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ONGOING SURVEY OF ORANGUTAN POPULATIONS ON SUMATRA On a mission to collect data in the middle of the rainforest Full view


Have you ever wondered how the number of wild orangutans is determined? We explain this by using the example of the two orangutan species on Sumatra.

The last population wide survey of orangutans on Sumatra was conducted by the SOCP between 2009 and 2011, and was published in 2016. As far as we are aware, this was the first ever survey of a great ape species’ entire global distribution. It revealed that only around 13,500 Sumatran orangutans and less than 800 Tapanuli orangutans still live in Sumatra. Thousands of years ago, orangutans could be found throughout South-East Asia, but today their habitat has been reduced to a few tropical rainforests in Borneo and in neighbouring Sumatra, which hosts two species. According to the calculations of our Sumatran Orangutan Conservation Programme (SOCP), around 110,000 ha of the Sumatran orangutan’s (primary) rainforest habitat were lost between 2002 and 2018. The Tapanuli orangutans lost around 4,330 hectares in the same period. Combined this corresponds to an area of the size of the Canton of Uri in Switzerland. Depending on food availability, there are areas where orangutans occur at higher or lower densities. The highest densities on Sumatra tend tob e in the lowland forests, especially the peat swamps. There their preferred food tree species (especially fruits) are most abundant, meaning orangutan densities are also highest. This is particularly tragic when one considers that these are also the very areas where deforestation for palm oil plantations is most common.

But how has the population developed in recent years? At the moment, our SOCP teams are conducting a large-scale survey across all orangutan habitat in Sumatra to determine the current population densitities and the overall numbers of orangutans remaining. This is especially useful as being 10 years after the previous surveys, it may help us identify population trends, i.e. increasing, decreasing or stable. We explain how:

  • Adhering to standard methodology: Our teams are working with the accepted methods for recording population numbers of great apes. The survey results are used by the World Conservation Union, IUCN, to deternine if species are threatened, endangered or critically endangered (as all orangutan species are now).
  • Preparing for the survey: Line transects are overlayed regularly throughout orangutan habitat. They are then visitied by teams on foot. Some are so remote they can take weeks to reach.
  • Looking for nests: Orangutans build sleeping nests high up in the trees, and it is much easier to find and record these nests than to find and count orangutans themselves. Two experienced conducters walk slowly along the straight line transect and record the perpendicular distances of all identified nests, It is then possible to deteremine the density of nests at each transect, and to convert that density into a density estimate of the orangutans themselves, since we know how many nests an orangutan makes each day, how long nests remain visible in the forest and what percentage of orangutans make nests. Oncew we know the density in an area, by extrapolating that over the area of habitat, we can estimate the total number of orangutans in a given forest.
  • Additional data collection: While the teams are looking for orangutan nests they also collect additional data, such as the quality of vegetation in the rainforest, using a UNESCO datasheet. They also ask locals about human-animal conflicts and possible hunting habits. Another SOCP team looks at threats tot he habitat and monitors forest loss by analysing satellite imagery.
  • Data analysis: A statistical model can then used to determine the factors that most influence the the density (and therefore numbers) of orangutans in a given area.

The field work partv oft he current survey is scheduled to run until the end 2021, They wer originally scheduled to finish sooner but the teams were unable to start as planned in 2020 due to the emergence oft he corona virus pandemic. After detailed and lengthy analyses we expect the results to be published in late 2022 or early 2023.

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