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The Endangered proboscis monkey Nasalis larvatus is endemic to the island of Borneo. Habitat loss is a major threat to this species, and an understanding of long-term demographic trends is crucial for its conservation. We assessed the population trends and group sizes of proboscis monkeys over 10 years in the Lower Kinabatangan floodplain in Sabah, Malaysian Borneo. Comparisons of observed populations between 2004 and 2014 revealed significantly reduced group sizes, which is probably a result of forest fragmentation. Three long-term studies over 34–73 months in specific areas showed fluctuating estimated densities in each area, but no overall population increase or decrease. Riparian forests are the most important habitat for these monkeys, and one reason for the relatively stable population could be that there were only minor losses of forest along rivers during 2004–2014 because protected areas have been established in the region in 2005. However, proboscis monkey habitat remains under threat in areas allocated for oil palm, and protection of these areas is paramount to maintaining this population.
The forests of Borneo and Sumatra are a major hot-spot for biodiversity. They are home to the only two species of Asian great apes that survive today: the Sumatran orang-utan (Pongo abelii) and the Bornean orang-utan (Pongo pygmaeus). However, huge tracks of natural forests are being degraded by extractive industries (mining, timber harvesting, etc) or replaced with small-scale subsistence crops and large-scale industrialised monoculture (oil palm, acacias, eucalyptus, etc) throughout the region. As a result, entire wildlife populations are displaced or wiped out.
The destruction of these forests is a multi-faceted disaster for the orang-utan and for myriad other species. Indeed, the intense degradation and destruction of food resources lead to starvation, increased sensitivity to disease and stress, or to decreased reproduction. During waves of forest conversion, animals that are not killed are displaced and take refuge in remaining forest patches, resulting in significant alteration of the dynamics and structure of surviving wild populations. The newly created man-made landscape results in increased contact and conflict between people and wildlife. Crop-raiding orang-utans are often perceived as pests and are killed, although they are officially protected in both Indonesia and Malaysia. Surprisingly, our work in Kinabatangan (Sabah, Malaysian Borneo) indicates that orang-utans show an unexpected resilience to these disasters. To a certain extent, the animals are able to adapt their behaviour and ecology to these new environmental conditions. From a conservation point of view, it is essential to investigate this resilience in order to design and promote wiser land management practices that will reconcile economic growth with the long-term survival of orang-utans and other animal species.
It is also crucial to investigate how local communities’ perceptions of wildlife are being influenced by recent landscape transformation. The communities living in the Kinabatangan floodplain used to consider orang-utans as non-detrimental and peaceful creatures living in the depths of the jungle. The recent increase in conflicts between crop-owners and orang-utans has greatly changed this perception: from being part of their natural heritage, orang-utans are now a competitor and a hindrance to economic growth. Reconciling people and wildlife and identifying ways for local communities and displaced animals to cohabit peacefully is the major conservation challenge in these fractured landscapes.
The Changing Environment of the Orang-utan
Great apes are our closest living relatives (Harrison 2010). Orang-utans (literally ‘men of the forest’) occur solely in Borneo (Pongo pygmaeus) and in Sumatra (Pongo abelii).
The oil palm industry is blamed for the demise of iconic species such as the orang-utan Pongo pygmaeus in Borneo but production of, and demand for, this commodity continue to expand. Therefore, a better understanding of how the orang-utan is adapting to human-transformed environments is crucial for conserving the species. Results from a combination of repeated ground transects, aerial presence/absence surveys, and interviews of workers in mature plantations of the lower Kinabatangan River floodplain (eastern Sabah) provide an overall picture of the current status of orang-utans in an established agro-industrial oil palm landscape. Our results show that orang-utans disperse into mature plantations, use oil palm trees for nesting, and feed on mature fruits. Most oil palm workers report orang-utans of all age–sex classes within the estates but fail to report any negative effect of the animals on productivity of mature palms ≥5 years. Our surveys also show that orang-utan presence in the mature oil palm landscape is correlated with proximity to natural forest patches. These results suggest that forest patches, even when small, fragmented and degraded, are required to sustain the species in human-transformed landscapes. Homogenous oil palm plantations are incompatible with viable populations of orang-utans. The cessation of further forest conversion to agriculture and the enforcement of better management practices are needed to reduce the threat of oil palm development to orang-utan survival.
Because of the difficulties encountered in detecting many large tropical forest-dwelling species in their natural habitat, precise figures concerning the distribution, number and trends of many populations remain deficient. In tropical forests, ground surveys are generally carried out by counting objects along straight lines. These counts require a strict compliance with the line-transect methodology before (proper design of the census), during (careful data collection) and after (accurate and correct data processing and analysis) the census itself. In addition, the major source of bias when estimating population size and/or trends comes from the extrapolation of estimates obtained in small sampling areas to the larger, and often incompletely known, distribution of the population. In the Kinabatangan floodplain (Sabah, Malaysia), helicopter surveys were useful in directly assessing the distribution of orang-utans and were a major advantage in the precise estimation of the size of the orang-utan population surviving in this region. Our survey showed that about 1100 orang-utans remain in the multiple-use forests of the Kinabatangan floodplain. These results provide new evidence on orang-utan adaptation to habitat disturbance and indicate the potential of the Kinabatangan multiple-use forests for orang-utan conservation. Helicopter surveys appear to be a promising alternative to ground survey for precise distribution assessment and for monitoring population trends of apes throughout their entire range in Asia and in some parts of Africa.
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