging others to use the rope as well.

So far, the researchers have found five out of six available primate species in the forest using the rope bridges to cross the road. Other than hoolock gibbon, all five other primates, including the endangered slow loris, have benefitted from this set-up. It remains to be seen whether the hoolock gibbons will respond to this intervention, even though they are arboreal mammals and are known to travel between areas simply by moving from tree to tree.

In Bangladesh, this is a fairly new conservation method, and only long-term studies will reveal whether this is indeed a solution to fragmented forests. But studies in other countries, such as the case of Hainan gibbons in Hainan Bawangling National Nature Reserve in China, have shown promise. According to an article titled "Rain Forest Canopy Bridges Aid Slow Lorises, Gibbons and Other Threatened Species," published in the Scientific American, aerial bridges have been used in Peru, the UK, India, Kenya, Brazil, Australia, and other countries to help a diverse groups of animals, ranging from marsupial gliders and squirrels to sloths and capuchin monkeys.

The work centring Hainan gibbons in China also points to a growing body of evidence proving the usefulness of artificial bridges to help arboreal animals cross fragmented landscapes to access habitat and unite populations, the article says quoting Dr Kylie Soanes, a conservation biologist in the University of Melbourne, Australia. And we now know that the footage from camera traps in Satchari have shown small arboreal mammals and primates using this aerial rope bridge to cross the forest.

This can be replicated at other forests in Bangladesh. For example, Madhupur forest, where a good population of rhesus macaque and capped langur is known to exist, could benefit from such rope bridges, says Hasan Al-Razi Chayan, one of the researchers who implemented the initiative in Satchari. The Tangail-Mymensingh highway cuts through the Madhupur forest at the moment. Forests in Chattogram, too, have become fragmented. Hasan's observation includes a population of gibbons in Kaptaimukh beat forest, who could use such a connecting bridge. According to Hasan, a river divides the forest into two, and there is a gibbon population on both sides. If ropes or bridges could connect both the sides, then it would ensure the gene flow, and a continuous variation will exist—thus making sure there is genetic diversity among the population.

But before going about and replicating the same idea everywhere, studies need to be conducted to find species-specific solutions. What works for one species or habitat—according to Rachel Nuwer, author of the aforementioned article in the Scientific American—may not be applicable for another. For some, a simple rope bridge may work, while some others may need lattice bridges.

The idea of artificial canopy bridges is already being modelled in Lawachara National Park, which is located in Moulvibazar and is a forest similar to Satchari. The researchers and conservationists of that project, which was initiated in September this year, are hopeful that their canopy bridges will help hoolock gibbons.

Regardless of which animal uses the rope bridges for safe travel, this is a step forward in the right direction. In an increasingly divided and changing forest scape, it is important to bridge the gaps. Much like we do in science, in human lives—connecting cities and towns with highways and air passage—to keep the flow of genes, we must facilitate natural migration and movement to make sure that the remaining wildlife continues to exist and flourish.

Abida Rahman Chowdhury is a journalist at The Daily Star.