Modelling Population Viability of Three Independent Javan Gibbon (Hylobates moloch) Populations on Java, Indonesia

Abstract

Population viability analysis is a predictive procedure that uses a combination of different modelling approaches to estimate species vulnerability to extinction. Javan gibbons (Hylobates moloch) are vulnerable to local extinction primarily due to loss of habitat and hunting for the illegal pet trade. Using the modelling software VORTEX, we assessed the status of Javan gibbons in 3 areas (Ujung Kulon National Park, Halimun-Salak National Park, and Dieng Mountains) which hold over half of the remaining estimated number of gibbons on Java. Ujung Kulon and Halimun-Salak are long-time protected areas, whereas Dieng Mountains remain unprotected. For each area, we calculated the probability of extinction over a 100-year time period by testing different area-specific scenarios (e.g., hunting, deforestation, and increase in carrying capacity). Our modelling suggests each of the populations has a high chance of becoming extinct within the next 100 years if hunting and deforestation persist. If these threats are eliminated, the model shows each of the populations are large enough to persist in the long term whilst maintaining high levels of current genetic diversity. We conclude that specific actions should be implemented to develop more inclusive conservation management practices, especially improving awareness regarding the illegal wildlife trade and increased protection of wild populations and their habitats.

Home range variation and site fidelity of Bornean southern gibbons [Hylobates albibarbis] from 2010-2018

“…These and other as yet unknown impacts are expected to continue and worsen unless the underlying causes can be tackled, adding an additional major threat to biodiversity and increasing extinction risk for many species. Understanding the complex use of space of these territorial animals is important in assessing both carrying capacity [43,89], how dispersing gibbons use space and establish a territory [9,90–92] and understading how reintroduced gibbon pairs will establish their core and HR [93–97].…”

The Dialectical Primatologist

“…In a population viability analysis, Jaima Smith and colleagues calculated the extinction probability over a 100-year time period under various scenarios involving variable degrees of deforestation and loss of gibbons due to illegal hunting/ trade. Smith et al (2017) determined that the gibbon population at UKNP is indeed viable, provided their estimates for annual hunting losses can be avoided at all costs. Unfortunately, as I have recorded at Sancang, relatively widespread reverence for the forest and its denizens does not produce an infallible immunity from illegal poaching.…”

“…As such, the prevention of losses from hunting to supply the illegal pet trade remains the top conservation priority. With respect to deforestation, a scenario involving modest losses (1.2% annually) will not result in extinction, but lead to a reduced, remnant population (n = 135) within the 100-year time frame of the model (Smith et al 2017). Although the results from our 12.5 kilometres of reconnaissance walk "ground-truthing" exercise remain preliminary (Malone et al 2020), the observation of newly created irrigated rice paddies and ongoing small-scale deforestation within the park boundary are deeply concerning (Figure 4.8).…”

Phylogeography, Population Genetics, and Conservation of Javan Gibbons (Hylobates moloch)

“…Halimun-Salak, and Mts. Dieng) found that inbreeding depression (3.14 lethal equivalents per diploid genome on juvenile mortality) had no measurable effect on extinction risk in the medium-long term (100 years) (Smith et al 2017). Supriatna et al (1999) and Andayani et al (2001, p. 774) noted that Breleasing confiscated gibbons … into the wild population on Java should be avoided except under the most stringent conditions of taxonomic identification.Ĥ owever, Javan gibbons have been released in at least five areas in recent years, viz.…”