"In other words," Goossens said, "the only remaining explanation for the declining numbers was recent exploitation of the forest and its fragmentation."
His results indicate that orangutan populations have declined by more than 95 percent, and most of the decline likely took place recently.
"As it happens, we know that the forest exploitation started about a century ago in 1890 and that it accelerated in the 1970s," added Goossens, whose findings are published this week in the journal PLoS Biology.
"I think the paper's right on track," said James Compton, regional director of TRAFFIC Southeast Asia, a wildlife-trade monitoring program run by the Washington, D.C.-based conservation nonprofit WWF.
"In Sabah, the conversion of forests to oil-palm plantations and the habitat fragmentation are having serious detrimental effects on the orangutans."
According to Eric Dinerstein, chief scientist at WWF, the results confirm the obviousorangutan numbers are declining due to loss of habitat.
What's more interesting, he says, is the discovery of high genetic diversity among the orangutans, despite their endangered status.
Since a small fraction of genetic diversity is lost every generation, the high diversity indicates that a large number of the apes previously mixed and bred, Dinerstein says.
Considering their current low numbers and the fact that orangutans breed every 10 to 15 years, the conservationist says, the finding suggests the orangutan decline is very recent.
"It gives us an idea of the road ahead," which is to protect and restore as much mature forest as possible in Borneo and Sumatra, he added.
The continuing decrease in forest cover has experts worried.
"After deforestation, oil-palm plantations take over and increase the fragmentation," said Lounès Chikhi, a population geneticist based at the Université Paul Sabatier in France and study co-author.
"The point is that in the smallest fragments inbreeding will quickly increase," Chikhi said, resulting in possible physical and health defects.
Despite the grim outlook, experts say there is hope for the orangutan.
"The good news is that the orangutans still possess enough genetic diversity to stabilize if immediate action is taken," Goossens, the Cardiff wildlife geneticist, said.
To start with, he says, the size of the orangutan refuge needs to be enlarged. Also "orangutan bridges" need to be built near river tributaries where large trees have been felled to facilitate the apes' movement between segments of forest.
Compton, of TRAFFIC Southeast Asia, says systematic zoning of the land will help balance the needs of the orangutans and agriculture in the region.
He calls the latest study "a turning point for the government and private sector to look at alternatives for sustainable agriculture."
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