About half of the orangutans on the island of Borneo were either killed or removed between 1999 and 2015, according to new research.

Key points: Surveys show only 70,000 - 100,000 orangutans remain in the wild in Borneo

Surveys show only 70,000 - 100,000 orangutans remain in the wild in Borneo Deforestation one factor, but greater losses seen in forested areas

Deforestation one factor, but greater losses seen in forested areas Hunting may be the biggest driver

"That's a huge amount of loss," said Professor Serge Wich from Liverpool John Moores University, a co-author of the study published today in the journal Current Biology.

"It's a higher amount than we thought, and that we were thinking based on previous studies," Professor Wich said.

Maria Voigt from the Max Plank Institute for Evolutionary Anthropology is another co-author of the research, which pooled data from 38 international institutions.

"We used a very broad compilation of orangutan survey data to model their distribution and found that they had declined by more than 100,000 individuals," Ms Voigt said.

"It's a 50 per cent loss."

Based on their data, Ms Voigt said there were "around 70,000 to 100,000" orangutans left in the wild in Borneo.

Human development has crept further into Bornean orangutans' native habitats ( Supplied: Serge Wich )

Professor Wich said the decline was calculated by combining existing surveys of orangutan nests.

"The study we've conducted took advantage of 16 years of survey data on orangutan locations in Borneo," he said.

"On a large island like Borneo, it's impossible to go to every single piece of forest."

The team filled in the gaps by looking at maps of land-use changes and other threats that have an impact on orangutan populations.

They examined how those changes affected populations in areas where they had nest-survey data.

Then they used those results to extrapolate how the species was faring right across the island.

"We've used a lot of land-use layers, [and] threat layers, like human population density, to try to predict what the density is in the areas where we did not go," Professor Wich said.

The data showed there were more orangutans on the island than was previously estimated, but also that a greater number were killed.

"In 1999, there were more orangutans than we thought," Professor Wich said.

"But it also means we've lost much more than we thought — so it's a bit of a double-edged sword in that way."

Deforestation vs hunting

Palm oil plantations and logging have both contributed to deforestation in Borneo ( Supplied: Marc Ancrenaz )

The study showed changes in land cover led to whole populations of orangutans being driven out of some areas.

"You have the steepest loss in the habitat loss areas, where you have deforestation and conversion," Ms Voigt said.

However, she said this only made up for about 10 per cent of the total number of orangutans lost.

In terms of raw numbers, more individuals were lost in the remaining forests and partially forested areas.

The researchers believe this has been driven mainly by hunting.

"This was a really important finding, as it supports previous research that killing and hunting is a huge problem, and it might even be the biggest driver during this period," Ms Voigt said.

Professor Wich said roughly 70 per cent of the total number of orangutans killed came from forested areas.

"Hunting is a widespread issue," he said.

"Orangutans are frequently being shot when they venture into plantations, when they venture into small-scale farm areas.

"We had not realised that the issue was this large though," Professor Wich said.

Adjunct Professor Erik Meijaard from the University of Queensland and the Brunei-based conservation group Borneo Futures was another co-author of the study.

He said he had been trying to highlight the impact of hunting over the past decade, after learning many people in Borneo were still killing orangutans for food.

"Those people frequently told me how good and sweet orangutan meat was," Dr Meijaard said.

"Our current study shows that we have collectively failed to address the most important threats, and therefore orangutan populations are plummeting on Borneo."

Many populations of orangutans are close to the brink of extinction ( Supplied: Marc Anzrenaz )

Further losses predicted

Professor Wich said the research team also modelled how the remaining orangutan population in Borneo would fare over coming years. The results were not promising.

"We examined the potential decline of orangutans through deforestation in the future, up until 2050," he said.

"It could be that in the period from now until then, we lose about 45,000 orangutans through deforestation alone.

"We didn't even incorporate the potential losses through hunting in that projection going forward.

"That's the main worry — that we're not giving it its due attention, and that therefore we're not developing conservation strategies to curb that hunting."

However, Ms Voigt said there were some promising signs for orangutans, such as a few populations that were being successfully protected in Malaysian forest fragments around plantations.

"There are high densities of orangutans [there], and also other wildlife," Ms Voigt said.

The researchers say some orangutan populations in Borneo have stabilised ( Supplied: Marc Ancrenaz )

She also said a string of recent stakeholder meetings and new action plans in both Malaysia and Indonesia could help the species.

Professor Wich said he hoped increased awareness would lead to improved conservation.

"There is a lot more awareness in Indonesia and Malaysia for environmental issues in general than ever before," he said.

"There are some promising collaborations between conservation and industry — be that oil palm industry or logging companies — where they try to maintain the orangutans in those areas.

"We need to be able to find a situation in which we protect orangutans in a matrix of different land uses.

"Orangutans are fairly flexible and they can probably survive in those matrices. But we can obviously only do that when they're not being killed."