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

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, 2,204 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.