Orangutans weren't always found at such low densities, unsurprisingly, humans to blame

From National Geographic blogs
Hunting a key factor in orangutan's decline, study suggests
by RICHARD THOMAS

Humans entering the forests of Borneo 150 years ago were six times more likely to encounter wild orangutans than they would today, a new study finds. The researchers suspect that heavy hunting over the years is to blame. The finding means our understanding of the lives and behaviors of the great ape is based on artificially low population densities. We may need to rethink what we know about our nearest animal relative.

Hunting appears to have been significantly underestimated as a key reason for the historical decline of orangutans, according to a new study published today.

An international team of scientists noted how animal collectors operating in the mid-19th Century in Borneo [an island shared by Indonesia and Malaysia] were able to shoot orangutans on a daily basis and speculated that 150 years ago, encounter rates with the forest primates must have been far higher than they are today.

To test the hypothesis, the researchers attempted to quantify historic encounter rates from information contained in hunting accounts and museum collections and comparing them to recent field studies.

"Even after allowing for variations in the size and length of hunting and survey expeditions and other variables, we estimated that daily encounter rates with orangutans have declined by about six-fold in areas with little or no forest disturbance," said Erik Meijaard, of People and Nature Consulting International in Indonesia, the lead author of the study.

Possible explanations for the decline were examined, including habitat loss and degradation, hunting, disease, and even changes in behavior, such as animals becoming more wary in the face of human persecution.

"Although there are gaps in the data, after examining several possible explanations, we concluded that high levels of hunting was the most likely cause of the reduced encounter rates over time," said Vincent Nijman, of Oxford Brookes University, a co-author of the study.

Despite legal protection, hunting of orangutans still occurs and may have had a more significant impact on wild populations than previously realized.

According to Meijaard, recent unpublished studies in Indonesian Borneo suggest that more than 1,000 orangutans are killed annually.

Orangutans are hunted for a variety of reasons, including food, as agricultural pests, and to obtain young individuals for the pet trade.

Hanging in the balance, an assessment of trade in gibbons and orangutan in Kalimantan, Indonesia, a TRAFFIC report published in 2005, found that the vast majority of orangutans in trade were young animals, suggesting that the adult females had been hunted.

"We need to understand better how orangutan populations are affected by different levels of hunting pressure," said Nijman.

"Indeed, our findings may force us to rethink the whole biology of orangutans. Much of our current ecological understanding is possibly based on field studies of animals living at densities below those that would be imposed by food availability."

"How would the species behave if natural densities were 5 or 10 times higher than those we currently observe?"

Declining orangutan encounter rates from Wallace to the present suggest the species was once more abundant by Erik Meijaard, Alan Welsh, Marc Ancrenaz, Serge Wich, Vincent Nijman and Andrew J. Marshall, is available from PLoS One.

Richard Thomas is the communications coordinator for TRAFFIC, the UK-based wildlife trade monotiring network. This article is reproduced on Nat Geo News Watch courtesy of TRAFFIC.

The views expressed here are those of Richard Thomas or TRAFFIC and not necessarily those of the National Geographic Society.

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REEFERNCE
Meijaard E, Welsh A, Ancrenaz M, Wich S, Nijman V, et al. (2010) Declining Orangutan Encounter Rates from Wallace to the Present Suggest the Species Was Once More Abundant. PLoS ONE 5(8): e12042. doi:10.1371/journal.pone.0012042

ABSTRACT
Background
Bornean orangutans (Pongo pygmaeus) currently occur at low densities and seeing a wild one is a rare event. Compared to present low encounter rates of orangutans, it is striking how many orangutan each day historic collectors like Alfred Russel Wallace were able to shoot continuously over weeks or even months. Does that indicate that some 150 years ago encounter rates with orangutans, or their densities, were higher than now?

Methodology/Principal Findings
We test this hypothesis by quantifying encounter rates obtained from hunting accounts, museum collections, and recent field studies, and analysing whether there is a declining trend over time. Logistic regression analyses of our data support such a decline on Borneo between the mid-19th century and the present. Even when controlled for variation in the size of survey and hunting teams and the durations of expeditions, mean daily encounter rates appear to have declined about 6-fold in areas with little or no forest disturbance.

Conclusions/Significance
This finding has potential consequences for our understanding of orangutans, because it suggests that Bornean orangutans once occurred at higher densities. We explore potential explanations—habitat loss and degradation, hunting, and disease—and conclude that hunting fits the observed patterns best. This suggests that hunting has been underestimated as a key causal factor of orangutan density and distribution, and that species population declines have been more severe than previously estimated based on habitat loss only. Our findings may require us to rethink the biology of orangutans, with much of our ecological understanding possibly being based on field studies of animals living at lower densities than they did historically. Our approach of quantifying species encounter rates from historic data demonstrates that this method can yield valuable information about the ecology and population density of species in the past, providing new insight into species' conservation needs.