764 A. Yanuar et al.


FIG. 1 Sungai Palin and Nanga Awen, indicating the location of the transects used to survey for orangutan nests in limited production forest and peat swamp forests, in Kapuas Hulu, west Kalimantan, Indonesia.


In both 2017 and 1991 the perpendicular distance of all


nests visible from each transect was measured with a tape. Density estimates for both sets of data were computed using the signed nest-count technique (Schaik et al., 1995). We used Distance 6.0 (Thomas et al., 2010) to estimate the effective strip width and thus determine the survey area and estimate nest density in both Sungai Palin and Nanga Awen. Distance uses a detection curve to estimate detectability (Buckland et al., 1993) as a function of the perpendicular dis- tance of nests from the centre of the transect (Cattau et al., 2014). We calculated nest density as Dn = n/L2w, where Dn is nest density, n is the number of nests counted on a tran- sect; L is transect length (in km), and w is the effective transect strip width (measured in m, converted to km), and orangutan density was calculated as Do = Dn/prt, where Do is orangutan density, p is the proportion of nest builders, r is daily nest production rate per orangutan and t is nest decay rate (Russon et al., 2001; Morrogh-Bernard et al., 2003; Johnson et al., 2005; Van Schaik et al., 2005). We used values of p = 0.85, r = 1.0 and t = 202, determined in studies (Ancrenaz et al., 2004) at sites with a similar habitat type and disturbance level to our survey area (peat-swamp forest of Lower Kinabatangan and surrounding areas in Malaysia; Ancrenaz et al., 2005). Weobserved a total of 76 orangutan nests along a total of


4.5 km of transects in Sungai Palin, and 71 nests along a total of 3.4 km of transects were observed in Nanga Awen. Mean nest densities were 222 ± SE 82.14 and 499 ± SE 162.43 per km2 in Sungai Palin and Nanga Awen, respectively (Table 1). The density estimate for Sungai Palin in 2017 is slightly lower than that for Nanga Awen in 1991 (Table 1), suggesting that orangutan density may have halved between these dates and that the orangutan population outside


protected areas remains relatively low. The building of new roads is one of the threats to the orangutan subpopulation on Sungai Palin (Local people, pers. comm., 2017). Estimated orangutan density in Sungai Palin is lower


than in the peat-swamp forest of Danau Sentarum National Park and the peat-swamp forest of the former Mega-Rice project in central Kalimantan, higher than that in the lowland and hill lowland forests of Betung Kerihun National Park, but similar to that in Sebangau National Park (Table 2). Many factors influence orangutan density in disturbed forests, including food availability, connectivity between forest remnants, and whether orangutans can sur- vive after habitat loss. The logging concession in the area of Nanga Lauk village has been inactive since at least 2003 (Local people, pers. comm., 2017), and there are no reports of local people hunting orangutans in the Nanga Lauk forest area, even though orangutans sometimes take honey from local beehives (Local people, pers. comm., 2017). Protection forests surrounding the Sungai Palin and


Nanga Awen landscape are under the jurisdiction of the district forestry agency, but are managed by the local com- munity as the Nanga Lauk Village Forest (Hutan Desa). The community is allowed to extract non-timber forest products only. This initiative to include the local community in man- aging forest land has helped to protect the orangutan, as evi- denced by the fact that logging ceased in 2003. The people of Nanga Lauk plan to expand the area of this Village Forest (Damayanti et al., 2016). More detailed surveys of the orangutan and its habitat


are required in this area, so that the orangutan population can be appropriately managed. As Sungai Palin and Nangan Awen lie between Danau Sentarum Wildlife Reserve and Betung Kerihun National Park, awareness amongst the


Oryx, 2020, 54(6), 763–766 © 2019 Fauna & Flora International doi:10.1017/S0030605319000875


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