Orangutan encounter rates have fallen six-fold in Borneo over the past 150 years, report researchers writing in the journal PLoS One.

Erik Meijaard, an ecologist with People and Nature Consulting International, and colleagues compared present-day encounter rates with collection rates from naturalists working in the mid-19th Century. They found orangutans are much rarer today even in pristine forest areas.

“Whereas some early explorers would see as many as eight orangutans in one tree or encounter three dozen along a river in a single day, today, in the same forests, spotting orangutan in the wild is rare,” Meijaard told mongabay.com via email.



Sumatran orangutan

The results suggest hunting is taking a toll on orangutan populations.

“Recent interviews of nearly 7000 villagers in Kalimantan [the Indonesian part of Borneo] revealed that over a thousand orangutans are still killed annually by local people, of which more than 50% is for food,” he said.

Meijaard and his co-authors highlight several factors that may have led to an increase in hunting in Borneo, including a particularly unusual one for Northern Borneo: a ban on head-hunting by colonial rulers in Malaysia.

“It remains unclear why hunting pressure would have increased in western Sabah a few hundred years ago. The timing coincides with the approximate end of head-hunting in Borneo, which had kept large parts of the island too dangerous to travel in,” the authors write. “Banks inferred that head-hunting provided wildlife a refuge, because large areas of forest were avoided by hunters fearful of roving bands of head-hunters. He noted that, immediately after the colonial ban on head-hunting was enforced, many forest areas became much safer to travel through, allowing hunters to travel further from villages… Also, it has been suggested that orangutan heads replaced human heads as trophies, which may also have added to hunting pressure on orangutans.”



Borneo orangutan

Looking at data from Indonesian Borneo, the researchers note a correlation between orangutan densities and the distance from villages: more remote forest areas have more orangutans.

The authors write that introduced disease could also be a factor in the apparent decline in orangutan populations, but note there is no evidence to date of epidemics in wild orangutans.

Danger of shifting baselines

The finding that orangutan densities were once considerably higher leads the authors to warn that the present state of knowledge on the red apes may be biased by a “shifting baseline” scenario: a situation where current degraded conditions are assumed to be the representative of past conditions.

“Insights from our study make us realize how much the population baseline may have shifted,” Meijaard told mongabay.com via email. “There may have been half a million orangutans 150 years ago, and even more before that.”

“Scientists have learned about orangutans by studying them under present-day conditions and densities, but this might be a bit like studying bushmen in the Kalahari to understand the behavior of a New Yorker.”

In other words, everything we think we know about orangutans may be based on incomplete knowledge.

“If our ideas turn out to be correct it would completely change the way we understand breeding, interactions between sexes, feeding habits, and ranging,” he said, adding that conservationists will need to consider these issues in their strategies to protect orangutans, which currently number around 50,000 between populations in Sumatra and Borneo.



Kalimantan, 2006

Kalimantan, 2009

“Conservation won’t get us back to the half million orangutans of [19th century biologist Alfred] Wallace’s day, but what should we be aiming for?”

Meijaard says the research also highlights the need to more effectively address hunting as a major threats to orangutans.

“The present study and another one that we still need to publish increasingly indicate that hunting by local people has been and still is a massive threat to orangutans in Kalimantan. Not a single program is addressing that issue though. Why? Is it too sensitive to blame poor Dayaks? Or is it too difficult? Whichever the reason, it is obvious that unless we can address hunting, pretty much all of Kalimantan’s orangutans that occur outside protected areas are doomed.”

CITATION: Erik Meijaard, Alan Welsh, Marc Ancrenaz, Serge Wich, Vincent Nijman, Andrew J. Marshall. Declining Orangutan Encounter Rates from Wallace to the Present Suggest the Species Was Once More Abundant. PLoS ONE August 11, 2010

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