09 19/09/2017

New study reveals decline in Bornean orangutan population

Populations have declined by 25% over the last 10 years

The innovative model assessed the different drivers linked to the species’ decline and showed that orangutans have lower survival rates in areas where forests have recently been converted to industrial agriculture.

Assessed as Critically Endangered by the IUCN, the Bornean orangutan (Pongo pygmaeus) is one of only two great ape species to be found in Asia. The species is protected under Malaysian and Indonesian law but an accurate assessment of the rate of its population decline or its drivers was still missing. We are partnering with HUTAN-Kinabatangan Orangutan Conservation Project, an organisation located in Borneo, to make a difference and help preserve orangutans.

Threats to the species include habitat loss, fragmentation due to forests being turned into agricultural, mining or infrastructure development sites, human-orangutan conflict, and hunting for bushmeat and wildlife trade. 

Orangutan in wild
Photo credit: HUTAN - Dzulirwan bin Takasi

Drivers of population decline are known to be hard to assess accurately as species’ surveys are often restricted to small geographic areas, conducted over short period of time and employ a diverse range of survey protocols. The new study by Santika et al., co-authored by Marc Ancrenaz the director of our project partners HUTAN, applied novel innovative methods integrating both field and interview survey data to get a deeper understanding of the species’ dynamic through time.

The analysis is the first robust population trend analysis for orangutans including quantitative assessments of drivers of change and revealed that the species has suffered a 25% decline over the past 10 years, contradicting some views and non-supported claims that indicated an increase in orangutans’ numbers.

Adapting a dynamic population model developed by Chandler & Clark, researchers were able to integrate both count data and presence-absence data of the species. The model developed required both spatial and temporal data including four broad levels: latent orangutan population density, observed orangutan occurrence, latent orangutan nest density, and observed orangutan nest density and occurrence.

Orangutan in tree
Photo credit: HUTAN - Mohd Daisah Kapar

Survival rates were shown to be lowest in areas with intermediate rainfall which is the optimal climate in this zone to support plant productivity and agriculture. This finding can be explained by the fact that orangutans favour the same climate and range as human populations do, putting them in direct competition for space and resources, which results in their destruction. 

Other factors impacting orangutans’ survival revealed by this study also include forest clearing and decrease in forest carrying capacity potentially due to climate change. The model also indicated that the species’ long-term abundance and survival rates per km2 is strongly determined by the extent of natural forest, suggesting that this factor alone can decrease orangutans’ survival rates.

The populations’ survival in Sabah were estimated especially at risk as the average size of forest patches where the species currently reside was assessed to be the lowest in Borneo. Even though the populations in Sabah are mainly found within the boundary of protected areas, they are facing high threats due to the protected areas small sizes and their lack of connectivity. However, the study also found that overall in Borneo the orangutans’ survival rates increase with proximity to a protected area indicating that natural forests can mitigate some of the threats affecting the great ape species.

Orangutan using bridge in wild
Orangutan on bridge built by Chester Zoo and HUTAN team. Photo credit: Clark Andkerson

Highlighting the urgency to determine specific management interventions, the team of researchers behind the study suggests that maintaining high forest cover and improving connectivity among remaining forest patches is crucial to increase orangutans’ survival. They also advise that working in close cooperation with plantation companies, smallholder farmers and local communities is essential to manage conflicts with the great ape.

At Chester Zoo we partner and support conservation projects in South East Asia, with a focus on orangutans in Malaysian Borneo, protecting the wildlife that is threatened by palm oil production. We’ve been working to raise awareness of the impact that the production of palm oil in South East Asia has had on rainforests all over the world and increase collaborations with like-minded organisations to work together to make sustainable palm oil the norm.

After starting small and auditing our own supply chain making sure that the food products that we stock in our shops and restaurants that contain palm oil were from certified sustainable sources, we are now taking a step forward by launching our new initiative: Sustainable Palm Oil City.

We have created the Sustainable Palm Oil City toolkit, which contains everything you need to join us. We’ll promote any business in the city that takes the pledge to make Chester the first Sustainable Palm Oil City in the world and will follow your journey to celebrate your successes! For more information and to join our launch event on 21 September contact us here.  

Full Reference: Santika et al. (2017). First integrative trend analysis for a great ape species in Borneo, Scientific Reports, 7, doi:10.1038/s41598-017-04435-9