Orangutans might survive sustainable logging

Like for all of its great ape cousins, the rise of Homo sapiens has not been pleasant for the Bornean orangutan, Pongo pygmaeus. The endangered “red ape,” found only in Borneo, is threatened by the continuing loss of its forest home. Hectare after hectare of primary forest is being lost either to logging or to palm oil plantations.

Primatologists have historically thought of orangutans as primarily arboreal, only rarely descending to the forest floor. With the recent expansion of logging and its associated clearing of roads, some researchers have noted orangutans spending more time with their feet on the ground. The assumption is that the clearing of roads creates large open spaces in the forest canopy. Without the ability to move from branch to branch, orangutans are increasingly forced to come down to the ground and cross roads in order to travel. It’s further assumed that being forced to be more terrestrial would negatively impact orangutan populations. It is conceivable that being on the ground makes them more susceptible to persecution by humans or to predation. It’s also, theoretically, outside of their species-typical behaviors.

Or is it? New research published last week in the journal Oryx suggests that orangutans may be more terrestrial than we thought, and have been all along. If true, that suggests that orangutans may be better able to thrive in the face of road construction and logging (if done sustainably) than has been assumed.

Simon Fraser University researcher Brent Loken, together with forestry researcher Chandradewana Boer from Indonesia’s Mulawarman University and Nunuk Kasyanto from an American NGO called Integrated Conservation, conducted a 2.5-year-long camera trap study in Wehea Forest, East Kalimantan, Indonesia, to test the assumption that orangutans prefer avoiding terrestriality when possible.

In all, they deployed 97 camera traps in three adjacent forest patches. One was mostly-undisturbed primary forest (within Wehea), one was a secondary forest in the process of recovering from logging which ended around 15 years prior to the study (also within Wehea), and a third was a patch of forest that was actively logged (“but not intensively”) just prior to data collection. In all, their camera trap array yielded information from 28,485 camera trap nights. Forty-four photos from the primary forest contained an orangutan, compared to 189 from the secondary forest, and 189 from the recently logged forest.

The data suggested that the red apes were indeed terrestrial in the recently logged forest, which is consistent with the notion that disturbances in the forest canopy can encourage terrestrial movements. However, the researchers also discovered a “high amount” of terrestriality in both of the other forest types as well. They say that indicates “that anthropogenic canopy disturbances are not the only driver of terrestriality.” Despite an entirely intact canopy, orangutans still walked on the ground in the primary forest.

“Our results indicate that terrestriality may be a regular strategy, employed almost equally by males and females as a means of locomotion,” the researchers conclude. Surprisingly, in the recent logged forest where orangutans displayed the highest amount of terrestriality, “most photographs of orangutans appeared to show the primates walking along the road rather than across it, which would be the case if they were forced to the ground by a break in the canopy.” Instead, it appears as if the apes are using roads quite often, and even in the relative absence of human impacts.

That suggests that orangutans might be more resilient to logging than was thought, as long as it is done sustainably. Sustainable logging efforts are probably going to be necessary in the formation of long-term, effective orangutan conservation programs. Still, how much logging they can tolerate remains an open question. “We must be careful [not] to reinforce the notion that orangutans can survive in any human altered landscape,” Loken and his colleagues warn. “Orangutans still need trees, and lots of them.” – Jason G. Goldman | 13 February 2015

Source: Brent Loken, Chandradewana Boer, & Nunuk Kasyanto (2015). Opportunistic behaviour or desperate measure? Logging impacts may only partially explain terrestriality in the Bornean orang-utan Pongo pygmaeus morio, Oryx, 1-4. DOI: http://dx.doi.org/10.1017/s0030605314000969

Header image: Flickr/Gaurika Wijeratne, CC BY-NC-ND 2.0.

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