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Canopy bridges keep rainforest animals connected over gas pipeline

  • Pipelines, roads, railways and transmission lines cause severe habitat fragmentation in the Amazon rainforest. A new study looked at canopy connectivity for large arboreal mammal populations using natural bridges above a new gas pipeline in Peru.
  • In 7,102 canopy camera trap nights, the crossing rate of natural bridges in the canopy above a new pipeline was surprisingly high: nearly 200 times that of the ground (3,100+ overhead versus 16 ground occurrences).
  • Researchers recorded 25 species from 12 mammal families using natural canopy bridges in 3,372 photo events, including night monkeys, kinkajous, olingos, dwarf porcupines, opossums and squirrels.
  • These results suggest natural and artificial canopy bridges could significantly improve habitat connectivity for rainforest arboreal species when new, or already existing, transportation, mining and energy corridors threaten fragmentation
Dwarf porcupines (Coendou ichillus) were documented in camera-trap images, 900 kilometers (560 miles) outside of their known range. Photo courtesy of SCBI-CCS

Hundreds of square miles of the Amazon are zoned for oil and gas exploration, with commercially viable reserves destined for national and international markets, including the US and the UK. But pipelines fragment the rainforest, dividing populations and disrupting the movements of species that spend their lives in the treetops. Now, a study using canopy camera trapping has shown that these impacts can be mitigated if natural canopy bridges are left in place when pipelines are constructed.

The study, in the Urubamba region of the Peruvian Amazon, was led by Tremaine Gregory of the Smithsonian Conservation Biology Institute. Before construction took place, Gregory and her colleague Farah Carrasco-Rueda, of the University of Florida, worked with the construction company to identify places where trees were large enough that their branches might be able to span the proposed pipeline. Engineering constraints meant that out of a possible 42 sites along a 5.2 kilometer (3.2 mile) stretch of pipeline, only 13 proved to be feasible. A 4 kilometer (2.5 mile) stretch of pipeline was left without any natural bridges as a control, so the effect of bridges on animal movements could be assessed.

Gregory’s team walked transects before construction began, and found that arboreal species frequently crossed the pipeline route. Once the pipeline was constructed, Gregory climbed the trees that were left to act as bridges, and installed camera traps to monitor their use. She placed cameras at all locations where animals could cross from one side to the other — some trees had multiple branches forming connections across the pipeline. More cameras were placed near the ground underneath the canopy bridges, and in the control area.

The traps monitored animal movements for a year, and upon analyzing the vast number of images generated from 7,102 canopy camera nights, Gregory said she and Carrasco-Rueda were “blown away by the data. The crossing rate in the canopy was nearly 200 times that of the ground (16 versus over 3,100).”

In total, they recorded 25 species from 12 mammal families using the natural canopy bridges in 3,372 photo events, with night monkeys (Aotus nigriceps), kinkajous (Potos flavus), olingos (Bassaricyon alleni), dwarf porcupines (Coendou ichillus), opossums (Caluromys lanatus), and squirrels (Hadrosciurus spadiceus) being the most frequently observed commuters. The team estimates that the bridges were used by at least 150 individuals.

A tamandua (Tamandua tetradactyla) with a baby on its back. Natural canopy bridges are already being incorporated into other pipeline projects in Peru, and Gregory hopes they will soon be a government requirement. Photo courtesy of SCBI-CCS

“The diversity of mammal species that used the bridges also represents a broad diversity of locomotor habits,” Gregory said. “For example, spider and woolly monkeys can brachiate — or swing by their arms — across slightly disconnected clearings, but dwarf porcupines do not leap, and therefore need branches that are in full contact. Both of these types of species used the bridges, as did many animals with dependent offspring on their backs, suggesting that the animals feel safe enough to cross with precious cargo.”

Six canopy species were observed on the ground a total of 16 times, but the absence of 19 species from ground-based camera traps indicates that most arboreal mammals were susceptible to the negative effects of the pipeline.

The range of species caught on camera encompassed the majority of arboreal mammal species in the region, suggesting that natural canopy bridges have widespread benefits, Gregory said.

“The greatest thing about this research is that it is of significant applied conservation importance,” said Andrew Whitworth, of Osa Conservation, Costa Rica, who previously led camera trap studies in the rainforest canopy of Manu National Park, Peru, but was not involved with the new study.

Night monkeys (Aotus nigriceps) making use of a natural canopy bridge across the pipeline. A year-long camera-trap study of 13 natural canopy bridges showed that they were an effective way to mitigate the fragmentation caused by pipelines. Photo courtesy of SCBI-CCS

Whitworth highlighted the black-faced spider monkey (Ateles chamek) and the Peruvian woolly monkey (Lagothrix cana) — which were each recorded just once — as particularly important bridge users, as they are classified as Endangered by the IUCN, and are sensitive to hunting, forest degradation and habitat fragmentation. “They are particularly sensitive as they have relatively long reproductive cycles and need to be able to traverse linear clearings to access different food sources and maintain genetic flow between communities.”

Whitworth emphasized the value of camera trapping to generate quantities of data “that would have been logistically unfeasible using human observers,” the results of which show “how essential, and cost-effective, maintaining canopy crossings can be for rainforest wildlife.”

Gregory said that other pipelines projects in Peru were beginning to incorporate natural bridges, and she’s “hopeful that they will soon be required by the government.”

Gray’s bald-faced saki monkeys (Pithecia irrorata). In total, 25 arboreal species from 12 mammal families were recorded using the natural bridges. Photo courtesy of SCBI-CCS

Because trees that are suddenly exposed to the more open habitat of the pipeline can experience environmental stresses, which can lead to branches breaking, Gregory suggests that “companies consider projects that incorporate both natural and artificial bridges, which can be maintained or replaced, if necessary.”

“Although this strategy is desperately needed in the Western Amazon right now, where linear clearings for unmarked roads and pipelines are fast expanding due to a ramping up in resource extraction (for logging and mining), this strategy could also help to restore connectivity in areas which have previously been cleared, where such mitigation strategies were never considered,” Whitworth said.

Gregory sees huge scope for natural canopy bridges worldwide. “Because forests across the tropics tend to have large arboreal mammal populations, we are hopeful that other countries will also adopt this simple, low-cost method.”


Gregory, T., Carrasco-Rueda, F., Alonso, A., Kolowski J., and Deichmann, J.L. (2017) Natural canopy bridges effectively mitigate tropical forest fragmentation for arboreal mammals. Scientific Reports 7: 3892 DOI:10.1038/s41598-017-04112-x

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