When roads cut through forests, they can become a death trap for wildlife. Canopy bridges, structures that connect trees on either side of roads, are considered a crucial lifeline for tree-dwelling animals, but few researchers have examined their long-term effectiveness.
A recently published study did just that, by analyzing three years of videos from camera traps installed on bridges in Costa Rica. Researchers found that wildlife use of the bridges increased steadily with time, and animals showed preferences for certain bridge designs and length.
Previously, most arboreal bridges had only been monitored for about a year at most, according to study lead author Marion Fischer, a Ph.D. student with the University of French Guiana. Without long-term data, it’s hard to know how useful bridges really are or what type works best, she told Mongabay in a video call.
So, Fischer worked with the Costa Rican NGO Osa Conservation to analyze thousands of videos recorded by camera traps that the organization installed on either side of 17 bridges across the Osa Peninsula, a biodiversity hotspot.
The team documented 2,231 animal crossings from December 2020 to June 2023. These included at least 13 species of arboreal mammals, including two species that are highly vulnerable to collisions with cars: The common opossum (Didelphis marsupialis) and the Mexican hairy dwarf porcupine (Coendou mexicanus).
White faced capuchin monkeys (Cebus imitator) and squirrel monkeys (Saimiri oerstedii) frequented the bridges as well. However, larger-bodied howler (Alouatta spp.) and spider monkeys (Ateles geoffroyi) were not documented using them. Lizards, small rodents and one snake were also seen in the camera traps. Birds used the bridges for perching.
The team looked at five different bridge designs, three made of rope and two plastic mesh ones, and found wildlife preferred the rope bridges.
The animals also seemed to prefer longer bridges over short ones, likely because many animals, including monkeys, could simply jump across the space connected by a short bridge, Fischer told Mongabay.
The study found that bridge use steadily increased over the three-year study period. Some of the bridges have been in place for a total of six years now, and just last year Fisher saw the first spider monkey using one of them.
“We always saw them [spider monkeys] around the bridges, we had some videos of them touching the ropes and interacting with the bridge, but never crossing,” she said. “And now, finally, after some time they use them. So, I think they needed time to trust the new structure.”
Costa Rica is advancing new legislation that will require wildlife crossings in ecologically sensitive areas. Fischer said it’s important that such wildlife infrastructure also come with long-term monitoring, “because then you can really see the long-term impact on the connectivity.”
Banner image: Screenshot of a capuchin monkey using an arboreal bridge in the Osa Peninsula. Image courtesy of Marion Fischer.
