Population status of Sumatran and Tapanuli orangutans: A comprehensive assessment 2021–2023

Hadi, AN ORCID: 0009-0008-3007-9377, Siregar, JP ORCID: 0009-0005-0104-7323, Anugrah, N, Kuswanda, W ORCID: 0000-0002-8496-3263, Boang Manalu, JH, Nasution, N, Fauzan, R, Wardani, NK, Subhan, Barata, UW, Rahman, DA ORCID: 0000-0001-5405-5400, Ishadamy, MY, Sitorus, SF, Andayani, N ORCID: 0000-0002-1888-193X, Supriatna, J ORCID: 0000-0001-9850-8395, Wich, S ORCID: 0000-0003-3954-5174, Kühl, HS ORCID: 0000-0002-4440-9161 and Buckley, BJW ORCID: 0000-0003-2186-9365 (2026) Population status of Sumatran and Tapanuli orangutans: A comprehensive assessment 2021–2023. Global Ecology and Conservation, 69. ISSN 2351-9894

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Abstract

Both the Sumatran orangutan (Pongo abelii) and Tapanuli orangutan (Pongo tapanuliensis) are Critically Endangered, yet no range-wide population assessment has been conducted since 2011. We present results from 208 systematic line-transect nest surveys across the full ranges of both species in northern Sumatra, Indonesia (2021–2023), analysed using distance sampling, Negative Binomial Generalised Linear Models with spatial autocorrelation and a matched-distribution framework enabling direct temporal comparison with the 2011 baseline. We estimate 11,694 (95% CI: 10,949–12,518) Sumatran orangutans across eight meta-populations and 716 (95% CI: 645–792) Tapanuli orangutans across three meta-populations (12,410 combined). For the Tapanuli orangutan, this constitutes the first systematic range-wide baseline; at 716 individuals, the species has no demographic buffer against additional mortality. The matched-distribution comparison indicates a 19.5% decline in Sumatran orangutans, equivalent to ∼1.8% per year. Forest structure and human population density were the strongest predictors of orangutan density across both species. Forest loss explained 76% of variation in population change across all meta-populations (R² = 0.76), but several meta-populations, notably Batu Ardan and Siranggas, declined substantially more than habitat loss predicted, suggesting non-habitat mortality contributes to losses in these areas. Highest densities were recorded in the Trumon–Singkil peat swamp. Across both species, 82% of orangutans occur within formally protected areas, 2204 individuals (18%) remain in landscapes with no formal conservation status, including the entire Tripa population. Gazettement alone is insufficient: enforcing existing legal protections, restoring connectivity between fragmented meta-populations and targeted intervention in unprotected landscapes are immediate priorities to avert further decline.

Item Type:	Article
Uncontrolled Keywords:	0502 Environmental Science and Management; 0602 Ecology; 3103 Ecology; 4104 Environmental management
Subjects:	G Geography. Anthropology. Recreation > GE Environmental Sciences Q Science > QL Zoology
Divisions:	Biological and Environmental Sciences (from Sep 19)
Publisher:	Elsevier
Date of acceptance:	17 June 2026
Date of first compliant Open Access:	25 June 2026
Date Deposited:	25 Jun 2026 09:31
Last Modified:	25 Jun 2026 09:31
DOI or ID number:	10.1016/j.gecco.2026.e04302
URI:	https://researchonline.ljmu.ac.uk/id/eprint/28898

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