![]() The advantages of using nest counts include: (a) nests are proxies for orang-utans (b) indicator of active habitat use as weaned individuals build nests on an almost daily basis to sleep at night or sometimes to rest during the day (c) higher encounter rates than encounters with great apes and (d) easier measurement of perpendicular distances as nests are stationary 16, 17.Ĭurrently, the standard survey protocol to estimate orang-utan density consists of: (a) counting all nests visible from a line transect or plot (b) generating nest density within the area surveyed and (c) converting nest density into orang-utan density using an algorithm 16, 18. For orang-utans, this means counting nests instead of individuals or groups. Researchers in general opted for indirect sign counts to generate population estimates due to constraints on direct counts 16, 17. The great apes are elusive, solitary and live in small population sizes which require greater effort to detect 13, 16. However, it is rarely feasible to acquire accurate population and density estimates from direct counts of orang-utans in the wild. This is in line with implementing the Orang-utan Population and Habitat Viability Assessment (PHVA) mitigation measures with the goals of maintaining high forest cover at orang-utan habitats and improving connectivity between forest patches with orang-utans 11, 12. The decline is likely to continue in the immediate future considering social and economic circumstances 13 and the economic importance of oil palm plantations in Malaysia and Indonesia 14.ĭue to limited data collection, continued monitoring of orang-utan abundance is crucial to assess their population status and rates of population decline 12, 15. The Bornean orang-utan populations suffered more than 25% decline between 19 11, 12 despite an increase in scientific interest and public support. Threats to orang-utan survival have intensified in the past 60 years due to rapid deforestation 4, 5, land use conversion into monoculture plantations 6, 7, habitat fragmentation 8, illegal wildlife trade and hunting of the species 9, 10. The world’s three orang-utan species, Sumatran ( Pongo abelii), Bornean ( Pongo pygmaeus) and Tapanuli orang-utans ( Pongo tapanuliensis) are listed as Critically Endangered under the IUCN Red List of Threatened Species, with the latter described and listed in 2017 1, 2, 3. Over the course of this research, two of the study sites were gazetted as extensions to the Lanjak-Entimau Wildlife Sanctuary for orang-utan conservation. We discussed the limitations and advantages of our study design, and made recommendations to improve the sampling scheme. Only three out of the seven estimates had <35% overlap to indicate strong reliability. We subsequently assess the strength or reliability of the generated estimates using identifiability tests. We visually inspected the posterior distributions of our parameters and compared precisions between study sites. The result was a combined estimate of 355 orang-utans with the 95% highest density interval (HDI) of 135 to 602 individuals. We used the N-mixture models to integrate suitability, abundance and detection models which account for zero inflation and imperfect detection for the analysis. Our survey teams marked new nests on the first survey and revisited the plots on two more occasions after about 21 and 42 days respectively. ![]() We used the Marked Nest Count (MNC) method to count new orang-utan nests at seven previously undocumented study sites in Sarawak, Malaysia. The integration of Bayesian analysis into existing great ape survey methods could be used to generate precise and reliable population estimates of Bornean orang-utans.
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