Bornean Orangutans are lowland forest specialists, rarely found above 500 m asl. In the 1950s, the habitat suitable for orangutans extended across ~255,000 km² of the island of Borneo (see below).
The two major reasons why most Bornean Orangutans populations are in sharp decline are (1) destruction, degradation and fragmentation of their habitats, and (2) hunting. Recurrent forest fires, especially in peat forests, cause additional sharp declines about once every decade. Bornean Orangutans decreased by more than 60% between 1950 and 2010, and a further 22% decline is projected to occur between 2010 and 2025 (see below). Combined, this equates to a loss of more than 82% over 75 years, 1950–2025. Given that a Bornean Orangutan's generation length is ~25 years (Wich et al. 2009), this decline will occur in a period of three generations. Each of the Pongo pygmaeus subspecies is roughly equally affected. Only one comprehensive quantitative survey of Bornean Orangutans has been conducted, in 2010, which prohibits quantitative assessment of changes in numbers for most populations. Temporal changes in population status are therefore best assessed via the proxies of habitat loss and hunting rates. A detailed rationale for a population decline of more than 86% between 1950 and 2025 follows.
The most accurate estimate of the geographic range of Bornean Orangutans showed that in 2010, 59.6% of the forest remaining in Borneo was suitable habitat (155,106 km² of 260,109 km² of forest: Wich et al. 2012, Gaveau et al. 2014). Considering that in 1973, 75.7% of Borneo (424,753 km²) was under natural forest (Gaveau et al. 2014), we estimate that 253,153 km² of forest was orangutan habitat at this time.
Mechanized logging in Borneo started in the early 1950s, and industrial logging and forest conversion intensified in the late 1960s. The rate of forest conversion is difficult to estimate prior to 1973 due to the lack of satellite imagery, but a recent spatial analysis evaluated forest persistence, clearance and logging spanning the 37 years between 1973 and 2010 (Gaveau et al. 2014). We take the rate of forest loss documented from 1973 to 2010 as conservative and de facto lower than if data were available from 1950.
1. Habitat loss and orangutan decline
During the period 1973–2010, 39% of Bornean forests were lost (Gaveau et al. 2014), representing a net loss of 98,730 km² of prime orangutan habitat. It is estimated that a further 37% of suitable orangutan habitat (155,106 km²) will be converted to plantations between 2010 and 2025, which accounts for the loss of an additional 57,140 km² of orangutan habitat (Wich et al. 2012). Compared to the baseline (253,153 km²), more than 155,867 km² or of 61.5% of orangutan habitat will be gone by 2025; see Table 1 in Supplementary Material.
The orangutan habitat remaining in 2010 (97,716 km²) was either protected or designated for timber production (Wich et al. 2012). Nonetheless, forest loss is expected to occur here too, owing to fires, encroachment and smallholder plantation development. Rates of forest loss measured at two sites with the largest Borneo orangutan populations are: 1.9% per year (1991–2000) and 1.5% per year (2000–2007) at Sebangau National Park (Husson et al. 2015); and 2.4% per year including the buffer zone, or 1.1% excluding the buffer zone (1988–2002) at Gunung Palung National Park (Curran et al. 2004). The rate of loss in production forests outside formally protected areas will undoubtedly be higher (Santika et al. 2015). Thus we conservatively estimate the ongoing rate of loss in this administrative type of forest to be 1.5% per year. This will represent another 19,821 km² of forest lost between 2010 and 2025: 20.2% of the orangutan habitat in 2010, or 8.7% in 1973.
2. Habitat degradation and orangutan decline
In addition to habitat loss, selective logging has degraded 56% of Bornean Orangutan habitat since 1973 (Gaveau et al. 2014). The impacts of logging on orangutan density are variable, from little change in lightly-logged forest to major negative impacts in heavily-logged forest (Ancrenaz et al. 2010). For example, selective artisanal logging reduced orangutan densities in peat-swamp forests by 21–30% (Husson et al. 2009), while mechanised logging in dryland forests is presumed to have a greater impact. Thus, 56% of the Bornean Orangutan range could undergo a 20% decrease in carrying capacity. This estimate is conservative, considering that in Kalimantan the total area of natural forest allocated for timber extraction is increasing. Reduction in carrying capacity due to logging would then equate to a loss of 7% of the 1973 population by 2010, and overall accounts for 4% of the total projected 1973–2025 population decline.
3. Hunting and orangutan decline
The widespread impacts of illegal hunting had not been quantified prior to a major questionnaire study throughout Kalimantan in 2008–2009 (Meijaard et al. 2011). The authors estimated that 630–1,357 Bornean Orangutans were killed in 2008 and that an average of 2,383–3,882 per year had been killed during the lifetimes of the survey respondents. The mean estimate (2,256 orangutans poached in Kalimantan each year) equates to 2.6% of the 2010 population for Kalimantan. Population losses due to hunting may be partially offset by population growth, which has a maximum theoretical rate of 2% annually (Marshall et al, 2009). The only study to have measured growth in a Bornean Orangutan population was carried out in Sebangau National Park in a population recovering from a logging-induced crash. Here, growth was relatively uninhibited and estimated to increase at an average annual rate of 1.6% between 2001 and 2013 (Husson et al. 2015). It is highly unlikely that a continuously-hunted population could recover at this rate, but in order to take into account uncertainties in determining the level of hunting, a 1.5% growth rate is applied, resulting in a net population decrease due to hunting of 1.1% annually. This equates to an additional loss (once habitat clearance and impacts of logging are factored in) of 18% of the 1973 population by 2010 and 7% of the 2010 population by 2025. Overall, poaching contributes 12% to the estimated 1973–2025 population decrease.
The combined impacts of habitat loss, habitat degradation and illegal hunting equate to an 86% population reduction between 1973 and 2025 which qualifies the species for listing as Critically Endangered. This estimate is relatively conservative, as it does not include additional future population losses anticipated due to stochastic effects that will reduce populations inhabiting increasingly small forest fragments.
Orangutan habitat loss and killing were already significant threats during the period 1950–1973, and the species was already declining at this time. However the paucity of data for this period prevents to estimate a specific rate of population decline for this specific period of time. However, if we assume that orangutan abundance in 1950 was similar to that in 1973 (which is not the case because any declines that occurred between 1950 and 1973 are not accounted for), our analysis shows that the species will suffer a more than 80% decline in three generations (1950–2025).