Most orangutans were, however, lost from forests, implying the importance of hunting

The most severe declines occurred in areas in which habitat was removed

We estimate that over 100,000 Bornean orangutans were lost between 1999 and 2015

Unsustainable exploitation of natural resources is increasingly affecting the highly biodiverse tropics []. Although rapid developments in remote sensing technology have permitted more precise estimates of land-cover change over large spatial scales [], our knowledge about the effects of these changes on wildlife is much more sparse []. Here we use field survey data, predictive density distribution modeling, and remote sensing to investigate the impact of resource use and land-use changes on the density distribution of Bornean orangutans (Pongo pygmaeus). Our models indicate that between 1999 and 2015, half of the orangutan population was affected by logging, deforestation, or industrialized plantations. Although land clearance caused the most dramatic rates of decline, it accounted for only a small proportion of the total loss. A much larger number of orangutans were lost in selectively logged and primary forests, where rates of decline were less precipitous, but where far more orangutans are found. This suggests that further drivers, independent of land-use change, contribute to orangutan loss. This finding is consistent with studies reporting hunting as a major cause in orangutan decline []. Our predictions of orangutan abundance loss across Borneo suggest that the population decreased by more than 100,000 individuals, corroborating recent estimates of decline []. Practical solutions to prevent future orangutan decline can only be realized by addressing its complex causes in a holistic manner across political and societal sectors, such as in land-use planning, resource exploitation, infrastructure development, and education, and by increasing long-term sustainability [].

Not by science alone: why orangutan conservationists must think outside the box.

Both Kalimantan and Sabah had the highest orangutan abundance in selectively logged forests, followed by primary forest. In Sarawak, the highest orangutan abundance was found in primary forests. The rate of orangutan decline across the three regions and these two land-use classes was less precipitous, but still high (49%–56%). The loss of orangutans in primary and selectively logged forests between 1999 and 2015 accounted for 67% of the total loss in Kalimantan (93,000 individuals; 95% CI: 26,500–162,300), 72% in Sabah (6,100 individuals; 95% CI: 2,400–10,000), and 83% of the total loss in Sarawak (900 individuals; 95% CI: 250–1,600).

By 2015, 50% of the orangutans estimated to have occurred on Borneo in 1999 were found in areas in which resource use had altered the environment. A comparison of distinct regions revealed that 50%, 60%, and 10% of the orangutans were affected by transformation into industrial oil palm or paper pulp plantations, deforestation, or selective logging in Kalimantan, Sabah, and Sarawak, respectively. Rates of orangutan decline were highest in areas deforested or converted to plantations (63%–75% loss) in both Kalimantan and Sabah ( Figure 3 ). In Sarawak, there were almost no industrial plantations and deforested areas within the orangutan metapopulation range, together affecting only 0.4% of the area and 2% of the orangutan population. Industrial plantations and deforestation contributed 7% (Kalimantan), 2% (Sabah), and less than 1% (Sarawak) to the overall estimated loss of orangutans in each of the three regions.

Percent area affected by resource use in orangutan metapopulations during the study period, forest and non-forest classes (pie charts), their spatial distribution (map), and total orangutan abundance and its change between the first study year (1999) and last study year (2015) (bar charts). Total metapopulation areas per province in square kilometers are given in the lower-right corner of the pie charts. Areas had been transformed into plantations (oil palm and paper pulp), deforested, or selectively logged between 1999 and 2015; were covered with forest (regrowth, primary or montane primary forest); were plantations already before the study period; or were another unspecified non-forest class. The percent orangutan abundance loss in comparison to 1999 is highlighted in rectangles in the bar charts. The “∗” indicates the absence of orangutans in the respective category. The error bars indicate the 95% confidence interval. On the x axes, the number “2000” is highlighted in blue to show the scale differences between the three areas. See also Figure S3

To identify possible causes for the estimated orangutan loss, we compared absolute abundance and density from the beginning and the end of the survey period between land-use types and assessed differences in change over time. We differentiated areas in which resource use had altered the environment and areas in which land-use remained unaltered during the study period. For land-use changes, we considered deforestation, conversion to industrial plantations (oil palm and paper pulp), and selective logging in natural forests. As stable land-use, we considered primary and montane primary forest, regrowth forests, industrial plantations established prior to the study period, and “other,” comprising non-forest areas.

We used predictions of forest cover from Struebig et al. [] for 2020 and 2050 to project future orangutan decline ( Figures 1 and 2 ). To this end, we assumed that orangutans cannot survive in areas without tree cover. The orangutan abundance in the three largest populations was projected to drop further and reach 31,100 individuals (95% CI: 22,500–44,000) in the Western Schwaner metapopulation area, 14,700 individuals (95% CI: 9,600–19,600) in Eastern Schwaner, and 6,100 individuals (95% CI: 3,800–10,000) in Karangan by 2050. The total future loss for all metapopulations was projected to be 45,300 (95% CI: 33,300–63,500). This projected future decline is only based on the direct consequence of habitat loss. It does not consider the effects of orangutan killing for food and in conflict and is therefore most likely an underestimate. All estimates are rounded to the nearest hundred.

Bornean orangutan density per 1 kmin the beginning and the end of the study period and for 2020 and 2050. Between 1999 and 2015, high-density areas (dark green) disappeared, and medium-density areas (light green) declined. Low-density areas (beige and purple) expanded. Future estimates are based on projected forest loss []; therefore, map representations between model estimates and future projections differ. Areas in which forest was projected to be lost also lose the resident orangutans. Hence, maps between 2015 and 2020 seem to lose many fragments inhabited by orangutans, but they already had low density before. Between 2020 and 2050, further areas were projected to lose forest, but the loss is less visible. See also Figures S1 and S2 and Tables S1–S3

Spatial Distribution of Estimated Orangutan Densities on Borneo for the Year 1999 and 2015 and Projections to 2020 and 2050

Figure 2 Spatial Distribution of Estimated Orangutan Densities on Borneo for the Year 1999 and 2015 and Projections to 2020 and 2050

Western Schwaner, the largest metapopulation, lost an estimated 42,700 individuals (95% confidence interval [CI]: 12,700–73,400) since 1999, with 40,700 (95% CI: 30,000–57,200) remaining in 2015. The second-largest population, Eastern Schwaner, lost 20,100 individuals (95% CI: 7,200–33,500) and was estimated to contain 16,800 (95% CI: 12,100–23,100) in 2015. In Karangan, the third-largest population, 8,200 individuals (95% CI: 1,900–15,400) were lost and 9,000 (5,900–14,200) remained in 2015. The total estimated loss of Bornean orangutans between 1999 and 2015 amounted to 148,500 individuals (95% CI: 48,100–252,300).

The three largest metapopulations were found in Kalimantan, the Indonesian part of Borneo, and have experienced a strong decline over the studied 16-year period ( Figure 1 ).

Orangutan abundance was estimated for the three largest metapopulations with a multi-model approach over the study period (1999 to 2015). Estimates of future orangutan abundance were based on forest cover projections for 2020 and 2050 by Struebig et al. [] and are indicated by a dashed line. Shaded areas and error bars represent the 95% confidence intervals. On the y axes, the number “10,000” is highlighted in blue to show the scale difference between the three populations. The map shows all identified metapopulations in gray. The three largest metapopulations are indicated by their color. State labels are as follows: Br, Brunei; Sb, Sabah; and Sk, Sarawak in Malaysia; WK, West; EK, East; NK, North; SK South; and CK, Central Kalimantan in Indonesia. See also Figures S1 and S2 and Tables S1–S3

Abundance of the Three Largest Orangutan Metapopulations between 1999 and 2015 and Projected Abundance for 2020 and 2050

Figure 1 Abundance of the Three Largest Orangutan Metapopulations between 1999 and 2015 and Projected Abundance for 2020 and 2050

With the aim of minimizing model uncertainty in spatial model predictions, we used multi-model inference and evaluated all possible combinations of covariates included in the full model ( Table S1 ). The complete set of all fitted models was then used to estimate the orangutan density distribution across the range. The estimated distribution was mapped to metapopulations delineated by experts at the Population and Habitat Viability Assessment Workshop (PHVA) for Bornean orangutans. In this context, the term “metapopulation” was used to identify larger entities that are bound by dispersal barriers, such as rivers, major roads, and areas without forests, and that include one or more orangutan subpopulations. Only 38 out of 64 identified metapopulations retained more than 100 individuals and can thus be considered to contain viable subpopulations [].

We built a predictive density distribution model to estimate Bornean orangutan abundance. The full model included survey year, climate, habitat cover, and human threat predictor variables (see STAR Methods and Key Resources Table ) and explained orangutan density significantly better than the null model including only the intercept (likelihood ratio test, χ= 1,440, df = 13, p < 0.001). Mean temperature and lowland and peatswamp forest cover had a significant positive relationship with orangutan density ( Figure S1 Table S1 ). Study year, rainfall variability, montane forest cover, and human population density negatively affected orangutan density ( Figure S1 Table S1 ). Intermediate levels of rainfall in dry months were related to higher densities of orangutans. The cover of deforested areas around transects was slightly positively correlated, but its confidence limits included zero. Topsoil organic carbon content, estimate of orangutan killing, and percentage of the population with hunting taboos were not significantly correlated with orangutan density.

To model Bornean orangutan (Pongo pygmaeus) density distribution and derive metapopulation abundances, we compiled orangutan field surveys. Estimates of orangutan density and abundance are usually derived from the observation of their nests [] on line transects []. A total of 36,555 orangutan nests were observed on 1,491 ground and 252 aerial transects that were surveyed between 1999 and 2015 throughout the Bornean orangutan range, with a total survey effort of 4,316 km (ground: 1,388 km; aerial: 2,928 km) and a median of 86 transects (interquartile range [IQR]: 28–156 transects) per year. The cumulative area of land surveyed contained 1,234 km. During the study period, the average yearly encounter rate significantly decreased from 22.5 to 10.1 nests/km (parameter estimate = −0.06, SE = 0.02, Z = −2.25, p = 0.04; the model contained the log-transformed mean nest encounter rate per year as response, weighted by the number of transects per year and the year as predictor).

Best Practice Guidelines for the Surveys and Monitoring of Great Ape Populations.

Discussion

16 Utami-Atmoko, S., Traylor-Holzer, K., Rifqi, M.A., Siregar, P.G., Achmad, B., Priadjati, A., Husson, S., Wich, S., Hadisiswoyo, P., Saputra, F., et al. (2017). Orangutan population and habitat viability assessment: final report. Report of the IUCN/SSC Conservation Breeding Specialist Group. http://www.cpsg.org/. 18 Goossens B.

Chikhi L.

Ancrenaz M.

Lackman-Ancrenaz I.

Andau P.

Bruford M.W. Genetic signature of anthropogenic population collapse in orang-utans. 8 Meijaard E.

Buchori D.

Hadiprakarsa Y.

Utami-Atmoko S.S.

Nurcahyo A.

Tjiu A.

Prasetyo D.

Nardiyono

Christie L.

Ancrenaz M.

et al. Quantifying killing of orangutans and human-orangutan conflict in Kalimantan, Indonesia. 9 Davis J.T.

Mengersen K.

Abram N.K.

Ancrenaz M.

Wells J.A.

Meijaard E. It’s not just conflict that motivates killing of orangutans. 10 Abram N.K.

Meijaard E.

Wells J.A.

Ancrenaz M.

Pellier A.-S.

Runting R.K.

Gaveau D.

Wich S.

Nardiyono

Tjiu A.

et al. Mapping perceptions of species’ threats and population trends to inform conservation efforts: the Bornean orangutan case study. 11 Santika T.

Ancrenaz M.

Wilson K.A.

Spehar S.

Abram N.

Banes G.L.

Campbell-Smith G.

Curran L.

d’Arcy L.

Delgado R.A.

et al. First integrative trend analysis for a great ape species in Borneo. 19 Ancrenaz, M., Gumal, M., Marshall, A.J., Meijaard, E., Wich, S.A., and Husson, S. (2016). Pongo pygmaeus. The IUCN Red List of Threatened Species 2016, e.T17975A17966347. https://doi.org/10.2305/IUCN.UK.2016-1.RLTS.T17975A17966347.en. 11 Santika T.

Ancrenaz M.

Wilson K.A.

Spehar S.

Abram N.

Banes G.L.

Campbell-Smith G.

Curran L.

d’Arcy L.

Delgado R.A.

et al. First integrative trend analysis for a great ape species in Borneo. 20 Campbell G.

Kuehl H.

N’Goran Kouamé P.

Boesch C. Alarming decline of West African chimpanzees in Côte d’Ivoire. 21 Kühl H.S.

Sop T.

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Hall J.S. Catastrophic decline of world’s largest primate: 80% loss of Grauer’s gorilla (Gorilla beringei graueri) population justifies Critically Endangered status. 23 Walsh P.D.

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et al. Catastrophic ape decline in western equatorial Africa. The unsustainable use of natural resources has caused a dramatic decline of Bornean orangutans. Only 38 out of 64 remaining metapopulations have more than 100 individuals, the assumed threshold for viability of Bornean orangutan populations []. Our findings suggest that more than 100,000 individuals have been lost in the 16 years between 1999 and 2015. All three analytical approaches employed in this study, based on field survey data, spatial covariate modeling, and remote sensing, corroborated the concluded impact of resource use and resulting decline of Bornean orangutans. The results are also very consistent with the genetic signature of a recent collapse found in an orangutan population in Sabah [] and evidence of large annual losses of orangutans through hunting and conflict killing in Kalimantan []. Our results substantiate the percentage loss estimated by Santika et al. [] and reinforce the recent uplisting of the Bornean orangutan as Critically Endangered on the IUCN Red List []. The numbers reported here are larger than past estimates [], but they are in line with findings reported for other great ape taxa [].

24 Elith J.

Leathwick J.R. Species distribution models: ecological explanation and prediction across space and time. We have established the density distribution of Bornean orangutans with a model-based approach that uses the relationships between predictor variables and observed orangutan abundance to predict abundance for unsurveyed sites. These predictions are useful for deducing trends at the regional to landscape scale [], but they may be limited at a local scale, where additional demographic and behavioral drivers can influence orangutan density distribution, e.g., ranging behavior in response to local food resources or conspecifics. Thus, our findings reveal patterns at large spatial scales, but great care should be taken when inferring from predictions at specific sites.

13 van Schaik C.P.

Priatna A.

Priatna D. Population estimates and habitat preferences of orangutans based on line transects of nests. 25 Ancrenaz M.

Calaque R.

Lackman-Ancrenaz I. Orangutan nesting behavior in disturbed forest of Sabah, Malaysia: implications for nest census. 26 Mathewson P.D.

Spehar S.N.

Meijaard E.

Nardiyono

Purnomo

Sasmirul A.

Sudiyanto

Oman

Sulhnudin

Jasary

et al. Evaluating orangutan census techniques using nest decay rates: implications for population estimates. 27 Marshall A.J.

Meijaard E. Orang-utan nest surveys: the devil is in the details. Another aspect of our study that requires critical assessment is the inference of orangutan abundance from nest counts. Nest decay time, an essential parameter to translate nest density into orangutan density, varies between survey sites. Although factors like rainfall, wood density, and complexity of nest architecture are known to influence nest decay time [], additional variability in decay time between sites is not fully understood []. We addressed this issue by using all available datasets on orangutan nest decay, comprising information on the lifespan of more than 1,000 nests (see STAR Methods ) across Borneo. If our findings of orangutan decline were an artifact of severely biased nest decay times, this would require nest decay time to have halved over the course of the study period. However, we found no indication of this and so do not consider this to be a limitation of our study.

28 Husson S.J.

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Gumal M.

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Simorangkir T. Orangutan distribution, density, abundance and impacts of disturbance. 29 Abram N.

Ancrenaz M. Orangutan, Oil Palm and RSPO: Recognising the Importance of the Threatened Forests of the Lower Kinabatangan, Sabah, Malaysian Borneo. 30 Russon A.E.

Kuncoro P.

Ferisa A. Orangutan behavior in Kutai National Park after drought and fire damage: adjustments to short- and long-term natural forest regeneration. 31 Wich S.A.

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Setia T.M.

Djoyosudharmo S.

Geurts M.L. Dietary and energetic responses of Pongo abelii to fruit availability fluctuations. 28 Husson S.J.

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Gumal M.

Hearn A.J.

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Simorangkir T. Orangutan distribution, density, abundance and impacts of disturbance. 32 Gaveau D.L.A.

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Pacheco P.

Meijaard E. Rapid conversions and avoided deforestation: examining four decades of industrial plantation expansion in Borneo. 33 Ancrenaz M.

Oram F.

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Meijaard E. Of Pongo, palms and perceptions: a multidisciplinary assessment of Bornean orang-utans Pongo pygmaeus in an oil palm context. 34 Meijaard E.

Albar G.

Nardiyono

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Ancrenaz M.

Spehar S. Unexpected ecological resilience in Bornean orangutans and implications for pulp and paper plantation management. 35 Spehar S.N.

Rayadin Y. Habitat use of Bornean Orangutans (Pongo pygmaeus morio) in an Industrial Forestry Plantation in East Kalimantan, Indonesia. 33 Ancrenaz M.

Oram F.

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Ahmad E.

Elahan H.

Kler H.

Abram N.K.

Meijaard E. Of Pongo, palms and perceptions: a multidisciplinary assessment of Bornean orang-utans Pongo pygmaeus in an oil palm context. 34 Meijaard E.

Albar G.

Nardiyono

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Spehar S. Unexpected ecological resilience in Bornean orangutans and implications for pulp and paper plantation management. 35 Spehar S.N.

Rayadin Y. Habitat use of Bornean Orangutans (Pongo pygmaeus morio) in an Industrial Forestry Plantation in East Kalimantan, Indonesia. Contrary to our expectations, the model coefficient for deforestation indicated a slightly positive relationship between deforestation in years prior to the survey and orangutan abundance. There are several possible explanations for this observation, suggesting that the model coefficient does not capture a causal relationship. First, surveys tend to be biased toward areas with known orangutan occurrence. Thus, our dataset possibly lacks sufficient variance for detecting the true impact of deforestation on orangutan density. Second, some studies have suggested that the number of orangutans in areas adjacent to deforested areas is temporally inflated, due to the displacement of individuals and subsequent refugee crowding []. Third, high dietary flexibility allows orangutans to be resilient in the face of some levels of disturbance []. This may delay the effects of deforestation on the observed density for several years, before populations eventually start to decline []. Irrespective of this, when we compare spatial model predictions and remotely sensed land-use change, the highest rates of orangutan decline were detected in areas with habitat removal (deforestation and conversion to industrial plantations). This shows that the predictive density distribution model has indirectly captured the deleterious effects of deforestation on orangutan abundance. Our finding suggests that deforestation and industrial oil palm and paper pulp plantations are responsible for about 9% (14,000 individuals) of the total loss of orangutan abundance. Whereas in the early years of the study it was mainly degraded land with low orangutan density that was converted to industrial plantations, after 2005 the conversion of forests to oil palm plantations has been increasing dramatically []. Some studies have suggested that orangutans can occur in oil palm or paper pulp plantations, when these are managed well and adjacent forest fragments are maintained []. However, it is unclear whether this is just a transient effect or whether orangutans can indeed persist over the long term [].

36 Ancrenaz M.

Sollmann R.

Meijaard E.

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Ross J.

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Chivers D.J. Fine-scale habitat use by orang-utans in a disturbed peat swamp forest, central Kalimantan, and implications for conservation management. 39 Wearn O.R.

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Bernard H.

Ewers R.M. Mammalian species abundance across a gradient of tropical land-use intensity: a hierarchical multi-species modelling approach. 36 Ancrenaz M.

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et al. Coming down from the trees: is terrestrial activity in Bornean orangutans natural or disturbance driven?. 40 Meijaard E.

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Runting R.K.

Mengersen K. People’s perceptions about the importance of forests on Borneo. 33 Ancrenaz M.

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Meijaard E. Of Pongo, palms and perceptions: a multidisciplinary assessment of Bornean orang-utans Pongo pygmaeus in an oil palm context. 34 Meijaard E.

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Nardiyono

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Spehar S. Unexpected ecological resilience in Bornean orangutans and implications for pulp and paper plantation management. The highest orangutan abundances were found in selectively logged forests in Kalimantan and Sabah and in primary forests in Sarawak. This finding is consistent with studies reporting that orangutans can occur in selectively logged or regenerating logging concessions, depending on the type and intensity of logging operations []. Consequently, successful orangutan conservation is necessarily situated in multi-functional landscapes [] and recognizes the importance of degraded and logged forests as well as forest fragments in plantation matrices [].

41 Struebig M.J.

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et al. Borneo Mammal Distribution Consortium

Targeted conservation to safeguard a biodiversity hotspot from climate and land-cover change. 33 Ancrenaz M.

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Kler H.

Abram N.K.

Meijaard E. Of Pongo, palms and perceptions: a multidisciplinary assessment of Bornean orang-utans Pongo pygmaeus in an oil palm context. 35 Spehar S.N.

Rayadin Y. Habitat use of Bornean Orangutans (Pongo pygmaeus morio) in an Industrial Forestry Plantation in East Kalimantan, Indonesia. 42 Meijaard E.

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Ancrenaz M. An Impact Analysis of RSPO Certification on Borneo Forest Cover and Orangutan Populations. 43 Meijaard E.

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Ancrenaz M. An Impact Analysis of RSPO Certification on Borneo Forest Cover and Orangutan Populations. Effective partnerships with logging companies, whose concessions harbor the majority of orangutans, are essential to curb orangutan loss []. Similarly, partnerships with oil palm and paper pulp producers are important to promote best practice guidelines for management []. Such partnerships have already been reported, e.g., by Meijaard et al. [], and could potentially provide co-benefits for biodiversity conservation in general []. The Roundtable on Sustainable Palm Oil (RSPO) and the Forest Stewardship Council (FSC) are examples of certification schemes that incentivize these partnerships, by enabling consumers to favor responsible natural resource management [].

44 Benítez-López A.

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Gonder M.K.

Humle T.

Leendertz F.H. Ebola in great apes – current knowledge, possibilities for vaccination, and implications for conservation and human health. The pervasive decline of orangutans in more intact habitat is consistent with various studies identifying hunting as the main driver of biodiversity loss in the tropics [], including Southeast Asia []. More specifically, our observation is supported by the results of extensive interview surveys in Kalimantan that show that, per year, on average 2,256 orangutans were hunted or killed due to conflict with humans []. The estimate of orangutan killing in the model is based on a Borneo-wide projection of hunting pressure derived from these interview surveys []. In the model, this predictor did not show an influence on orangutan density. It is possible that our survey dataset lacks sufficient variance for detecting the impact of killing on orangutan density or the available predictor layer does not well represent the actual pressure, as the relationship between density and hunting or killing is very complex and non-linear. Human population density, on the other hand, had a significant negative influence on orangutan densities in the model and may have already captured the effect of orangutan killing. Orangutans are also present in the national and international wildlife trade. Traded orangutans are usually young orphans, and for each orphan, adult individuals have been killed []. Due to the low reproductive rate of the species, even very low offtake rates of reproductive females (∼1% per year) will drive populations to extinction []. In the absence of plausible alternative explanations for the observed loss of orangutans in seemingly intact habitats, such as the occurrence of widespread and highly lethal infectious diseases as observed among African apes [], killing is the most likely explanation. From this perspective, our prediction of a further loss of 45,300 orangutans over the next 35 years, based solely on projections of forest cover change, is most likely an underestimate. Furthermore, many individuals currently occur in fragmented, small populations that are assumed not to be viable and will most likely disappear in the near future.

Knowledge about the density distribution of key species is essential to explore the consequences of land-use change, exploitation of natural resources, development of infrastructure, and climate change. It is also needed to evaluate which conservation interventions are most effective in reducing decline and loss of biodiversity.

In essence, natural resources are being exploited at unsustainably high rates across tropical ecosystems, including Borneo. As a consequence, more than 100,000 Bornean orangutans vanished between 1999 and 2015. The major causes are habitat degradation and loss in response to local to global demand for natural resources, including timber and agricultural products, but very likely also direct killing. Our findings are alarming. To prevent further decline and continued local extinctions of orangutans, humanity must act now: biodiversity conservation needs to permeate into all political and societal sectors and must become a guiding principle in the public discourse and in political decision-making processes.