Nearly half of all primate species are threatened with extinction, with habitat destruction being the biggest threat to their survival. Studies on the impact of habitat changes on primate populations are limited and often based on inferences because primates are long-lived mammals with slow life cycles, and generally respond very slowly to environmental changes. Such information is, however, essential for developing effective management plans for long-term conservation.

Proboscis monkeys Nasalis larvatus: female and young (left) and male (right). © Ikki Matsuda/Chubu University

The proboscis monkey Nasalis larvatus is endemic to the island of Borneo. The species is classified as Endangered according to the IUCN Red List of Threatened Species, and listed in Appendix I of CITES. Despite these levels of protection, lowland swamp forest habitats vital to this species are decreasing at alarming rates, mainly through logging and forest conversion to oil palm plantations.

Riverine forest, the habitat of the proboscis monkey. © Ikki Matsuda/Chubu University

In the study ‘Population dynamics of the proboscis monkey Nasalis larvatus in the Lower Kinabatangan, Sabah, Borneo, Malaysia’, published in Oryx—The International Journal of Conservation, we assessed the population trends of proboscis monkeys over 10 years in the Lower Kinabatangan floodplain in Sabah, Malaysian Borneo. Comparisons of directly counted 2004 and 2014 population sizes revealed subtle changes, where population densities fluctuated. Although the total number of observed monkeys showed no general increase or decrease, group sizes were significantly smaller in 2014 compared to 2004.

One reason for the relatively stable population is the reduced losses of important forests along the rivers where proboscis monkeys are generally found as a result of increased protection measures in the Kinabatangan floodplain over the study decade. Our analysis of habitat changes showed that, in protected reserves, forests within an 800 m buffer from riverbanks had remained relatively intact. This suggests that the protection of riparian forests can contribute immensely to the stability of proboscis monkey populations. However, larger losses of interior forests and some losses within the riparian zone meant that habitats had generally become more degraded and fragmented, and this could have contributed to limited population growth and reduced group sizes, which is consistent with findings from other primate studies.

A proboscis monkey (in red circle) in a narrow riparian forest strip. © John Sha

Although the protection of forests within the proboscis monkeys’ range had proved effective in mitigating serious negative impacts, this was not the case in unprotected forests, where ongoing forest loss could eventually lead to a fifth of the population being threatened. At least a third of these forests has been allocated for oil palm cultivation but much of these are unsuitable for oil palm because they are subject to seasonal or daily flooding. Further efforts to more effectively conserve high value habitats and to restore riparian areas could go a long way towards ensuring the long-term viability of this species.

The paper ‘Population dynamics of the proboscis monkey Nasalis larvatus in the Lower Kinabatangan, Sabah, Borneo, Malaysia’ is now freely available until the end of the year.

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